Downloading the JDK

To get to the download page, point your browser to http://java.oracle.com/technetwork/java. Then follow the appropriate links to download the JDK 9 for your operating system.

When you get to the Java download page, you find links to download the JDK or the JRE. Follow the JDK link; the JRE link gets you only the Java Runtime Environment, not the complete Java Development Kit.

The JDK download comes in two versions: an online version that requires an active Internet connection to install the JDK and an offline version that lets you download the JDK installation file to your computer and install it later.

I recommend that you use the offline version. It installs faster, and you can reinstall the JDK later if you need to without having to download it again.

The exact size of the offline version depends on the platform, but most versions are between 50MB and 60MB. As a result, the download takes a few hours if you don’t have a high-speed Internet connection. With a cable, DSL, or T1 connection, the download takes less than five minutes.

Installing the JDK

After you download the JDK file, you can install it by running the executable file you downloaded. The procedure varies slightly depending on your operating system, but basically, you just run the JDK installation program file after you download it, as follows:

• On a Windows system, open the folder in which you saved the installation program and double-click the installation program’s icon.
• On a Linux or Solaris system, use console commands to change to the directory to which you downloaded the file and then run the program.

After you start the installation program, it prompts you for any information that it needs to install the JDK properly, such as which features you want to install and what folder you want to install the JDK in. You can safely choose the default answer for each option.

Perusing the JDK folders

When the JDK installs itself, it creates several folders on your hard drive. The locations of these folders vary depending on your system, but in all 32-bit versions of Windows, the JDK root folder is in the path Program FilesJava on your boot drive. On 64-bit versions of Windows, the root folder will be either Program FilesJava or Program Files (x86)Java. The name of the JDK root folder also varies, depending on the Java version you’ve installed. For version 1.9, the root folder is named jdk-9.

Table 2-1 lists the subfolders created in the JDK root folder. As you work with Java, you’ll refer to these folders frequently.

In addition to these folders, the JDK installs several files in the JDK root folder. I list these files in Table 2-2.

Setting the path

After you install the JDK, you need to configure your operating system so that it can find the JDK command-line tools. To do that, you must set the Path environment variable — a list of folders that the operating system uses to locate executable programs. To do this on a Windows 7 or earlier system, follow these steps. You must be logged in as an administrator to make the changes described in this procedure. On Windows 8, Java installs as a desktop application that works like the Windows 7 configuration.

1. Click the Start menu, then right-click Computer.

   This brings up the System Properties page.

2. Click the Advanced System Settings link.

3. Click the Environment Variables button.

   The Environment Variables dialog box appears, as shown in Figure 2-1.

4. In the System Variables list, scroll to the Path variable, select it, and then click the Edit button.

   A little dialog box pops up to let you edit the value of the Path variable.

5. Add the JDK bin folder to the beginning of the Path value.

   Use a semicolon to separate the bin folder from the rest of the information that may already be in the path.

   Note: The name of the bin folder may vary on your system, as in this example:

   c:Program FilesJavajdk-9in;other directories…

6. Click OK three times to exit.

   The first OK gets you back to the Environment Variables dialog box; the second OK gets you back to the System Properties dialog box; and the third OK closes the System Properties dialog box.

For Linux or Solaris, the procedure depends on which shell you’re using. For more information, consult the documentation for the shell you’re using.

Compiling a program

You can compile a program from a command prompt by using the javac command. Before you can do that, however, you need a program to compile. Follow these steps:

1. Using any text editor, type the following text in a file, and save it as HelloApp.java:

   public class HelloApp
{
public static void main(String[] args)
{
System.out.println("Hello, World!");
}
}

   Pay special attention to capitalization. If you type Public instead of public, for example, the program won’t work. (If you don’t want to bother with typing, you can download the sample programs from this book’s website at www.dummies.com/go/javaaiofd5e.)

2. Save the file in any directory you want.

3. Open a command prompt, use a cd command to change to the directory you saved the program file in, and then enter the command javac HelloApp.java.

   This command compiles the program (javac) and creates a class file named HelloApp.class.

Assuming that you typed the program exactly right, the javac command doesn’t display any messages at all. If the program contains any errors, however, you get one or more error messages onscreen. If you typed Public instead of public despite my warning earlier in this section, the compiler displays the following error message:

C:javasamples>javac HelloApp.java
HelloApp.java:1: error: class, interface, or enum expected
Public class HelloApp
^
1 error

C:javasamples>

The compiler error message indicates that an error is in line 1 of the HelloApp.java file. If the compiler reports an error message like this one, your program contains a coding mistake. You need to find the mistake, correct it, and compile the program again.

Compiling more than one file

Normally, the javac command compiles only the file that you specify on the command line, but you can coax javac into compiling more than one file at a time by using any of the techniques I describe in the following paragraphs:

• If the Java file you specify on the command line contains a reference to another Java class that’s defined by a java file in the same folder, the Java compiler automatically compiles that class too.

  Suppose you have a java program named TestProgram, which refers to a class called TestClass, and the TestClass.java file is located in the same folder as the TestProgram.java file. When you use the javac command to compile the TestProgram.java file, the compiler automatically compiles the TestClass.java file too.

