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This limitation of liability shall apply to any claim or cause whatsoever whether such claim or cause arises in contract, tort or otherwise. Logger and System. UncaughtExceptionHandler Interface The java. Furthermore, it has held that distinction for many years. Java leapt to the forefront of Internet programming with its first release. Each subsequent version has solidified that position. Today, it is still the first and best choice for developing web-based applications.
It is also a powerful, general-purpose programming language suitable for a wide variety of purposes. Simply put: much of the modern world runs on Java code. Java really is that important. Since its original 1. Most importantly, it has not just followed the trends, it has helped create them. Since this book was first published in , it has gone through several editions, each reflecting the ongoing evolution of Java.
As a result, this edition of the book contains a substantial amount of new material, updates, and changes. Of special interest are the discussions of two key features that have been added to Java since the previous edition of this book. The first is local variable type inference because it streamlines some types of local variable declarations. To support local variable type inference, the context-sensitive, reserved type name var has been added to the language.
The second key new Java feature is the reworking of the version number to reflect what is expected to be a faster release cycle, which started with JDK As explained in Chapter 1, Java feature releases are now anticipated to take place every six months.
This is important because it is now possible for new features to be added to Java at a more rapid pace than in the past. Although introduced in the previous edition of this book, there are two recently added Java features that are still having a strong impact on Java programmers. The first is modules, which enable you to specify the relationships and dependencies of the code that comprises an application.
The addition of modules by JDK 9 represents one of the most profound changes ever made to the Java language. For example, it resulted in the addition of 10 context-sensitive keywords. Modules also significantly impacted the Java API library because its packages are now organized into modules.
Furthermore, to support modules, new tools have been added, existing tools have been updated, and a new file format has been defined. Because of their importance, the entirety of Chapter 16 is devoted to modules. The second recently added feature is JShell. JShell is a tool that offers an interactive environment in which it is easy to experiment with code snippets without having to write an entire program. Both beginners and experienced professionals will find it quite useful.
An introduction to JShell is found in Appendix B. A Book for All Programmers This book is for all programmers, whether you are a novice or an experienced pro. The beginner will find its carefully paced discussions and many examples especially helpful. For both, it offers a lasting resource and handy reference.
Significant portions of the Java API library are also examined. The book is divided into four parts, each focusing on a different aspect of the Java programming environment. Part I presents an in-depth tutorial of the Java language. It begins with the basics, including such things as data types, operators, control statements, and classes.
It then moves on to inheritance, packages, interfaces, exception handling, and multithreading. Next, it describes annotations, enumerations, autoboxing, generics, and lambda expressions. The final chapter in Part I covers modules. Part III offers three chapters that introduce Swing. Part IV contains two chapters that show examples of Java in action. The first discusses Java Beans. The second presents an introduction to servlets.
Patrick Naughton was one of the creators of the Java language. He also helped write the first edition of this book. For example, among many other contributions, much of the material in Chapters 21, 23, and 27 was initially provided by Patrick. His insights, expertise, and energy contributed greatly to the success of that book. Joe helped on several of my books, and his input has always been top-notch. Danny Coward is the technical editor for this edition of the book.
Danny has worked on several of my books, and his advice, insights, and suggestions have always been of great value and much appreciated. Like the successful computer languages that came before, Java is a blend of the best elements of its rich heritage combined with the innovative concepts required by its unique mission. While the remaining chapters of this book describe the practical aspects of Java—including its syntax, key libraries, and applications—this chapter explains how and why Java came about, what makes it so important, and how it has evolved over the years.
Although Java has become inseparably linked with the online environment of the Internet, it is important to remember that Java is first and foremost a programming language. Much of the character of Java is inherited from these two languages.
From C, Java derives its syntax. Moreover, the creation of Java was deeply rooted in the process of refinement and adaptation that has been occurring in computer programming languages for the past several decades. For these reasons, this section reviews the sequence of events and forces that led to Java. As you will see, each innovation in language design was driven by the need to solve a fundamental problem that the preceding languages could not solve.
Java is no exception. Its impact should not be underestimated, because it fundamentally changed the way programming was approached and thought about. The creation of C was a direct result of the need for a structured, efficient, high-level language that could replace assembly code when creating systems programs.
For example, although FORTRAN could be used to write fairly efficient programs for scientific applications, it was not very good for system code. Assembly language can be used to produce highly efficient programs, but it is not easy to learn or use effectively. Further, debugging assembly code can be quite difficult.
Instead, they relied upon the GOTO as a primary means of program control. While languages like Pascal are structured, they were not designed for efficiency, and failed to include certain features necessary to make them applicable to a wide range of programs. Specifically, given the standard dialects of Pascal available at the time, it was not practical to consider using Pascal for systems-level code.
So, just prior to the invention of C, no one language had reconciled the conflicting attributes that had dogged earlier efforts. Yet the need for such a language was pressing. A great deal of effort was being expended in academic circles in an attempt to create a better computer language. But, and perhaps most importantly, a secondary force was beginning to be felt. Computer hardware was finally becoming common enough that a critical mass was being reached.
No longer were computers kept behind locked doors. For the first time, programmers were gaining virtually unlimited access to their machines. This allowed the freedom to experiment. It also allowed programmers to begin to create their own tools. The creation of C is considered by many to have marked the beginning of the modern age of computer languages. It successfully synthesized the conflicting attributes that had so troubled earlier languages.
The result was a powerful, efficient, structured language that was relatively easy to learn. Prior to the invention of C, computer languages were generally designed either as academic exercises or by bureaucratic committees. C is different. It was designed, implemented, and developed by real, working programmers, reflecting the way that they approached the job of programming.
Its features were honed, tested, thought about, and rethought by the people who actually used the language. The result was a language that programmers liked to use. Indeed, C quickly attracted many followers who had a near-religious zeal for it. As such, it found wide and rapid acceptance in the programmer community. In short, C is a language designed by and for programmers. As you will see, Java inherited this legacy. Since C is a successful and useful language, you might ask why a need for something else existed.
The answer is complexity. Throughout the history of programming, the increasing complexity of programs has driven the need for better ways to manage that complexity. Approaches to programming have changed dramatically since the invention of the computer. For example, when computers were first invented, programming was done by manually toggling in the binary machine instructions by use of the front panel.
As long as programs were just a few hundred instructions long, this approach worked. As programs grew, assembly language was invented so that a programmer could deal with larger, increasingly complex programs by using symbolic representations of the machine instructions. As programs continued to grow, high-level languages were introduced that gave the programmer more tools with which to handle complexity.
While FORTRAN was an impressive first step, at the time it was hardly a language that encouraged clear and easy-to-understand programs. The s gave birth to structured programming. This is the method of programming championed by languages such as C. The use of structured languages enabled programmers to write, for the first time, moderately complex programs fairly easily. However, even with structured programming methods, once a project reaches a certain size, its complexity exceeds what a programmer can manage.
By the early s, many projects were pushing the structured approach past its limits. To solve this problem, a new way to program was invented, called object-oriented programming OOP. Object-oriented programming is discussed in detail later in this book, but here is a brief definition: OOP is a programming methodology that helps organize complex programs through the use of inheritance, encapsulation, and polymorphism.
Once the size of a program exceeds a certain point, it becomes so complex that it is difficult to grasp as a totality. While the precise size at which this occurs differs, depending upon both the nature of the program and the programmer, there is always a threshold at which a program becomes unmanageable. Instead, it was an enhancement to an already highly successful one. Indeed, for a brief moment it seemed as if programmers had finally found the perfect language. However, just as in the past, forces were brewing that would, once again, drive computer language evolution forward.
Within a few years, the World Wide Web and the Internet would reach critical mass. This event would precipitate another revolution in programming. It took 18 months to develop the first working version. Between the initial implementation of Oak in the fall of and the public announcement of Java in the spring of , many more people contributed to the design and evolution of the language. Somewhat surprisingly, the original impetus for Java was not the Internet! Instead, the primary motivation was the need for a platform-independent that is, architecture-neutral language that could be used to create software to be embedded in various consumer electronic devices, such as microwave ovens and remote controls.
As you can probably guess, many different types of CPUs are used as controllers. The problem is that compilers are expensive and time-consuming to create. An easier—and more cost-efficient— solution was needed. In an attempt to find such a solution, Gosling and others began work on a portable, platform-independent language that could be used to produce code that would run on a variety of CPUs under differing environments.
