Java in a Nutshell, 5th Edition

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 ...