Java Tech - Chapter 4

In this demo we use the StartApplet3.java starter, which includes a class called Test and its subclass Test1, which in turn is subclassed by Test2.

Each class includes two constructors. We use this() and super() to create different paths for the sequence of constructors invoked in the building of the Test2 objects.

Two Test2 objects are created, each from a different starting constructor. Each constructor prints out a message to indicate it was invoked. Write down the sequence of messages before looking at the printout to see if you understand how the sequence of constructors is built up.

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

public class ConstructApplet extends java.applet.Applet
{
  public void init () {
    System.out.println ("First test2 object");
    Test2 test2 = new Test2 (1.2, 1.3);
    System.out.println ();

    System.out.println ("Second test2 object");
    test2 = new Test2 (true, 1.2, 1.3);
  }

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

/** Test is our base class. **/
class Test
{
  int i;
  double d;
  boolean flag;

  Test ()  {
    d = 1.1;
    flag = true;
    System.out.println ("In Test  ()");
  }

  Test (int j) {
    this ();
    i = j;
    System.out.println ("In Test  (int j)");
  }
}

/** Test 1 is a subclass of Test. **/
class Test1 extends Test
{
  int k;

  Test1 (boolean b) {
    super (3);
    flag = b;
    System.out.println ("In Test1 (boolean b)");
  }

  Test1 (boolean b, int j) {
    this (b);
    k = j;
    System.out.println ("In Test1 (boolean b, int j)");
  }
}

/** And Test2 is a subclass of Test1. **/
class Test2 extends Test1
{
  double x,y;

  Test2 (double x, double y) {
    super (false);
    this.x = x;
    this.y = y;
    System.out.println ("In Test2 (double x, double y)");
  }

  Test2 (boolean b, double x, double y) {
    super (b,5);
    flag = b;
    System.out.println ("In Test2 (boolean b, double x, double y)");
  }
}

Latest Update: Oct. 21, 2004