Home : Course Map : Chapter 3 : Java :
Demo 1: Creating Objects
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Chapter 3

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Static Members
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Value&Reference
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The demos below illustrate how to create a class object. This can also by referred to as instantiating (creating an instance of) a class.

For the demos here and in the rest of this chapter, we will use the the applet starter StartApplet2.java and the application starter StartApp2.java. Each includes the class Test for inserting methods and data fields. In the main routine, which is where the processing begins, we create an instance of the Test class and invoke its methods, access its data, etc..

The first applet - DataFieldApplet - demonstrates a class with only a data field. In the init() method we create an instance of class Test and then use the reference "test" and the "." operator to access the "i" variable in the object:

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

public class DataFieldApplet extends java.applet.Applet
{
  public void init ()
  {
     //Create an instance of the Test class
     Test test = new Test();

     // Access the variable i in the Test object.
     int j = test.i;
     System.out.println ("test.i = "+j);
     System.out.println ();
  }

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

// Use Test to examine various aspects of class design
class Test
{
    int i = 5;
}

Application version:
DataFieldApp.java

 

The second applet - MethodApplet - demonstrates a class with both a data field and a method. In the init() method we create an instance of class Test and then use the reference "test" and the "." operator to access the "aMethod(int j)" method in the object:

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

public class MethodApplet extends java.applet.Applet
{
  public void init () {
     //Create an instance of the Test class
     Test test = new Test ();

     // Invoke a method
     test.aMethod (3);
  }

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

// Use Test to examine various aspects of class design
class Test
{
    int i;
    void aMethod (int j) {
      i = j;
      System.out.println ("In aMethod()");
      System.out.println ("i = "+i);
      System.out.println ();
    }
}

Application version:
MethodApp.java

 

Latest update: Oct. 19, 2004

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