Programming Tutorials

static keyword in Java

By: Abinaya in Java Tutorials on 2007-09-08  

There will be times when you will want to define a class member that will be used independently of any object of that class. Normally a class member must be accessed only in conjunction with an object of its class. However, it is possible to create a member that can be used by itself, without reference to a specific instance. To create such a member, precede its declaration with the keyword static. When a member is declared static, it can be accessed before any objects of its class are created, and without reference to any object. You can declare both methods and variables to be static. The most common example of a static member is main(). main() is declared as static because it must be called before any objects exist.

Instance variables declared as static are, essentially, global variables. When objects of its class are declared, no copy of a static variable is made. Instead, all instances of the class share the same static variable.

Methods declared as static have several restrictions:

• They can only call other static methods.
• They must only access static data.
• They cannot refer to this or super in any way. (The keyword super relates to inheritance.)

If you need to do computation in order to initialize your static variables, you can declare a static block which gets executed exactly once, when the class is first loaded. The following example shows a class that has a static method, some static variables, and a static initialization block:

// Demonstrate static variables, methods, and blocks.
class UseStatic {
    static int a = 3;
    static int b;

    static void meth(int x) {
        System.out.println("x = " + x);
        System.out.println("a = " + a);
        System.out.println("b = " + b);
    }

    static {
        System.out.println("Static block initialized.");
        b = a * 4;
    }

    public static void main(String args[]) {
        meth(42);
    }
}

As soon as the UseStatic class is loaded, all of the static statements are run. First, a is set to 3, then the static block executes (printing a message), and finally, b is initialized to a * 4 or 12. Then main() is called, which calls meth(), passing 42 to x. The three println() statements refer to the two static variables a and b, as well as to the local variable x.