2.11. Java File Structure

This chapter has taken us from the smallest to the largest elements of Java syntax, from individual characters and tokens to operators, expressions, statements, and methods, and on up to classes and packages. From a practical standpoint, the unit of Java program structure you will be dealing with most often is the Java file. A Java file is the smallest unit of Java code that can be compiled by the Java compiler. A Java file consists of:

• An optional package directive

• Zero or more import or import static directives

• One or more type definitions

These elements can be interspersed with comments, of course, but they must appear in this order. This is all there is to a Java file. All Java statements (except the package and import directives, which are not true statements) must appear within methods, and all methods must appear within a type definition.

Java files have a couple of other important restrictions. First, each file can contain at most one class that is declared public. A public class is one that is designed for use by other classes in other packages. This restriction on public classes only applies to top-level classes; a class can contain any number of nested or inner classes that are declared public. We'll see more about the public modifier and nested classes in Chapter 3.

The second restriction concerns the filename of a Java file. If a Java file contains a public class, the name of the file must be the same as the name of the class, with the extension .java appended. Thus, if Point is defined as a public class, its source code must appear in a file named Point.java. Regardless of whether your classes are public or not, it is good programming practice to define only one per file and to give the file the same name as the class.

When a Java file is compiled, each of the classes it defines is compiled into a separate class file that contains Java byte codes to be interpreted by the Java Virtual Machine. A class file has the same name as the class it defines, with the extension .class appended. Thus, if the file Point.java defines a class named Point, a Java compiler compiles it to a file named Point.class. On most systems, class files are stored in directories that correspond to their package names. Thus, the class com.davidflanagan.examples.Point is defined by the class file com/davidflanagan/examples/Point.class.

The Java interpreter knows where the class files for the standard system classes are located and can load them as needed. When the interpreter runs a program that wants to use a class named com.davidflanagan.examples.Point, it knows that the code for that class is located in a directory named com/davidflanagan/examples/ and, by default, it "looks" in the current directory for a subdirectory of that name. In order to tell the interpreter to look in locations other than the current directory, you must use the -classpath option when invoking the interpreter or set the CLASSPATH environment variable. For details, see the documentation for the Java interpreter, java, in Chapter 8.