• You can list more than one filename in the javac command. The following command compiles three files:

  javac TestProgram1.java TestProgram2.java TestProgram3.java

• You can use a wildcard to compile all the files in a folder, like this:

  javac *.java

• If you need to compile a lot of files at the same time but don’t want to use a wildcard (perhaps you want to compile a large number of files but not all the files in a folder), you can create an argument file, which lists the files to compile. In the argument file, you can type as many filenames as you want, using spaces or line breaks to separate them. Here’s an argument file named TestPrograms that lists three files to compile:

  TestProgram1.java
TestProgram2.java
TestProgram3.java

  You can compile all the programs in this file by using an @ character followed by the name of the argument file on the javac command line, like this:

  javac @TestPrograms

Using Java compiler options

The javac command has a gaggle of options that you can use to influence the way it compiles your programs. For your reference, I list these options in Table 2-3.

To use one or more of these options, type the option before or after the source filename. Either of the following commands, for example, compiles the HelloApp.java file with the -verbose and -deprecation options enabled:

javac HelloWorld.java -verbose -deprecation
javac -verbose -deprecation HelloWorld.java

Don’t get all discombobulated if you don’t understand what all these options do. Most of them are useful only in unusual situations. The options you’ll use the most are

• -classpath or -cp: Use this option if your program makes use of class files that you’ve stored in a separate folder.
• -deprecation: Use this option if you want the compiler to warn you whenever you use API methods that have been deprecated. (Deprecated methods are older methods that once were part of the Java standard API but are on the road to obsolescence. They still work but may not function in future versions of Java.)
• -source: Use this option to limit the compiler to previous versions of Java. Note, however, that this option applies only to features of the Java language itself, not to the API class libraries. If you specify -source 1.4, for example, the compiler won’t allow you to use new Java language features that were introduced in a version later than 1.4, such as generics, enhanced for loops, or Lambda expressions. But you can still use the new API features that were added with version 1.5, such as the Scanner class.
• -help: Use this option to list the options that are available for the javac command.

Running a Java program

When you successfully compile a Java program, you can run the program by typing the java command followed by the name of the class that contains the program’s main method. The JRE loads, along with the class you specify, and then runs the main method in that class. To run the HelloApp program, for example, type this command:

C:javasamples>java HelloApp

The program responds by displaying the message "Hello, World!".

The class must be contained in a file with the same name as the class, and its filename must have the extension .class. You usually don’t have to worry about the name of the class file because it’s created automatically when you compile the program with the javac command. Thus, if you compile a program in a file named HelloApp.java, the compiler creates a class named HelloApp and saves it in a file named HelloApp.class.

Using the javap command

The javap command is called the Java disassembler because it takes class files apart and tells you what’s inside them. You won’t use this command often, but using it to find out how a particular Java statement works is fun sometimes. You can also use it to find out what methods are available for a class if you don’t have the source code that was used to create the class.

Here’s the information you get when you run the javap HelloApp command:

C:javasamples>javap HelloApp
Compiled from "HelloApp.java"
public class HelloApp{
public HelloApp();
public static void main(java.lang.String[]);
}

As you can see, the javap command indicates that the HelloApp class was compiled from the HelloApp.java file and that it consists of a HelloApp public class and a main public method.

You may want to use two options with the javap command. If you use the -c option, the javap command displays the actual Java bytecodes created by the compiler for the class. And if you use the -verbose option, the bytecodes (plus a ton of other fascinating information about the innards of the class) are displayed.

If you become a big-time Java guru, you can use this type of information to find out exactly how certain Java features work. Until then, you probably should leave the javap command alone except for those rare occasions when you want to impress your friends with your in-depth knowledge of Java. (Just hope that when you do, they don’t ask you what the aload or invokevirtual instruction does.)

Java SE API Docs

The API and documentation links at http://download.java.net/java/jdk98/docs/api/index.html take you to the complete documentation for all currently supported versions of the Java API, in English as well as Japanese. Figure 2-2 shows the English Java SE 8 API documentation page.

You can use this page to find complete information for any class in the API. By default, all the Java classes are listed in the frame that appears in the bottom-left corner of the page. You can limit this display to the classes in a particular package by choosing the package from the menu in the top-left corner of the page.

Click the class you’re looking for in the class list to call up its documentation page. Figure 2-3 shows the documentation page for the String class. If you scroll down this page, you find complete information about everything you can do with this class, including an in-depth discussion of what the class does, a list of the various methods it provides, and a detailed description of what each method does. In addition, you find links to similar classes.

Java Language Specification

If you’re interested in learning details about some element of the Java language itself (rather than the information about a class in the API class library), visit the Java Programming Language reference page at http://docs.oracle.com/javase/specs/index.html. That link takes you to a set of pages that describes — in sometimes excruciating and obscure detail — exactly how each element of the Java language works.

Frankly, this documentation isn’t that much help for beginning programmers. It was written by computer scientists for computer scientists. You can tell just by looking at the table of contents that it isn’t for novices. The first chapter is called “Introduction” (that’s not so bad), but Chapters 2 and 3 are titled “Grammars” and “Lexical Structure,” respectively, and matters just get more arcane from there.

That’s why you’re reading this book, after all. You won’t even find a single sentence more about lexical structure in this book (other than this one, of course). Even so, at some time in your Java journeys, you may want to get to the bottom of the rules that govern such strange Java features as anonymous inner classes. When that day arrives, grab a six-pack of Jolt Cola, roll up your sleeves, and open the Java Language Specification pages.