This effort ultimately led to the creation of Java. About the time that the details of Java were being worked out, a second, and ultimately more important, factor was emerging that would play a crucial role in the future of Java. This second force was, of course, the World Wide Web. Had the Web not taken shape at about the same time that Java was being implemented, Java might have remained a useful but obscure language for programming consumer electronics.
However, with the emergence of the World Wide Web, Java was propelled to the forefront of computer language design, because the Web, too, demanded portable programs. Most programmers learn early in their careers that portable programs are as elusive as they are desirable. While the quest for a way to create efficient, portable platform-independent programs is nearly as old as the discipline of programming itself, it had taken a back seat to other, more pressing problems. Further, because at that time much of the computer world had divided itself into the three competing camps of Intel, Macintosh, and UNIX, most programmers stayed within their fortified boundaries, and the urgent need for portable code was reduced.
However, with the advent of the Internet and the Web, the old problem of portability returned with a vengeance. After all, the Internet consists of a diverse, distributed universe populated with various types of computers, operating systems, and CPUs.
Even though many kinds of platforms are attached to the Internet, users would like them all to be able to run the same program. What was once an irritating but low-priority problem had become a high-profile necessity. By , it became obvious to members of the Java design team that the problems of portability frequently encountered when creating code for embedded controllers are also found when attempting to create code for the Internet.
In fact, the same problem that Java was initially designed to solve on a small scale could also be applied to the Internet on a large scale. This realization caused the focus of Java to switch from consumer electronics to Internet programming. This is by intent. First, Java was designed, tested, and refined by real, working programmers.
It is a language grounded in the needs and experiences of the people who devised it. Second, Java is cohesive and logically consistent. Third, except for those constraints imposed by the Internet environment, Java gives you, the programmer, full control. If you program well, your programs reflect it. If you program poorly, your programs reflect that, too.
Put differently, Java is not a language with training wheels. It is a language for professional programmers. Java has significant practical and philosophical differences. Java was designed to solve a certain set of problems. Both will coexist for many years to come. As mentioned at the start of this chapter, computer languages evolve for two reasons: to adapt to changes in environment and to implement advances in the art of programming.
The environmental change that prompted Java was the need for platform-independent programs destined for distribution on the Internet. However, Java also embodies changes in the way that people approach the writing of programs. In the final analysis, though, it was not the individual features of Java that made it so remarkable.
Rather, it was the language as a whole. Java was the perfect response to the demands of the then newly emerging, highly distributed computing universe. Java was to Internet programming what C was to system programming: a revolutionary force that changed the world. The C Connection The reach and power of Java continues to be felt in the world of computer language development.
Many of its innovative features, constructs, and concepts have become part of the baseline for any new language. The success of Java is simply too important to ignore. Created by Microsoft to support the. For example, both share the same general syntax, support distributed programming, and utilize the same object model. How Java Impacted the Internet The Internet helped catapult Java to the forefront of programming, and Java, in turn, had a profound effect on the Internet.
In addition to simplifying web programming in general, Java innovated a new type of networked program called the applet that changed the way the online world thought about content. Java also addressed some of the thorniest issues associated with the Internet: portability and security. An applet is a special kind of Java program that is designed to be transmitted over the Internet and automatically executed inside a Java-compatible web browser.
If the user clicks a link that contains an applet, the applet will download and run in the browser. Applets were intended to be small programs. They were typically used to display data provided by the server, handle user input, or provide simple functions, such as a loan calculator, that execute locally, rather than on the server.
In essence, the applet allowed some functionality to be moved from the server to the client. The creation of the applet was important because, at the time, it expanded the universe of objects that could move about freely in cyberspace. In general, there are two very broad categories of objects that are transmitted between the server and the client: passive information and dynamic, active programs.
For example, when you read your e-mail, you are viewing passive data. By contrast, the applet is a dynamic, self-executing program. Such a program is an active agent on the client computer, yet it is initiated by the server. In the early days of Java, applets were a crucial part of Java programming.
They illustrated the power and benefits of Java, added an exciting dimension to web pages, and enabled programmers to explore the full extent of what was possible with Java. Although it is likely that there are still applets in use today, over time they became less important. For reasons that will be explained, beginning with JDK 9, the phase-out of applets began, with applet support being removed by JDK Security As desirable as dynamic, networked programs are, they can also present serious problems in the areas of security and portability.
Obviously, a program that downloads and executes on the client computer must be prevented from doing harm. It must also be able to run in a variety of different environments and under different operating systems. As you will see, Java solved these problems in an effective and elegant way. At the core of the problem is the fact that malicious code can cause its damage because it has gained unauthorized access to system resources. In order for Java to enable programs to be safely downloaded and executed on the client computer, it was necessary to prevent them from launching such an attack.
Java achieved this protection by enabling you to confine an application to the Java execution environment and prevent it from accessing other parts of the computer. You will see how this is accomplished shortly. The ability to download programs with a degree of confidence that no harm will be done may have been the single most innovative aspect of Java. Portability Portability is a major aspect of the Internet because there are many different types of computers and operating systems connected to it.
If a Java program were to be run on virtually any computer connected to the Internet, there needed to be some way to enable that program to execute on different systems. In other words, a mechanism that allows the same application to be downloaded and executed by a wide variety of CPUs, operating systems, and browsers is required. It is not practical to have different versions of the application for different computers.
The same application code must work on all computers. Therefore, some means of generating portable executable code was needed. As you will soon see, the same mechanism that helps ensure security also helps create portability. Rather, it is bytecode. In essence, the original JVM was designed as an interpreter for bytecode. This may come as a bit of a surprise since many modern languages are designed to be compiled into executable code because of performance concerns. However, the fact that a Java program is executed by the JVM helps solve the major problems associated with web-based programs.
Here is why. Translating a Java program into bytecode makes it much easier to run a program in a wide variety of environments because only the JVM needs to be implemented for each platform. Once a JRE exists for a given system, any Java program can run on it. Remember, although the details of the JVM will differ from platform to platform, all understand the same Java bytecode.
If a Java program were compiled to native code, then different versions of the same program would have to exist for each type of CPU connected to the Internet. This is, of course, not a feasible solution. Thus, the execution of bytecode by the JVM is the easiest way to create truly portable programs.
The fact that a Java program is executed by the JVM also helps to make it secure. Because the JVM is in control, it manages program execution. Thus, it is possible for the JVM to create a restricted execution environment, called the sandbox, that contains the program, preventing unrestricted access to the machine.
Safety is also enhanced by certain restrictions that exist in the Java language. In general, when a program is compiled to an intermediate form and then interpreted by a virtual machine, it runs slower than it would run if compiled to executable code. However, with Java, the differential between the two is not so great. Because bytecode has been highly optimized, the use of bytecode enables the JVM to execute programs much faster than you might expect.
Although Java was designed as an interpreted language, there is nothing about Java that prevents on-the-fly compilation of bytecode into native code in order to boost performance. When a JIT compiler is part of the JVM, selected portions of bytecode are compiled into executable code in real time, on a pieceby-piece, demand basis.
It is important to understand that an entire Java program is not compiled into executable code all at once. Instead, a JIT compiler compiles code as it is needed, during execution. Furthermore, not all sequences of bytecode are compiled—only those that will benefit from compilation. The remaining code is simply interpreted. However, the just-in-time approach still yields a significant performance boost. Even when dynamic compilation is applied to bytecode, the portability and safety features still apply, because the JVM is still in charge of the execution environment.
One other point: Beginning with JDK 9, some Java environments will also support an ahead-of-time compiler that can be used to compile bytecode into native code prior to execution by the JVM, rather than on-the-fly. Because of the highly specialized nature of ahead-oftime compilation, it is not discussed further in this book. Over those years, many changes have taken place. As you would expect, Java has also changed and so, too, has the way that Java is used.
Perhaps nothing illustrates the ongoing evolution of Java better than the applet. As explained previously, in the early years of Java, applets were a crucial part of Java programming. They not only added excitement to a web page, they were also a highly visible part of Java, which added to its charisma.
