How Java compiles and interprets code

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How Java compiles and interprets code

When you develop a Java application, you develop one or more classes. For each class, you write the Java statements that direct the operation of the class. Then, you use a Java tool to translate the Java statements into instructions that can be run by the computer. This process is illustrated in figure 1-3.

To start, you can use any text editor to enter and edit the Java source code for a class. These are the Java statements that tell the application what to do. Then, you use the Java compiler to compile the source code into a format known as Java bytecodes. At this point, the bytecodes can be run on any platform that has a Java interpreter to interpret (or translate) the Java bytecodes into code that can be understood by the underlying operating system. The Java interpreter is a concrete implementation of an abstract specification known as the Java virtual machine (JVM). As a result, these two terms are often used interchangeably.

Since Java interpreters are available for all major operating systems, you can run Java on most platforms. This is what gives Java applications their platform independence. In contrast, C++ requires a specific compiler for each type of platform that its programs are going to run on.

In addition, most modern web browsers can be Java enabled. This allows applets, which are bytecodes that are downloaded from the Internet or an intranet, to run within a web browser. To make this work, Sun developed theJava Plug-in. This piece of software is similar to other browser plug-ins such as Apple QuickTime. It allows the browser to run Sun's current version of the Java interpreter. You'll learn more about this in chapter 18.

How Java compiles and interprets code

Description

• When you develop a Java application, you develop one or more classes.
• You can use any text editor to create, edit, and save the source code for a Java class. Source code files have the java extension.
• The Java compiler translates Java source code into a platform-independent format known as Java bytecodes. Files that contain Java bytecodes have the class extension.
• The Java interpreter executes Java bytecodes. Since Java interpreters exist for all major operating systems, Java bytecodes can be run on most platforms. A Java interpreter is an implementation of a Java virtual machine (JVM).
• Most modern web browsers can be Java enabled. This lets applets run within these browsers. Sun provides a tool known as the Java Plug-in that allows you to specify the version of the Java interpreter that you want to use.

Introduction to Java ...................................................................... 4
Toolkits and platforms ....................................................................... 4
Java compared to C++ ...................................................................... 4
Java compared to C# ........................................................................ 4
Applications, applets, and servlets ....................................................... 6
How Java compiles and interprets code ................................................ 8
How to prepare your system for using Java .................................. 10
How to install the JDK ..........................................................................10
A summary of the directories and files of the JDK ...................................12
How to set the command path ...............................................................14
How to set the class path ..................................................................... 16
How to use TextPad to work with Java ........................................... 18
How to install TextPad ......................................................................... 18
How to use TextPad to save and edit source code ................................... 20
How to use TextPad to compile source code ........................................... 22
How to use TextPad to run an application ............................................... 22
Common error messages and solutions ................................................. 24
How to use the command prompt to work with Java ...................... 26
How to compile source code ................................................................. 26
How to run an application ..................................................................... 26
How to compile source code with a switch .............................................. 28
Essential DOS skills for working with Java ............................................. 30
How to use the documentation for the Java SE API ....................... 32
How to install the API documentation ..................................................... 32
How to navigate the API documentation ................................................. 34
Introduction to Java IDEs ................................................................ 36
The Eclipse IDE for Java ....................................................................... 36
The NetBeans IDE ................................................................................ 38
The BlueJ IDE ...................................................................................... 38
Perspective ....................................................................................... 40

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How to prepare your system for using Java

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How to prepare your system for using Java

Before you can develop Java applications, the JDK must be installed on your system. In addition, your system may need to be configured to work with the JDK. Once you install the JDK, you'll be ready to create your first Java application.

How to install the JDK

Figure 1-4 shows how to install the JDK. To start, you download the exe file for the setup program for the most recent version of the JDK from the Java web site. Then, you navigate to the directory that holds the exe file, run the setup file, and respond to the resulting dialog boxes.

Since Sun periodically updates the Java web site, we've kept the procedure shown in this figure somewhat general. As a result, you may have to do some searching to find the current version of the JDK. In general, you can start by looking at the downloads for Java SE. Then, you can find the most current version of the JDK for your operating system.

