Typical Java Development Environment

We now explain the commonly used steps in creating and executing a Java application using a Java development environment (illustrated in Fig. 1.1).

Figure 1.1. Typical Java development environment.

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Java programs normally go through five phasesedit, compile, load, verify and execute. We discuss these phases in the context of the J2SE Development Kit (JDK) version 5.0 from Sun Microsystems, Inc., which will be wrapped with the book as an accompanying CD once Sun releases the final version of the JDK 5.0. If the CD for your book does not have the JDK on it, you can download both the JDK and its documentation now from java.sun.com/j2se/5.0/download.jsp. For help with the download, visit servlet.java.sun.com/help/download. Carefully follow the installation instructions for the JDK provided on the CD (or at java.sun.com/j2se/5.0/install.html) to ensure that you set up your computer properly to compile and execute Java programs. Complete installation instructions can also be found on Sun's Java Web site at

java.sun.com/learning/new2java/index.html

[Note: This Web site provides installation instructions for Windows, UNIX/Linux and Mac OS X. If you are not using one of these operating systems, refer to the manuals for your system's Java environment or ask your instructor how to accomplish these tasks based on your computer's operating system. In addition, please keep in mind that Web links occasionally break as companies evolve their Web sites. If you encounter a problem with this link or any other links referenced in this book, please check our Web site (www.deitel.com) for errata and please notify us by e-mail at deitel@deitel.com. We will respond promptly.]

Phase 1: Creating a Program

Phase 1 consists of editing a file with an editor program (normally known simply as an editor). You type a Java program (typically referred to as source code) using the editor, make any necessary corrections and save the program on a secondary storage device, such as your hard drive. Java source-code file names end with the .java extension, which indicates that a file contains Java source code. We assume that the reader knows how to edit a file.

Two editors widely used on UNIX/Linux systems are vi and emacs. On Windows, a simple editing program like Windows Notepad will suffice. Many freeware and shareware editors are also available for download from the Internet on sites like www.download.com.

For organizations that develop substantial information systems, integrated development environments (IDEs) are available from many major software suppliers, including Sun Microsystems. IDEs provide many tools that support the software development process, including editors for writing and editing programs and debuggers for locating logic errors in programs.

Several popular IDEs are NetBeans (www.netbeans.org), jEdit (www.jedit.org), Eclipse (www.eclipse.org), JBuilder (www.borland.com), JCreator (www.jcreator.com), BlueJ (www.blueJ.org) and jGRASP (www.jgrasp.org). Sun Microsystems has the Sun Java Studio (wwws.sun.com/software/sundev/jde/), which is an enhanced version of Net-Beans. [Note: NetBeans v. 3.6, jEdit v. 4.1, jGRASP v. 1.7 and BlueJ v. 1.3.5 are included on the CD that accompanies this book. These IDEs are designed to execute on most major platforms. Our example programs should operate properly with any Java integrated development environment that supports the JDK 5.0. We also provide free Dive Into™ guides for various IDEs on our Web site at www.deitel.com/books/jHTP6/index.html.]

Phase 2: Compiling a Java Program into Bytecodes

In Phase 2, the programmer uses the command javac (the Java compiler) to compile a program. For example, to compile a program called Welcome.java, you would type

javac Welcome.java

in the command window of your system (i.e., the MS-DOS prompt in Windows 95/98/ ME, the Command Prompt in Windows NT/2000/XP, the shell prompt in UNIX/Linux or the Terminal application in Mac OS X). If the program compiles, the compiler produces a .class file called Welcome.class that contains the compiled version of the program.

The Java compiler translates the Java source code into bytecodes that represent the tasks to be performed during the execution phase (Phase 5). Bytecodes are executed by the Java Virtual Machine (JVM)a part of the JDK and the foundation of the Java platform. A virtual machine(VM) is a software application that simulates a computer, but hides the underlying operating system and hardware from the programs that interact with the VM. If the same VM is implemented on many computer platforms, applications that it executes can be used on all those platforms. The JVM is one of the most widely used virtual machines.

Unlike machine language, which is dependent on specific computer hardware, byte-codes are platform-independent instructionsthey are not dependent on a particular hardware platform. So Java's bytecodes are portablethat is, the same bytecodes can execute on any platform containing a JVM that understands the version of Java in which the bytecodes were compiled. The JVM is invoked by the java command. For example, to execute a Java application called Welcome, you would type the command

java Welcome

in a command window to invoke the JVM, which would then initiate the steps necessary to execute the application. This begins Phase 3.

Phase 3: Loading a Program into Memory

In Phase 3, the program must be placed in memory before it can executea process known as loading. The class loader takes the .class files containing the program's bytecodes and transfers them to primary memory. The class loader also loads any of the .class files provided by Java that your program uses. The .class files can be loaded from a disk on your system or over a network (e.g., your local college or company network, or the Internet).

Phase 4: Bytecode Verification

In Phase 4, as the classes are loaded, the bytecode verifier examines their bytecodes to ensure that they are valid and do not violate Java's security restrictions. Java enforces strong security, to make sure that Java programs arriving over the network do not damage your files or your system (as computer viruses and worms might).

Phase 5: Execution

In Phase 5, the JVM executes the program's bytecodes, thus performing the actions specified by the program. In early Java versions, the JVM was simply an interpreter for Java bytecodes. This caused most Java programs to execute slowly because the JVM would interpret and execute one bytecode at a time. Today's JVMs typically execute bytecodes using a combination of interpretation and so-called just-in-time (JIT) compilation. In this process, The JVM analyzes the bytecodes as they are interpreted, searching for hot spotsparts of the bytecodes that execute frequently. For these parts, a just-in-time (JIT) compilerknown as the Java HotSpot compilertranslates the bytecodes into the underlying computer's machine language. When the JVM encounters these compiled parts again, the faster machine-language code executes. Thus Java programs actually go through two compilation phasesone in which source code is translated into bytecodes (for portability across JVMs on different computer platforms) and a second in which, during execution, the bytecodes are translated into machine language for the actual computer on which the program executes.

Problems That May Occur at Execution Time

Programs might not work on the first try. Each of the preceding phases can fail because of various errors that we will discuss throughout this book. For example, an executing program might attempt to divide by zero (an illegal operation for whole-number arithmetic in Java). This would cause the Java program to display an error message. If this occurs, you would have to return to the edit phase, make the necessary corrections and proceed through the remaining phases again to determine that the corrections fix the problem(s). [Note: Most programs in Java input or output data. When we say that a program displays a message, we normally mean that it displays that message on your computer's screen. Messages and other data may be output to other devices, such as disks and hardcopy printers, or even to a network for transmission to other computers.]

Common Programming Error 1.1