However, applets rely on a Java browser plug-in. Thus, for an applet to work, the browser must support it. Recently, support for the Java browser plug-in has been waning. Simply put, without browser support, applets are not viable. Because of this, beginning with JDK 9, the phase-out of applets was begun, with support for applets being deprecated. In the language of Java, deprecated means that a feature is still available but flagged as obsolete.
Thus, a deprecated feature should not be used for new code. The phase-out became complete with the release of JDK 11 because support for applets was removed. Called Java Web Start, it enabled an application to be dynamically downloaded from a web page. It was a deployment mechanism that was especially useful for larger Java applications that were not appropriate for applets. The difference between an applet and a Web Start application is that a Web Start application runs on its own, not inside the browser.
It does, however, require that a stand-alone JRE that supports Web Start is available on the host system. Given that neither applets nor Java Web Start are supported by modern versions of Java, you might wonder what mechanism should be used to deploy a Java application. At the time of this writing, part of the answer is to use the jlink tool added by JDK 9. It can create a complete run-time image that includes all necessary support for your program, including the JRE.
Although a detailed discussion of deployment strategies is outside the scope of this book, it is something that you will want to pay close attention to going forward. A Faster Release Schedule Another major change has recently occurred in Java, but it does not involve changes to the language or the run-time environment. Rather, it relates to the way that Java releases are scheduled.
In the past, major Java releases were typically separated by two or more years. However, subsequent to the release of JDK 9, the time between major Java releases has been decreased. Today, it is anticipated that a major release will occur on a strict time-based schedule, with the expected time between such releases being just six months. Each six-month release, now called a feature release, will include those features ready at the time of the release.
This increased release cadence enables new features and enhancements to be available to Java programmers in a timely fashion. Furthermore, it allows Java to respond quickly to the demands of an ever-changing programming environment.
Simply put, the faster release schedule promises to be a very positive development for Java programmers. Currently, feature releases are scheduled for March and September of each year. The next release JDK 11 was in September Again, it is anticipated that every six months a new feature release will take place. You will want to consult the Java documentation for the latest release schedule information.
At the time of this writing, there are a number of new Java features on the horizon. Because of the faster release schedule, it is very likely that several of them will be added to Java over the next few years. You will want to review the information and release notes provided by each six-month release in detail. It is truly an exciting time to be a Java programmer! Not long after the initial release of Java, it became obvious that Java would also be useful on the server side.
The result was the servlet. A servlet is a small program that executes on the server. Servlets are used to create dynamically generated content that is then served to the client. For example, an online store might use a servlet to look up the price for an item in a database.
The price information is then used to dynamically generate a web page that is sent to the browser. Although dynamically generated content was available through mechanisms such as CGI Common Gateway Interface , the servlet offered several advantages, including increased performance. Because servlets like all Java programs are compiled into bytecode and executed by the JVM, they are highly portable.
Thus, the same servlet can be used in a variety of different server environments. The only requirements are that the server support the JVM and a servlet container. Today, server-side code in general constitutes a major use of Java. Although the fundamental forces that necessitated the invention of Java are portability and security, other factors also played an important role in molding the final form of the language.
Simple Java was designed to be easy for the professional programmer to learn and use effectively. Assuming that you have some programming experience, you will not find Java hard to master. If you already understand the basic concepts of objectoriented programming, learning Java will be even easier. Object-Oriented Although influenced by its predecessors, Java was not designed to be sourcecode compatible with any other language.
This allowed the Java team the freedom to design with a blank slate. One outcome of this was a clean, usable, pragmatic approach to objects. The object model in Java is simple and easy to extend, while primitive types, such as integers, are kept as high-performance nonobjects. Robust The multiplatformed environment of the Web places extraordinary demands on a program, because the program must execute reliably in a variety of systems.
Thus, the ability to create robust programs was given a high priority in the design of Java. To gain reliability, Java restricts you in a few key areas to force you to find your mistakes early in program development. At the same time, Java frees you from having to worry about many of the most common causes of programming errors.
Because Java is a strictly typed language, it checks your code at compile time. However, it also checks your code at run time. Many hardto-track-down bugs that often turn up in hard-to-reproduce run-time situations are simply impossible to create in Java.
Knowing that what you have written will behave in a predictable way under diverse conditions is a key feature of Java. To better understand how Java is robust, consider two of the main reasons for program failure: memory management mistakes and mishandled exceptional conditions that is, run-time errors.
Memory management can be a difficult, tedious task in traditional programming environments. This sometimes leads to problems, because programmers will either forget to free memory that has been previously allocated or, worse, try to free some memory that another part of their code is still using. Java virtually eliminates these problems by managing memory allocation and deallocation for you. In fact, deallocation is completely automatic, because Java provides garbage collection for unused objects.
Java helps in this area by providing object-oriented exception handling. In a well-written Java program, all run-time errors can—and should—be managed by your program. Multithreaded Java was designed to meet the real-world requirement of creating interactive, networked programs.
To accomplish this, Java supports multithreaded programming, which allows you to write programs that do many things simultaneously. The Java run-time system comes with an elegant yet sophisticated solution for multiprocess synchronization that enables you to construct smoothly running interactive systems.
Architecture-Neutral A central issue for the Java designers was that of code longevity and portability. Operating system upgrades, processor upgrades, and changes in core system resources can all combine to make a program malfunction. The Java designers made several hard decisions in the Java language and the Java Virtual Machine in an attempt to alter this situation. Interpreted and High Performance As described earlier, Java enables the creation of cross-platform programs by compiling into an intermediate representation called Java bytecode.
This code can be executed on any system that implements the Java Virtual Machine. Most previous attempts at cross-platform solutions have done so at the expense of performance. As explained earlier, the Java bytecode was carefully designed so that it would be easy to translate directly into native machine code for very high performance by using a just-in-time compiler. Java run-time systems that provide this feature lose none of the benefits of the platform-independent code.
In fact, accessing a resource using a URL is not much different from accessing a file. This feature enables a program to invoke methods across a network. Dynamic Java programs carry with them substantial amounts of run-time type information that is used to verify and resolve accesses to objects at run time.
This makes it possible to dynamically link code in a safe and expedient manner. This is crucial to the robustness of the Java environment, in which small fragments of bytecode may be dynamically updated on a running system. Unlike most other software systems that usually settle into a pattern of small, incremental improvements, Java continued to evolve at an explosive pace. Soon after the release of Java 1. The features added by Java 1.
Java 1. It also deprecated rendered obsolete several features originally defined by Java 1. Thus, Java 1. It may seem odd that the first release of Java 2 used the 1. The reason is that it originally referred to the internal version number of the Java libraries, but then was generalized to refer to the entire release.
Java 2 added support for a number of new features, such as Swing and the Collections Framework, and it enhanced the Java Virtual Machine and various programming tools. Java 2 also contained a few deprecations. The most important affected the Thread class in which the methods suspend , resume , and stop were deprecated. J2SE 1. In general, programs written for version 1. Although version 1. The release of J2SE 1. This release contained several important upgrades, enhancements, and additions.
It also made changes to the Collections Framework and the networking classes. In addition, numerous small changes were made throughout. Despite the significant number of new features, version 1. The next release of Java was J2SE 5, and it was revolutionary.
Unlike most of the previous Java upgrades, which offered important, but measured improvements, J2SE 5 fundamentally expanded the scope, power, and range of the language. Each item in the list represented a significant addition to the Java language. Some, such as generics, the enhanced for, and varargs, introduced new syntax elements. Others, such as autoboxing and auto-unboxing, altered the semantics of the language.
Annotations added an entirely new dimension to programming. In all cases, the impact of these additions went beyond their direct effects. They changed the very character of Java itself. However, the new features were so significant that a shift from 1.
Instead, Sun elected to increase the version number to 5 as a way of emphasizing that a major event was taking place. However, in order to maintain consistency, Sun decided to use 1. The next release of Java was called Java SE 6. Sun once again decided to change the name of the Java platform.
The internal, developer version number is 1. Java SE 6 added no major features to the Java language proper, but it did enhance the API libraries, added several new packages, and offered improvements to the run time. It also went through several updates during its in Java terms long life cycle, with several upgrades added along the way.
Java SE 7 contained many new features, including significant additions to the language and the API libraries. Upgrades to the Java run-time system that support nonJava languages were also included, but it is the language and library additions that were of most interest to Java programmers.