By the way, all of the examples in this book have been tested against version 6 of the JDK. Since Java has a good track record of being upwards compatible, however, these examples should work equally well with later versions of the JDK.

The Java web site

java.sun.com

How to download the JDK from the Java web site

1. Go to the Java web site.
2. Locate the download page for Java SE 6.
3. Click on the Download button for JDK 6 and follow the instructions.
4. Save the exe file for the setup program to your hard disk.

How to install the JDK

• Run the exe file and respond to the resulting dialog boxes. When you're prompted for the JDK directory, use the default directory. For most Windows systems, the default directory is C:\Program Files\Java\jdk1.6.0 for Java SE 6.

Notes

• For more information about installing the JDK, you can refer to the Java web site.
• If you are installing the Windows version of the JDK, you can perform either an offline or an online installation. An online installation is faster because only a small setup file is downloaded. However, you must remain online during the entire installation so the required files can be installed.

A summary of the directories and files of the JDK

Figure 1-5 shows the directories and files that are created when you install the JDK. Here, the JDK is stored in the C:\Program Files\Java\jdk1.6.0 directory. This directory has multiple subdirectories, but the bin, jre, lib, and docs directories are the most important.

The bin directory holds all the tools necessary for developing and testing a program, including the Java compiler. Later in this chapter, you'll learn how to use these tools to compile and run Java applications. The lib directory contains libraries and support files required by the development tools.

The jre directory contains the Java interpreter, or Java Runtime Environment (JRE), that's needed to run Java applications once they've been compiled. Although the JDK uses this internal version of the JRE, you can also download a standalone version of the JRE from the Java web site. Once you're done developing a Java application, for example, you can distribute the standalone JRE to other computers so they can run your application.

The docs directory can be used to store the Java documentation. Later in this chapter, you'll learn how to download and install this documentation.

In the JDK directory, you can find an HTML readme file that contains much of the information that's presented in this figure as well as more technical and detailed information about the JDK. You can view the HTML file with a web browser.

The JDK directory also contains the src.zip file. This is a compressed file that holds the source code for the JDK. If you want to view the source code, you can extract the source files from this zip file. If you're curious to see the Java code of the JDK, you may want to do that once you understand Java better.

When you work with Windows, you'll find that it sometimes uses the termsfolder and subfolder to refer to directories and subdirectories. For consistency, though, we use the term directory throughout this book. In practice, these terms are often used interchangeably.

The default directory for the JDK on a Windows machine

C:\Program Files\Java\jdk1.6.0\bin

Four important subdirectories of the JDK

Two important files stored in the JDK directory

Description

• The Java Runtime Environment (JRE) is the Java interpreter that allows you to run
  compiled programs in Java. The jre directory is an internal copy of the runtime environ-
  ment that works with the JDK. You can also download a standalone version of the JRE
  for computers that don't have the JDK installed on them.

Figure 1-5

Introduction to Java ...................................................................... 4
Toolkits and platforms ....................................................................... 4
Java compared to C++ ...................................................................... 4
Java compared to C# ........................................................................ 4
Applications, applets, and servlets ....................................................... 6
How Java compiles and interprets code ................................................ 8
How to prepare your system for using Java .................................. 10
How to install the JDK ..........................................................................10
A summary of the directories and files of the JDK ...................................12
How to set the command path ...............................................................14
How to set the class path ..................................................................... 16
How to use TextPad to work with Java ........................................... 18
How to install TextPad ......................................................................... 18
How to use TextPad to save and edit source code ................................... 20
How to use TextPad to compile source code ........................................... 22
How to use TextPad to run an application ............................................... 22
Common error messages and solutions ................................................. 24
How to use the command prompt to work with Java ...................... 26
How to compile source code ................................................................. 26
How to run an application ..................................................................... 26
How to compile source code with a switch .............................................. 28
Essential DOS skills for working with Java ............................................. 30
How to use the documentation for the Java SE API ....................... 32
How to install the API documentation ..................................................... 32
How to navigate the API documentation ................................................. 34
Introduction to Java IDEs ................................................................ 36
The Eclipse IDE for Java ....................................................................... 36
The NetBeans IDE ................................................................................ 38
The BlueJ IDE ...................................................................................... 38
Perspective ....................................................................................... 40

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