Java Tech - Chapter 3

The example below illustrates how a static property, here a double type variable, can be accessed before an instance of that class is created.

We also show that if that static variable is changed, it changes for all instances of that class. Whereas, an instance variable ("m"), will hold different values in different classes.

StaticApplet.java
(Output goes to browser's Java console.)

public class StaticApplet extends java.applet.Applet
{
  public void init() {

    System.out.println ("   Static vs Instance Variables");

    // Access static variable via the class name Test.
    System.out.println (" Before an instance of Test created:");
    System.out.println ("  pi = " + Test.pi);

    //Create an instance of the Test class
    Test test = new Test ();
    Test testa = new Test (456);

    System.out.println (" Create two instances of Test:");
    // Access instance variable
    System.out.println ("  1st instance j = " + test.m);

    System.out.println ("  2nd instance j = " + testa.m);

    // If a new value assigned to a static variable
    Test.pi = 3.1416;
    System.out.println (" Set Test.pi to 3.1416");
    System.out.println (" Value of pi in each Test object after change:");

    // It changes for all instances.
    System.out.println ("  test.pi  = " + test.pi);
    System.out.println ("  testa.pi = " + testa.pi);

  }

  // Paint message in Applet window.
  public void paint (java.awt.Graphics g) {
    g.drawString ("StaticApplet", 10, 20);
}
}

// This Test class inclues a static variable.
class Test
{
  static double pi = 3.14;
  int m;

  Test ()
  {}

  Test (int j) {
    m = j;
  }
}

Application version:
StaticApp.java