The new language features were developed as part of Project Coin. The purpose of Project Coin was to identify a number of small changes to the Java language that would be incorporated into JDK 7. In fact, for many programmers, these changes may well have been the most important new features in Java SE 7. For example, streams can be closed automatically when they are no longer needed.
In particular, the try-with-resources statement has profoundly affected the way that stream-based code is written. Also, the ability to use a String to control a switch statement was a long-desired improvement that simplified coding in many situations. However, the changes added by Java SE 7 fundamentally expanded its capabilities. So significant were the changes, that the term NIO. Parallel programming is the name commonly given to the techniques that make effective use of computers that contain more than one processor, including multicore systems.
The advantage that multicore environments offer is the prospect of significantly increased program performance. Of course, not all algorithms lend themselves to parallelization, but for those that do, a significant improvement in execution speed can be obtained. It has an internal version number of 1. JDK 8 was a significant upgrade to the Java language because of the inclusion of a far-reaching new language feature: the lambda expression.
The impact of lambda expressions was, and will continue to be, profound, changing both the way that programming solutions are conceptualized and how Java code is written. As explained in detail in Chapter 15, lambda expressions add functional programming features to Java.
In the process, lambda expressions can simplify and reduce the amount of source code needed to create certain constructs, such as some types of anonymous classes. The inclusion of lambda expressions has also had a wide-ranging effect on the Java libraries, with new features being added to take advantage of them. One of the most important was the new stream API, which is packaged in java.
The stream API supports pipeline operations on data and is optimized for lambda expressions. Another new package was java. It defines a number of functional interfaces, which provide additional support for lambda expressions. Other new lambda-related features are found throughout the API library. Another lambda-inspired feature affects interface.
Beginning with JDK 8, it is now possible to define a default implementation for a method specified by an interface. If no implementation for a default method is created, then the default defined by the interface is used. This feature enables interfaces to be gracefully evolved over time because a new method can be added to an interface without breaking existing code.
It can also streamline the implementation of an interface when the defaults are appropriate. Other new features in JDK 8 include a new time and date API, type annotations, and the ability to use parallel processing when sorting an array, among others.
The next release of Java was Java SE 9. With the release of JDK 9, the internal version number is also 9. JDK 9 represented a major Java release, incorporating significant enhancements to both the Java language and its libraries. The primary new JDK 9 feature was modules, which enable you to specify the relationship and dependencies of the code that comprises an application.
The inclusion of modules caused a new syntax element and several keywords to be added to Java. Furthermore, a tool called jlink was added to the JDK, which enables a programmer to create a run-time image of an application that contains only the necessary modules. A new file type, called JMOD, was created. Modules also have a profound affect on the API library because, beginning with JDK 9, the library packages are now organized into modules.
Although modules constitute a major Java enhancement, they are conceptually simple and straightforward. Furthermore, because pre-module legacy code is fully supported, modules can be integrated into the development process on your timeline. There is no need to immediately change any preexisting code to handle modules. In short, modules added substantial functionality without altering the essence of Java.
In addition to modules, JDK 9 included many other new features. One of particular interest is JShell, which is a tool that supports interactive program experimentation and learning. Another interesting upgrade is support for private interface methods.
JDK 9 added a search feature to the javadoc tool and a new tag called index to support it. As a general rule, in any Java release, it is the new features that receive the most attention. However, there was one high-profile aspect of Java that was deprecated by JDK 9: applets.
Beginning with JDK 9, applets were no longer recommended for new projects. As explained earlier in this chapter, because of waning browser support for applets and other factors , JDK 9 deprecated the entire applet API. As explained earlier, beginning with JDK 10, Java releases are anticipated to occur on a strict timebased schedule, with the time between major releases expected to be just six months.
The primary new language feature added by JDK 10 was support for local variable type inference. With local variable type inference, it is now possible to let the type of a local variable be inferred from the type of its initializer, rather than being explicitly specified. To support this new capability, the context-sensitive identifier var was added to Java as a reserved type name.
It can also simplify declarations in cases in which the type is difficult to discern or cannot be explicitly specified. Local variable type inference has become a common part of the contemporary programming environment. Its inclusion in Java helps keep Java up-to-date with evolving trends in language design.
Along with a number of other changes, JDK 10 also redefined the Java version string, changing the meaning of the version numbers so that they better align with the new time-based release schedule. It was released in September , which was six months after JDK The primary new language feature in JDK 11 is support for the use of var in a lambda expression. Also, another execution mode was added to the Java launcher that enables it to directly execute simple single-file programs.
JDK 11 also removes some features. Perhaps of the greatest interest because of its historical significance is the removal of support for applets. Recall that applets were first deprecated by JDK 9. With the release of JDK 11, applet support has been removed.
As the execution environment has continued to evolve, both applets and Java Web Start were rapidly losing relevance. Instead, this GUI framework has become a separate open-source project. Because these features are no longer part of the JDK, they are not discussed in this book.
One other point about the evolution of Java: Beginning in , the process of open-sourcing Java began. Today, open-source implementations of the JDK are available. Open-sourcing further contributes to the dynamic nature of Java development. Java remains the vibrant, nimble language that the programming world has come to expect. The material in this book has been updated for JDK Many new Java features, updates, and additions are described throughout.
As the preceding discussion has highlighted, however, the history of Java programming is marked by dynamic change. You will want to review the new features in each subsequent Java release. Simply put: The evolution of Java continues! A Culture of Innovation Since the beginning, Java has been at the center of a culture of innovation. Its original release redefined programming for the Internet. Portable code made the Web come alive. The world of Java has never stood still for very long.
Rather, they work together to form the language as a whole. However, this interrelatedness can make it difficult to describe one aspect of Java without involving several others. Often a discussion of one feature implies prior knowledge of another. For this reason, this chapter presents a quick overview of several key features of Java.
The material described here will give you a foothold that will allow you to write and understand simple programs. Most of the topics discussed will be examined in greater detail in the remaining chapters of Part I. In fact, all Java programs are to at least some extent object-oriented. OOP is so integral to Java that it is best to understand its basic principles before you begin writing even simple Java programs.
Therefore, this chapter begins with a discussion of the theoretical aspects of OOP. Two Paradigms All computer programs consist of two elements: code and data. Furthermore, a program can be conceptually organized around its code or around its data. The first way is called the process-oriented model. This approach characterizes a program as a series of linear steps that is, code.
The process-oriented model can be thought of as code acting on data. Procedural languages such as C employ this model to considerable success. However, as mentioned in Chapter 1, problems with this approach appear as programs grow larger and more complex. To manage increasing complexity, the second approach, called objectoriented programming, was conceived.
Object-oriented programming organizes a program around its data that is, objects and a set of well-defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code. As you will see, by switching the controlling entity to data, you can achieve several organizational benefits. Abstraction An essential element of object-oriented programming is abstraction. Humans manage complexity through abstraction. For example, people do not think of a car as a set of tens of thousands of individual parts.
They think of it as a welldefined object with its own unique behavior. This abstraction allows people to use a car to drive to the grocery store without being overwhelmed by the complexity of the individual parts. They can ignore the details of how the engine, transmission, and braking systems work. Instead, they are free to utilize the object as a whole. A powerful way to manage abstraction is through the use of hierarchical classifications. This allows you to layer the semantics of complex systems, breaking them into more manageable pieces.
From the outside, the car is a single object. Once inside, you see that the car consists of several subsystems: steering, brakes, sound system, seat belts, heating, cellular phone, and so on. In turn, each of these subsystems is made up of more specialized units. The point is that you manage the complexity of the car or any other complex system through the use of hierarchical abstractions.
Hierarchical abstractions of complex systems can also be applied to computer programs. The data from a traditional process-oriented program can be transformed by abstraction into its component objects. A sequence of process steps can become a collection of messages between these objects. Thus, each of these objects describes its own unique behavior.
You can treat these objects as concrete entities that respond to messages telling them to do something. This is the essence of object-oriented programming. Object-oriented concepts form the heart of Java just as they form the basis for human understanding. It is important that you understand how these concepts translate into programs.
As you will see, object-oriented programming is a powerful and natural paradigm for creating programs that survive the inevitable changes accompanying the life cycle of any major software project, including conception, growth, and aging. For example, once you have well-defined objects and clean, reliable interfaces to those objects, you can gracefully decommission or replace parts of an older system without fear.
