2.1 An Empty Program

Key terms: method header, method body, keyword, main class

We begin with the simplest possible Java program — one that does nothing. You might not expect this to be very enlightening, but it will show you the technical infrastructure required for every Java program, regardless of its size or purpose.

A program is a sequence of instructions, but instructions in Java are not freestanding — they must occur within a method. The entry point to a Java application is a special method named main, which always looks exactly like this:

public static void main(String[] args) {

}

The first line is the method header, and the method body (here empty) follows, enclosed in curly braces.

The words public, static, and void are keywords — reserved words with specific meanings in the language. The original release of Java defined 47 keywords, and recent versions define 51, with an additional 17 contextually reserved — that is, reserved only in specific contexts.

The meanings of the keywords in the header and the parenthetical code will become clear later; for now, it suffices to know that the main method is always written the way you see it here. Java is case sensitive, so pay close attention to capitalization — only the first letter in String is capitalized, for example.

Note: In recent versions of Java, the public modifier is optional for main; it is included in the program listings of this book for consistency with standard practice and legacy code.

The main method cannot be compiled in isolation because, by definition, methods belong to classes — there are no stand-alone methods in Java. We must therefore place the definition of our main method within a class. For that purpose, we can simply choose a name for the class, say SimpleClass:

public class SimpleClass {

    public static void main(String[] args) {

    }

}

In general, classes define blueprints for creating objects: they specify the attributes and methods that characterize those objects. However, there is an exception to this general scheme. Sometimes a class serves only as a container for the main method, in which case it is called a main class.

To write a program in Java, then, is to define a main class with a main method containing the instructions to be executed. The preceding code fragment represents a complete Java program: a public main class with an empty main method.