In this section, we’ll say a few words about javac, the Java compiler that is supplied as part of Sun’s SDK. (If you are happily working in another development environment, you may want to skip ahead to the next section.) The javac compiler is written entirely in Java, so it’s available for any platform that supports the Java runtime system. The ability to support its own development environments is an important stage in a language’s development. Java makes this bootstrapping automatic by supplying a ready-to-run compiler at the same cost as porting the interpreter.

javac turns Java source code into a compiled class that contains Java virtual machine byte-code. By convention, source files are named with a .java extension; the resulting class files have a .class extension. Each source code file is a single compilation unit. As you’ll see in Chapter 6, classes in a given compilation unit share certain features, such as package and import statements.

javac allows you one public class per file and insists the file have the same name as the class. If the filename and class name don’t match, javac issues a compilation error. A single file can contain multiple classes, as long as only one of the classes is public. Avoid packing many classes into a single source file. Including non-public classes in a .java file is one easy way to tightly couple such classes to a public class. But you might also consider using inner classes (see Chapter 6).

Now for an example. Place the following source code in file BigBird.java:

package animals.birds;  
  
public class BigBird extends Bird {  
    ...  
}

Then compile it with:

% javac BigBird.java

Unlike the Java interpreter, which takes just a class name as its argument, javac needs a filename to process. The previous command produces the class file BigBird.class in the same directory as the source file. While it’s useful to have the class file in the same directory as the source for testing a simple example, for most real applications you’ll need to store the class file in an appropriate place in the class path.

You can use the -d option to javac to specify an alternative directory for storing the class files it generates. The specified directory is used as the root of the class hierarchy, so .class files are placed in this directory or in a subdirectory below it, depending on whether the class is contained in a package. (The compiler creates intermediate subdirectories automatically, if necessary.) For example, we can use the following command to create the BigBird.class file at /home/vicky/Java/classes/animals/birds/BigBird.class:

% javac -d /home/vicky/Java/classes BigBird.java

You can specify multiple .java files in a single javac command; the compiler creates a class file for each source file. But you don’t need to list source files for other classes that your class references, as long as the other classes have already been compiled. During compilation, Java resolves other class references using the class path. If our class refers to other classes in animals.birds or other packages, the appropriate paths should be listed in the class path at compile time, so that javac can find the appropriate class files.

The Java compiler is more intelligent than your average compiler, replacing some of the functionality of a make utility. For example, javac compares the modification times of the source and class files for all referenced classes and recompiles them as necessary. A compiled Java class remembers the source file from which it was compiled, so as long as the source file is in the same directory as the class file, javac can recompile the source if necessary. If, in the previous example, class BigBird references another class, animals.furry.Grover, javac looks for the source file Grover.java in an animals.furry package and recompiles it if necessary to bring the Grover.class class file up-to-date.

By default, however, javac checks only source files that are referenced directly from other source files. This means that if you have an out-of-date class file that is referenced only by an up-to-date class file, it may not be noticed and recompiled. You can force javac to walk the entire graph of objects using the -depend option. But be warned, this can increase compilation time significantly. And this technique still won’t help if you want to keep class libraries or other collections of classes up to date even if they aren’t being referenced at all. For that you should consider a make utility.

Finally, it’s important to note that javac can compile an application even if only the compiled versions of referenced classes are available. You don’t need source code for all of your objects. Java class files contain all the data type and method signature information that source files contain, so compiling against binary class files is as type-safe (and exception-safe) as compiling with Java source code.