The Three OOP Principles All object-oriented programming languages provide mechanisms that help you implement the object-oriented model. They are encapsulation, inheritance, and polymorphism. Encapsulation Encapsulation is the mechanism that binds together code and the data it manipulates, and keeps both safe from outside interference and misuse.
One way to think about encapsulation is as a protective wrapper that prevents the code and data from being arbitrarily accessed by other code defined outside the wrapper. Access to the code and data inside the wrapper is tightly controlled through a well-defined interface. To relate this to the real world, consider the automatic transmission on an automobile. It encapsulates hundreds of bits of information about your engine, such as how much you are accelerating, the pitch of the surface you are on, and the position of the shift lever.
You, as the user, have only one method of affecting this complex encapsulation: by moving the gear-shift lever. Thus, the gear-shift lever is a well-defined indeed, unique interface to the transmission. Further, what occurs inside the transmission does not affect objects outside the transmission. For example, shifting gears does not turn on the headlights! Because an automatic transmission is encapsulated, dozens of car manufacturers can implement one in any way they please.
This same idea can be applied to programming. The power of encapsulated code is that everyone knows how to access it and thus can use it regardless of the implementation details—and without fear of unexpected side effects. In Java, the basis of encapsulation is the class.
Although the class will be examined in great detail later in this book, the following brief discussion will be helpful now. A class defines the structure and behavior data and code that will be shared by a set of objects. Each object of a given class contains the structure and behavior defined by the class, as if it were stamped out by a mold in the shape of the class. For this reason, objects are sometimes referred to as instances of a class.
Thus, a class is a logical construct; an object has physical reality. When you create a class, you will specify the code and data that constitute that class. Collectively, these elements are called members of the class. Specifically, the data defined by the class are referred to as member variables or instance variables.
The code that operates on that data is referred to as member methods or just methods. In properly written Java programs, the methods define how the member variables can be used. This means that the behavior and interface of a class are defined by the methods that operate on its instance data.
Since the purpose of a class is to encapsulate complexity, there are mechanisms for hiding the complexity of the implementation inside the class. Each method or variable in a class may be marked private or public. I never attempt to make money on the stock market. I buy on the assumption that they could close the market the next day and not reopen it for ten years.
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Leonardo Da Vinci Stay on top of your finances. Don't leave that up to others. Leif Garrett Let him who would enjoy a good future waste none of his present. Roger Babson Never spend your money before you have it. Charlie Munger Fortune sides with him who dares. Virgil Read the best books first, or you may not have a chance to read them at all.
About the Author Best-selling author Herbert Schildt has written extensively about programming for over three decades an.
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Satyaraj Pantham. Getting Started Guide 10g Golden Hands. Local variable type inference has become a common part of the contemporary programming environment. It was released in September , which is six months after JDK The primary new language feature in JDK 11 is its support for the use of var in a lambda expression.
JDK 11 also removes some features. Perhaps of greatest interest, because of its historical significance, is the removal of support for applets. Recall that applets were first deprecated by JDK 9. With the release of JDK 11, applet support has been removed. Because these features have been removed from the JDK, they are not discussed in this book.
Open sourcing further contributes to the dynamic nature of Java development. Java remains the vibrant, nimble language that the programming world has come to expect. The material in this book has been updated through JDK As the preceding discussion has highlighted, however, the history of Java programming is marked by dynamic change. As you advance in your study of Java, you will want to watch for new features of each subsequent Java release. Simply put: The evolution of Java continues!
It contains 16 chapters, each discussing an aspect of Java. This book is unique because it includes several special elements that reinforce what you are learning. Self Test Each chapter concludes with a Self Test that lets you test your knowledge. The answers are in Appendix A. These contain additional information or interesting commentary about a topic. Try This Elements Each chapter contains one or more Try This elements, which are projects that show you how to apply what you are learning.
Thus, if you have never programmed before, you can use this book. If you do have some previous programming experience, you will be able to advance a bit more quickly. Keep in mind, however, that Java differs in several key ways from other popular computer languages. It is important not to jump to conclusions. Thus, even for the experienced programmer, a careful reading is advised. Danny has worked on several of my books, and his advice, insights, and suggestions have always been of great value and much appreciated.
Its creation in the early days of the Web helped shape the modern form of the Internet, including both the client and server sides. Its innovative features advanced the art and science of programming, setting a new standard in computer language design. Although Java is a language often associated with Internet programming, it is by no means limited in that regard.
Thus, if you are new to programming, Java is an excellent language to learn. Moreover, to be a professional programmer today implies the ability to program in Java—it is that important. In the course of this book, you will learn the basic skills that will help you master it. The purpose of this chapter is to introduce you to Java, beginning with its history, its design philosophy, and several of its most important features.
By far, the hardest thing about learning a programming language is the fact that no element exists in isolation. Instead, the components of the language work in conjunction with each other. This interrelatedness is especially pronounced in Java. In fact, it is difficult to discuss one aspect of Java without involving others. To help overcome this problem, this chapter provides a brief overview of several Java features, including the general form of a Java program, some basic control structures, and simple operators.
It does not go into too many details, but, rather, concentrates on general concepts common to any Java program. As you advance through this book, you will see that many aspects of Java are either a direct or indirect result of the historical forces that shaped the language.
Thus, it is fitting that we begin our examination of Java by exploring how Java relates to the larger programming universe. Somewhat surprisingly, the original impetus for Java was not the Internet! As you can probably guess, many different types of CPUs are used as controllers.
The trouble was that at the time most computer languages were designed to be compiled into machine code that was targeted for a specific type of CPU. The problem, however, is that compilers are expensive and time consuming to create. This effort ultimately led to the creation of Java. About the time that the details of Java were being worked out, a second, and ultimately more important, factor emerged that would play a crucial role in the future of Java.
This second force was, of course, the World Wide Web. Had the Web not taken shape at about the same time that Java was being implemented, Java might have remained a useful but obscure language for programming consumer electronics. However, with the emergence of the Web, Java was propelled to the forefront of computer language design, because the Web, too, demanded portable programs.
Most programmers learn early in their careers that portable programs are as elusive as they are desirable. However, with the advent of the Internet and the Web, the old problem of portability returned with a vengeance.
After all, the Internet consisted of a diverse, distributed universe populated with many types of computers, operating systems, and CPUs. By it became obvious to members of the Java design team that the problems of portability frequently encountered when creating code for embedded controllers are also found when attempting to create code for the Internet.
This realization caused the focus of Java to switch from consumer electronics to Internet programming. Rather, it is a continuum in which each new language is influenced in one way or another by what has come before.
In this regard, Java is no exception. Before moving on, it is useful to understand where Java fits into the family tree of computer languages. From C, Java inherits its syntax. The modern age of programming began with C. By inheriting and building on that rich heritage, Java provides a powerful, logically consistent programming environment that takes the best of the past and adds new features related to the online environment and advances in the art of programming.
Programmers do not face major rifts when switching from one language to another. It is a language grounded in the needs and experiences of the people who devised it. Java was designed to solve a certain set of problems. They will coexist for many years to come. How Java Impacted the Internet The Internet helped catapult Java to the forefront of programming, and Java, in turn, had a profound effect on the Internet.
First, the creation of Java simplified Internet programming in general, acting as a catalyst that drew legions of programmers to the Web. Second, Java innovated a new type of networked program called the applet that changed the way the online world thought about content. Finally, and perhaps most importantly, Java addressed some of the thorniest issues associated with the Internet: portability and security.
It also provided mechanisms that enabled programs to be readily delivered over the Internet. If the user clicks a link that contains an applet, the applet will download and run in the browser automatically. Applets were intended to be small programs, typically used to display data provided by the server, handle user input, or provide simple functions, such as a loan calculator. The key feature of applets is that they execute locally, rather than on the server.
In essence, the applet allowed some functionality to be moved from the server to the client. The creation of the applet was important because, at the time, it expanded the universe of objects that could move about freely in cyberspace. In general, there are two very broad categories of objects that are transmitted between the server and the client: passive information and dynamic active programs.
Such a program is an active agent on the client computer, yet it is delivered by the server. In the early days of Java, applets were a crucial part of Java programming. They illustrated the power and benefits of Java, added an exciting dimension to web pages, and enabled programmers to explore the full extent of what was possible with Java. Finally, applet support was removed by JDK A: A few years after the creation of Java, Microsoft developed the C language. This is important because C is closely related to Java.
