The Anatomy of a Java Program

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

• First line declares HelloWorld class. class is the syntax for declaring a class, and prepending with the public modifer means the class will be visible outside HelloWorld's package.
• The code between the curly braces, { ... } define the contents of the HelloWorld class, in this case a single method, main

The main Method

In order to make a class executable with the java command, it must have a main method:

public static void main(String[] args) { ... }

• public means method can be called from outside the class
• static means the method can be called without instantiating an object of the class. Static methods (and variables) are sometimes called class methods.

• void is the return type. In particular, main returns nothing.

  • Sometimes such subprograms are called procedures and distinguished from functions, which return values.

• After the method name, main, comes the parameter list. main takes a single parameter of type String[] - an array of Strings. args is the name of the parameter, which we can refer to within the body of main

Methods

The main method is a special method that is used as the entry point for a Java program. We can define other methods as well. Consider this method from :

public static String extractLastName(String name) {
    int commaPos = name.indexOf(",");
    int len = name.length();
    String lastName = name.substring(0, commaPos).trim();
    return lastName;
}

Similar to our main method but:

• returns a String value
• takes a single parameter of type String

Method Parameters

In this method:

public static String extractLastName(String name) {
    int commaPos = name.indexOf(",");
    int len = name.length();
    String lastName = name.substring(0, commaPos).trim();
    return lastName;
}

name is a parameter (or formal parameter), a local scope variable within the extractLastName method. It is bound to a value when the method is called. In the statement:

String lastName = extractLastName(fullName);

the right-hand side, extractLastName(fullName), is a method invocation (or method call). We say that fullName is the argument (or actual parameter) to this invocation of the extractLastName method.

Local Variables

Method parameters and variables declared inside the method are local to the method, invisible outside the method. Local variables "shadow" variables of the same name in an enclosing scope

public class Methods {

    // Note that static variables cannot appear inside methods
    static String message = "Global message.";
    static int a = 5;

    public static int add(int a, int b) {
        String message = "Adding " + a + " and " + b;
        System.out.println(message);
        return a + b;
    }
...

In the add method, the parameter a shadows the static variable a, and the local variable message shadows the static variable message

Methods as Expressions

Methods that return values are expressions which can be used anywhere a value of the method's return type can be used. Given:

public static int add(int a, int b) { ... }

which returns an int, this:

x + (x + y)

is equivalent to this:

x + add(x, y)

See Methods.java.

Closing Thougts

Methods are subprograms with

• input (parameters),
• processing (a sequence of statements), and
• output (return value).

Methods are a powerful form of procedural abstraction, another step in the building of complex programs from simple parts.