This means that if you already know C , then learning Java will be especially easy. Conversely, if C is in your future, then your knowledge of Java will come in handy. Security As desirable as dynamic, networked programs are, they also present serious problems in the areas of security and portability.
Obviously, a program that downloads and executes on the client computer must be prevented from doing harm. It must also be able to run in a variety of different environments and under different operating systems. As you will see, Java addressed these problems in an effective and elegant way. As you are likely aware, every time that you download a program, you are taking a risk because the code you are downloading might contain a virus, Trojan horse, or other harmful code. At the core of the problem is the fact that malicious code can cause damage because it has gained unauthorized access to system resources.
In order for Java to enable programs to be safely downloaded and executed on the client computer, it was necessary to prevent them from launching such an attack. Java achieved this protection by enabling you to confine an application to the Java execution environment and prevent it from accessing other parts of the computer.
You will see how this is accomplished shortly. Portability Portability is a major aspect of the Internet because there are many different types of computers and operating systems connected to it. If a Java program were to be run on virtually any computer connected to the Internet, there needed to be some way to enable that program to execute on different types of systems. In other words, a mechanism that allows the same application to be downloaded and executed by a wide variety of CPUs, operating systems, and browsers is required.
It is not practical to have different versions of the same application for different computers. The same application code must work in all computers. Therefore, some means of generating portable code was needed. As you will soon see, the same mechanism that helps ensure security also helps create portability. Rather, it is bytecode. In essence, the original JVM was designed as an interpreter for bytecode. Here is why. Translating a Java program into bytecode makes it much easier to run a program in a wide variety of environments because only the JRE which includes the JVM needs to be implemented for each platform.
Once a JRE exists for a given system, any Java program can run on it. If a Java program were compiled to native code, then different versions of the same program would have to exist for each type of CPU connected to the Internet. This is, of course, not a feasible solution. Thus, the execution of bytecode by the JVM is the easiest way to create truly portable programs. The fact that a Java program is executed by the JVM also helps to make it secure.
Because the JVM is in control, it manages program execution. Thus, it is possible for the JVM to create a restricted execution environment, called the sandbox, that contains the program, preventing unrestricted access to the machine.
Safety is also enhanced by certain restrictions that exist in the Java language. When a program is interpreted, it generally runs slower than the same program would run if compiled to executable code. However, with Java, the differential between the two is not so great. Because bytecode has been highly optimized, the use of bytecode enables the JVM to execute programs much faster than you might expect.
That is, a JIT compiler compiles code as it is needed during execution. Furthermore, not all sequences of bytecode are compiled—only those that will benefit from compilation. The remaining code is simply interpreted. Even when dynamic compilation is applied to bytecode, the portability and safety features still apply because the JVM is still in charge of the execution environment.
Ask the Expert Q: I have heard about a special type of Java program called a servlet. What is it? A: A Java servlet is a small program that executes on a server. Servlets dynamically extend the functionality of a web server. Not long after the initial release of Java, it became obvious that Java would also be useful on the server side. The result was the servlet.
Over those years, many changes have taken place. As you would expect, Java has also changed and so, too, has the way that Java is used. Perhaps nothing illustrates the ongoing evolution of Java better than the applet. As explained previously, in the early years of Java, applets were a crucial part of Java programming. They not only added excitement to a web page, they were a highly visible part of Java, which added to its charisma. Thus, for an applet to work, it must be supported by the browser.
Simply put, without browser support, applets are not viable. In the language of Java, deprecated means that a feature is still available but flagged as obsolete. Thus, a deprecated feature should not be used for new code. Called Java Web Start, it enabled an application to be dynamically downloaded from a web page. It was a deployment mechanism that was especially useful for larger Java applications that were not appropriate for applets. The difference between an applet and a Web Start application is that a Web Start application runs on its own, not inside the browser.
Given that neither applets nor Java Web Start are viable options for modern versions of Java, you might wonder what mechanism should be used to deploy a Java application. At the time of this writing, one part of the answer is to use the jlink tool added by JDK 9. As you might guess, deployment is a rather advanced topic that is outside the scope of this book.
They are not deployed over the Internet. Rather, it relates to the way that Java releases are scheduled. In the past, major Java releases were typically separated by two or more years. However, subsequent to the release of JDK 9, the time between major Java releases has been decreased. Each major release, now called a feature release, will include those features ready at the time of the release.
This increased release cadence enables new features and enhancements to be available to Java programmers in a timely fashion. Simply put, the faster release schedule promises to be a very positive development for Java programmers.
Currently, feature releases are scheduled for March and September of each year. The next release JDK 11 was in September Again, it is anticipated that every six months a new feature release will take place. You will want to consult the Java documentation for latest release schedule information. At the time of this writing, there are a number of new Java features on the horizon.
Because of the faster release schedule, it is very likely that several of them will be added to Java over the next few years. It is truly an exciting time to be a Java programmer! Although the fundamental forces that necessitated the invention of Java are portability and security, other factors played an important role in molding the final form of the language.
The key considerations were summed up by the Java design team in the following list of buzzwords. Later in this book, you will see how to put these concepts into practice. OOP is a powerful way to approach the job of programming.
Programming methodologies have changed dramatically since the invention of the computer, primarily to accommodate the increasing complexity of programs. As long as programs were just a few hundred instructions long, this approach worked.
As programs grew, assembly language was invented so that a programmer could deal with larger, increasingly complex programs, using symbolic representations of the machine instructions. The s gave birth to structured programming. This is the method encouraged by languages such as C and Pascal. The use of structured languages made it possible to write moderately complex programs fairly easily. Although structured languages are a powerful tool, even they reach their limit when a project becomes too large.
Consider this: At each milestone in the development of programming, techniques and tools were created to allow the programmer to deal with increasingly greater complexity. Each step of the way, the new approach took the best elements of the previous methods and moved forward. Prior to the invention of OOP, many projects were nearing or exceeding the point where the structured approach no longer works.
The result was a different way of organizing a program. In the most general sense, a program can be organized in one of two ways: around its code what is happening or around its data what is being affected. Using only structured programming techniques, programs are typically organized around code. Thus, a data type defines precisely what sort of operations can be applied to that data.
Encapsulation Encapsulation is a programming mechanism that binds together code and the data it manipulates, and that keeps both safe from outside interference and misuse. Within the box are all necessary data and code. When code and data are linked together in this fashion, an object is created.
In other words, an object is the device that supports encapsulation. Within an object, code, data, or both may be private to that object or public. Private code or data is known to and accessible by only another part of the object.
That is, private code or data cannot be accessed by a piece of the program that exists outside the object. When code or data is public, other parts of your program can access it even though it is defined within an object. Typically, the public parts of an object are used to provide a controlled interface to the private elements of the object. Although the class will be examined in great detail later in this book, the following brief discussion will be helpful now.
A class defines the form of an object. It specifies both the data and the code that will operate on that data. Java uses a class specification to construct objects. Objects are instances of a class. Thus, a class is essentially a set of plans that specify how to build an object.
The code and data that constitute a class are called members of the class. Specifically, the data defined by the class are referred to as member variables or instance variables. The code that operates on that data is referred to as member methods or just methods. The specific action is determined by the exact nature of the situation. A simple example of polymorphism is found in the steering wheel of an automobile.
The steering wheel i. Therefore, once you know how to operate the steering wheel, you can drive any type of car. The same principle can also apply to programming. You might have a program that requires three different types of stacks. In this case, the algorithm that implements each stack is the same, even though the data being stored differs.
However, because of polymorphism, in Java you can create one general set of stack routines that works for all three specific situations. This way, once you know how to use one stack, you can use them all. Polymorphism helps reduce complexity by allowing the same interface to be used to specify a general class of action. You need only remember and utilize the general interface. Inheritance Inheritance is the process by which one object can acquire the properties of another object.
This is important because it supports the concept of hierarchical classification. If you think about it, most knowledge is made manageable by hierarchical i. For example, a Red Delicious apple is part of the classification apple, which in turn is part of the fruit class, which is under the larger class food. That is, the food class possesses certain qualities edible, nutritious, etc. In addition to these qualities, the fruit class has specific characteristics juicy, sweet, etc.
The apple class defines those qualities specific to an apple grows on trees, not tropical, etc. A Red Delicious apple would, in turn, inherit all the qualities of all preceding classes, and would define only those qualities that make it unique.
Without the use of hierarchies, each object would have to explicitly define all of its characteristics. Using inheritance, an object need only define those qualities that make it unique within its class. It can inherit its general attributes from its parent.
Thus, it is the inheritance mechanism that makes it possible for one object to be a specific instance of a more general case. This is the version for Java SE SE stands for Standard Edition. It is also the version described in this book. Because JDK 11 contains features that are not supported by earlier versions of Java, it is recommended that you use JDK 11 or later to compile and run the programs in this book.
However, depending on the environment in which you are working, an earlier JDK may already be installed. If this is the case, then newer Java features will not be available. In general, you should first find the JDK you want to use.
For example, at the time of this writing, the Oracle JDK can be downloaded from www. Also at the time of this writing, an open source version is available at jdk. Next, download the JDK of your choice and follow its instructions to install it on your computer. After you have installed the JDK, you will be able to compile and run programs. The JDK supplies two primary programs. The first is javac, which is the Java compiler. The second is java, which is the standard Java interpreter and is also referred to as the application launcher.
It is not a windowed application. It is also not an integrated development environment IDE. However, it seems that it might add substantial overhead to relatively small ones. A: No. Although it is true that Java follows a strict object model, you have wide latitude as to the degree to which you employ it. An IDE can be very helpful when developing and deploying commercial applications.
As a general rule, you can also use an IDE to compile and run the programs in this book if you so choose. The reasons for this are easy to understand. First, the JDK is readily available to all readers. Second, the instructions for using the JDK will be the same for all readers. If you are using an IDE, you will need to follow its instructions. Because of differences between IDEs, no general set of instructions can be given.
Enter the program. Compile the program. Run the program. Entering the Program The programs shown in this book are available from www. However, if you want to enter the programs by hand, you are free to do so. In this case, you must enter the program into your computer using a text editor, not a word processor. Word processors typically store format information along with text. This format information will confuse the Java compiler. If you are using a Windows platform, you can use WordPad or any other programming editor that you like.
For most computer languages, the name of the file that holds the source code to a program is arbitrary. However, this is not the case with Java. The first thing that you must learn about Java is that the name you give to a source file is very important. For this example, the name of the source file should be Example. In Java, a source file is officially called a compilation unit. It is a text file that contains among other things one or more class definitions. For now, we will be using source files that contain only one class.
The Java compiler requires that a source file use the. As you can see by looking at the program, the name of the class defined by the program is also Example. This is not a coincidence. In Java, all code must reside inside a class. By convention, the name of the main class should match the name of the file that holds the program. You should also make sure that the capitalization of the filename matches the class name.
The reason for this is that Java is case sensitive. At this point, the convention that filenames correspond to class names may seem arbitrary. However, this convention makes it easier to maintain and organize your programs. Furthermore, as you will see later in this book, in some cases, it is required. Compiling the Program To compile the Example program, execute the compiler, javac, specifying the name of the source file on the command line, as shown here: The javac compiler creates a file called Example.
Remember, bytecode is not executable code. Bytecode must be executed by a Java Virtual Machine. Thus, the output of javac is not code that can be directly executed. To actually run the program, you must use the Java interpreter, java. This is why it is a good idea to give your Java source files the same name as the class they contain—the name of the source file will match the name of the.
When you execute the Java interpreter as just shown, you are actually specifying the name of the class that you want the interpreter to execute. It will automatically search for a file by that name that has the. If it finds the file, it will execute the code contained in the specified class. Before moving on, it is important to mention that beginning with JDK 11, Java provides a way to run some types of simple programs directly from a source file, without explicitly invoking javac.
This technique, which can be useful in some situations, is described in Appendix C. For the purposes of this book, it is assumed that you are using the normal compilation process just described. Of course, you will need to find the path to Java on your computer, which may differ from the one just shown. Also the specific version of the JDK may differ. You will need to consult the documentation for your operating system on how to set the path, because this procedure differs between OSes.
The program begins with the following lines: This is a comment. The contents of a comment are ignored by the compiler. Instead, a comment describes or explains the operation of the program to anyone who is reading its source code. In this case, the comment describes the program and reminds you that the source file should be called Example. Of course, in real applications, comments generally explain how some part of the program works or what a specific feature does.
Java supports three styles of comments. The one shown at the top of the program is called a multiline comment. Anything between these two comment symbols is ignored by the compiler. As the name suggests, a multiline comment may be several lines long. The next line of code in the program is shown here: This line uses the keyword class to declare that a new class is being defined.
Example is the name of the class. The elements between the two braces are members of the class. The next line of code is shown here: This line begins the main method. As mentioned earlier, in Java, a subroutine is called a method. As the comment preceding it suggests, this is the line at which the program will begin executing. In general, Java applications begin execution by calling main.
The public keyword is an access modifier. An access modifier determines how other parts of the program can access the members of the class. When a class member is preceded by public, then that member can be accessed by code outside the class in which it is declared. The opposite of public is private, which prevents a member from being used by code defined outside of its class.
In this case, main must be declared as public, since it must be called by code outside of its class when the program is started. The keyword static allows main to be called before an object of the class has been created. This is necessary because main is called by the JVM before any objects are made. The keyword void simply tells the compiler that main does not return a value. As you will see, methods may also return values. All of these concepts will be discussed in detail in subsequent chapters.
As stated, main is the method called when a Java application begins. Any information that you need to pass to a method is received by variables specified within the set of parentheses that follow the name of the method.
These variables are called parameters. If no parameters are required for a given method, you still need to include the empty parentheses. In main there is only one parameter, String args[ ], which declares a parameter named args. This is an array of objects of type String.
Arrays are collections of similar objects. Objects of type String store sequences of characters. This program does not make use of this information, but other programs shown later in this book will. The next line of code is shown here. Notice that it occurs inside main. In this case, println displays the string that is passed to it. As you will see, println can be used to display other types of information, too.
The line begins with System. While too complicated to explain in detail at this time, briefly, System is a predefined class that provides access to the system, and out is the output stream that is connected to the console. Thus, System. Notice that the println statement ends with a semicolon. Many statements in Java end with a semicolon. As you will see, the semicolon is an important part of the Java syntax.
One last point: Java is case sensitive. Forgetting this can cause you serious problems. For example, if you accidentally type Main instead of main, or PrintLn instead of println, the preceding program will be incorrect. Furthermore, although the Java compiler will compile classes that do not contain a main method, it has no way to execute them. So, if you had mistyped main, the compiler would still compile your program. However, the Java interpreter would report an error because it would be unable to find the main method.
As you may know from your previous programming experience, it is quite easy to accidentally type something incorrectly when entering code into your computer. Fortunately, if you enter something incorrectly into your program, the compiler will report a syntax error message when it tries to compile it. The Java compiler attempts to make sense out of your source code no matter what you have written.
For this reason, the error that is reported may not always reflect the actual cause of the problem. In the preceding program, for example, an accidental omission of the opening curly brace after the main method causes the compiler to report the following two errors: Clearly, the first error message is completely wrong because what is missing is not a semicolon, but a curly brace. The messages may be misleading. Also, look at the last few lines of code in your program that precede the line being flagged.
Sometimes an error will not be reported until several lines after the point at which the error actually occurred. A variable is a named memory location that can be assigned a value. Further, the value of a variable can be changed during the execution of a program. That is, the content of a variable is changeable, not fixed. The following program creates two variables called myVar1 and myVar2: When you run this program, you will see the following output: This program introduces several new concepts.
First, the statement declares a variable called myVar1 of type integer. In Java, all variables must be declared before they are used. Further, the type of values that the variable can hold must also be specified. This is called the type of the variable. In this case, myVar1 can hold integer values.
These are whole number values. In Java, to declare a variable to be of type integer, precede its name with the keyword int. Thus, the preceding statement declares a variable called myVar1 of type int. The next line declares a second variable called myVar2: Notice that this line uses the same format as the first line except that the name of the variable is different.
In addition to int, Java supports several other data types. The following line of code assigns myVar1 the value In Java, the assignment operator is the single equal sign. It copies the value on its right side into the variable on its left. This approach can be generalized. The next line of code assigns myVar2 the value of myVar1 divided by 2: This line divides the value in myVar1 by 2 and then stores that result in myVar2.
Thus, after the line executes, myVar2 will contain the value The value of myVar1 will be unchanged. Like most other computer languages, Java supports a full range of arithmetic operators, including those shown here: Here are the next two lines in the program: Two new things are occurring here.
This string is not followed by a new line. This means that when the next output is generated, it will start on the same line. The print method is just like println , except that it does not output a new line after each call. Second, in the call to println , notice that myVar2 is used by itself. One more point about declaring variables before we move on: It is possible to declare two or more variables using the same declaration statement. Just separate their names by commas.
However, a variable of type int can hold only whole numbers. Thus, it cannot be used when a fractional component is required. For example, an int variable can hold the value 18, but not the value Fortunately, int is only one of several data types defined by Java. Of the two, double is the most commonly used. To declare a variable of type double, use a statement similar to that shown here: Here, x is the name of the variable, which is of type double. A: Java supplies different data types so that you can write efficient programs.
Second, the amount of memory required for one type of data might be less than that required for another. By supplying different types, Java enables you to make best use of system resources. Finally, some algorithms require or at least benefit from the use of a specific type of data. However, when the double variable x is divided by 4, the fractional component is preserved, and the proper answer is displayed. There is one other new thing to notice in the program.
To print a blank line, simply call println without any arguments. Even though you do not know much about Java at this point, you can still put what you have learned to work to create a practical program. In this project, we will create a program that converts gallons to liters. The program will work by declaring two double variables.
One will hold the number of the gallons, and the second will hold the number of liters after the conversion. There are 3. Thus, to convert gallons to liters, the gallon value is multiplied by 3. The program displays both the number of gallons and the equivalent number of liters. Create a new file called GalToLit. Compile the program using the following command line: 4. Run the program using this command: You will see this output: 5. As it stands, this program converts 10 gallons to liters.
However, by changing the value assigned to gallons, you can have the program convert a different number of gallons into its equivalent number of liters. However, it is possible to alter this flow through the use of the various program control statements supported by Java. Although we will look closely at control statements later, two are briefly introduced here because we will be using them to write sample programs.
The Java if statement works much like the IF statement in any other language. It determines the flow of program execution based on whether some condition is true or false. A Boolean expression is one that evaluates to either true or false. If condition is true, then the statement is executed. If condition is false, then the statement is bypassed.
Here is an example: In this case, since 10 is less than 11, the conditional expression is true, and println will execute. However, consider the following: In this case, 10 is not less than 9. Thus, the call to println will not take place. Java defines a full complement of relational operators that may be used in a conditional expression. They are shown here: Notice that the test for equality is the double equal sign.
Here is a program that illustrates the if statement: The output generated by this program is shown here: Notice one other thing in this program. As mentioned earlier, when you need two or more variables of the same type, they can be declared in one statement. Just separate the variable names by commas. The for Loop You can repeatedly execute a sequence of code by creating a loop. Loops are used whenever you need to perform a repetitive task because they are much simpler and easier than trying to write the same statement sequence over and over again.
Java supplies a powerful assortment of loop constructs. The one we will look at here is the for loop. The condition is a Boolean expression that tests the loop control variable. If the outcome of that test is true, statement executes and the for loop continues to iterate. If it is false, the loop terminates. The iteration expression determines how the loop control variable is changed each time the loop iterates. Here is a short program that illustrates the for loop: The output generated by the program is shown here: In this example, count is the loop control variable.
It is set to zero in the initialization portion of the for. If the outcome of this test is true, the println statement is executed, and then the iteration portion of the loop is executed, which increases count by 1. This process continues until the conditional test is false, at which point execution picks up at the bottom of the loop. As a point of interest, in professionally written Java programs, you will almost never see the iteration portion of the loop written as shown in the preceding program.
That is, you will seldom see statements like this: The reason is that Java includes a special increment operator that performs this operation more efficiently. The increment operator increases its operand by one. By use of the increment operator, the preceding statement can be written like this: Thus, the for in the preceding program will usually be written like this: You might want to try this.
As you will see, the loop still runs exactly the same as it did before. Java also provides a decrement operator, which is specified as — —. This operator decreases its operand by one. A code block is a grouping of two or more statements. This is done by enclosing the statements between opening and closing curly braces.
Once a block of code has been created, it becomes a logical unit that can be used any place that a single statement can. Consider this if statement: Here, if w is less than h, both statements inside the block will be executed. Thus, the two statements inside the block form a logical unit, and one statement cannot execute without the other also executing.
The key point here is that whenever you need to logically link two or more statements, you do so by creating a block. Code blocks allow many algorithms to be implemented with greater clarity and efficiency. Code blocks do not add any overhead whatsoever. In fact, because of their ability to simplify the coding of certain algorithms, their use generally increases speed and efficiency.
In this case, the target of the if statement is a block of code and not just a single statement. If the condition controlling the if is true as it is in this case , the three statements inside the block will be executed. Try setting i to zero and observe the result. You will see that the entire block is skipped. As you will see later in this book, blocks of code have additional properties and uses. However, the main reason for their existence is to create logically inseparable units of code.
It is often used to terminate a statement. In essence, the semicolon indicates the end of one logical entity. As you know, a block is a set of logically connected statements that are surrounded by opening and closing braces. A block is not terminated with a semicolon. Instead, the end of the block is indicated by the closing brace. Java does not recognize the end of the line as a terminator.
For this reason, it does not matter where on a line you put a statement. For example, is the same as the following, to Java: Furthermore, the individual elements of a statement can also be put on separate lines. For example, the following is perfectly acceptable: Breaking long lines in this fashion is often used to make programs more readable.
It can also help prevent excessively long lines from wrapping. However, over the years, a common and accepted indentation style has developed that allows for very readable programs. This book follows that style, and it is recommended that you do so as well.
Using this style, you indent one level after each opening brace, and move back out one level after each closing brace. This new version will print a table of conversions, beginning with 1 gallon and ending at gallons. After every 10 gallons, a blank line will be output. This is accomplished through the use of a variable called counter that counts the number of lines that have been output.
Pay special attention to its use. Create a new file called GalToLitTable. Enter the following program into the file: 3. These keywords, combined with the syntax of the operators and separators, form the definition of the Java language.
In general, keywords cannot be used as names for a variable, class, or method. See Chapter 15 for details. Also, beginning with JDK 9, an underscore by itself is considered a keyword in order to prevent its use as the name of something in your program. In the early days of Java, several other keywords were reserved for possible future use.
In addition to the keywords, Java reserves four other names. Three have been part of Java since the start: true, false, and null. These are values defined by Java. You may not use these words for the names of variables, classes, and so on. See Chapter 5 for details on var. Identifiers can be from one to several characters long. Variable names may start with any letter of the alphabet, an underscore, or a dollar sign. Next may be either a letter, a digit, a dollar sign, or an underscore.
Thus, 12x is invalid, for example. In general, you cannot use the Java keywords as identifier names. Also, you should not use the name of any standard method, such as println, as an identifier. Beyond these two restrictions, good programming practice dictates that you use identifier names that reflect the meaning or usage of the items being named. These methods are accessed through System. System is a class predefined by Java that is automatically included in your programs.
The standard classes also provide support for a graphical user interface GUI. Thus, Java as a totality is a combination of the Java language itself, plus its standard classes. As you will see, the class libraries provide much of the functionality that comes with Java. Indeed, part of becoming a Java programmer is learning to use the standard Java classes. Throughout this book, various elements of the standard library classes and methods are described.
However, the Java library is something that you will also want to explore more on your own. Chapter 1 Self Test 1. Where do Java programs begin execution? What is a variable? Which of the following variable names is invalid? How do you create a multiline comment? Show the general form of the if statement. Show the general form of the for loop. How do you create a block of code?
Write a program that computes your effective weight on the moon. Display 12 feet of conversions, inch by inch. Output a blank line every 12 inches. One meter equals approximately If you make a typing mistake when entering your program, what sort of error will result? Does it matter where on a line you put a statement? These elements define the limits of a language and determine the kind of tasks to which it can be applied.
Fortunately, Java supports a rich assortment of both data types and operators, making it suitable for any type of programming.
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