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Easy Java Lecture 4: OOP programming concepts: example. Teach/learn online

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Object-oriented programming concepts: example


by: Anatoliy Malyarenko

Abstract

In this lecture, you will learn, how the concepts of object-oriented programming can be implemented into Java code.

Preparation

Start RealJ. Create a new applet project named ClickMe. Close the project. Point you browser to
http://java.sun.com/docs/books/tutorial/java/concepts/practical.html

Download the files ClickMe.java and Spot.java to the folder where your project lives.
Open project and add file Spot.java to the project. Build and run applet. A red spot appears when you click the mouse within the applet's bounds.

Objects in the ClickMe applet

Many objects play a part in this applet. The two most obvious ones are the ones you can see: the applet itself and the spot, which is red on-screen.

The browser creates the applet object when it encounters the applet tag in the HTML code containing the applet. The applet tag provides the name of the class from which to create the applet object. In this case, the class name is ClickMe.

The ClickMe applet in turn creates an object to represent the spot on the screen. Every time you click the mouse in the applet, the applet moves the spot by changing the object's x and y location and repainting itself. The spot does not draw itself; the applet draws the spot, based on information contained within the spot object.

Besides these two obvious objects, other, non visibleobjects play a part in this applet. Three objects represent the three colours used in the applet (black, white, and red); an event object represents the user action of clicking the mouse, and so on.

Classes in the ClickMe applet

Because the object that represents the spot on the screen is very simple, let's look at its class, named Spot. It declares three instance variables: size contains the spot's radius, x contains the spot's current horizontal location, and y contains the spot's current vertical
location:

Code:

public class Spot {
   //instance variables
   public int size;
   public int x, y;
   //constructor
   public Spot(int intSize) {
      size = intSize;
      x = -1;
   y = -1;
   }
}

Additionally, the class has a constructor -- a subroutine used to initialise new objects created from the class. You can recognise a constructor because it has the same name as the class. The constructor initialises all three of the object's variables. The initial value of size is provided as an argument to the constructor by the caller. The x and y variables are set to -1 indicating that the spot is not on-screen when the applet starts up.

The applet creates a new spot object when the applet is initialised. Here's the relevant code from the applet class:

Code:
   
private Spot spot = null;
private static final int RADIUS = 7;
...
spot = new Spot(RADIUS);

The first line shown declares a variable named spot whose data type is Spot, the class from which the object is created, and initialises the variable to null. The second line declares an integer variable named RADIUS whose value is 7. Finally, the last line shown creates the object; new allocates memory space for the object. Spot(RADIUS) calls the constructor you saw previously and passes in the value of RADIUS. Thus the spot object's size is set to 7.

The figure on the left is a representation of the Spot class. The figure on the right is a spot object.

Messages in the ClickMe applet

As you know, object A can use a message to request that object B do something, and a message has three components:

  • The object to which the message is addressed.
  • The name of the method to perform.
  • Any parameters the method needs.

Here are a few lines of code from the ClickMe applet:

Code:

g.setColor(Color.white);
g.fillRect(0,
            0,
            getSize().width - 1,
            getSize().height - 1);

Both are messages from the applet to an object named g -- a Graphics object that knows how to draw simple on-screen shapes and text. This object is provided to the applet when the browser instructs the applet to draw itself. The first line sets the colour to white; the second fills a rectangle the size of the applet, thus painting the extent of the applet's area white.

The following figure highlights each message component in the first message:

Inheritance in the ClickMe applet

To run in a browser, an object must be an applet. This means that the object must be an instance of a class that derives from the Applet class provided by the Java platform.

The ClickMe applet object is an instance of the ClickMe class, which is declared like this:

Code:

public class ClickMe extends Applet
               implements MouseListener {               
   ...
}

The extends Applet clause makes ClickMe a subclass of Applet. ClickMe inherits a lot of capability from its superclass, including the ability to be initialised, started, and stopped by the browser, to draw within an area on a browser page, and to register to receive mouse
events. Along with these benefits, the ClickMe class has certain obligations: its painting code must be in a method called paint, its initialisation code must be in a method called init, and so on.

Code:

public void init() {
   ... // ClickMe’s initialisation code here
}
public void paint(Graphics g) {
   ... // ClickMe’s painting code here
}

Interfaces in the ClickMe applet

The ClickMe applet responds to mouse clicks by displaying a red spot at the click location. If an object wants to be notified of mouse clicks, the Java platform event system requires that the object implement the MouseListener interface. The object must also register
as a mouse listener.

The MouseListener interface declares five different methods each of which is called for a different kind of mouse event: when the mouse is clicked, when the mouse moves outside of the applet, and so on. Even though the applet is interested only in mouse clicks it must implement all five methods. The methods for the events that it isn't interested in are empty.

The complete code for the ClickMe applet is shown below.

Code:

import java.applet.Applet;
import java.awt.*;

import java.awt.event.*;

public class ClickMe extends Applet
      implements MouseListener {
   private Spot spot = null;
   private static final int RADIUS = 7;
   public void init() {
      addMouseListener(this);
   }

   public void paint(Graphics g) {
      // draw a black border
      // and a white background
      g.setColor(Color.white);
      g.fillRect(0,
                  0,
                  getSize().width - 1,
                  getSize().height - 1);
      g.setColor(Color.black);
      g.drawRect(0,
                  0,
                  getSize().width - 1,
                  getSize().height - 1);
      // draw the spot
      g.setColor(Color.red);
      if (spot != null) {
         g.fillOval(spot.x - RADIUS,
         spot.y - RADIUS,
         RADIUS * 2, RADIUS * 2);
      }      
   }
   public void mousePressed(MouseEvent event) {
      if (spot == null) {
         spot = new Spot(RADIUS);
      }
      spot.x = event.getX();
      spot.y = event.getY();
      repaint();
   }
   public void mouseClicked(MouseEvent event) {}
   public void mouseReleased(MouseEvent event){}
   public void mouseEntered(MouseEvent event) {}
   public void mouseExited(MouseEvent event) {}

API documentation

The ClickMe applet inherits a lot of capability from its superclass. To learn more about how ClickMe works, you need to learn about its superclass, Applet. How do you find that information? You can find detailed descriptions of every class in the API documentation, which
constitute the specification for the classes that make up the Java platform.

The API documentation for the Java 2 Platform is online at java.sun.com. It's helpful to have the API documentation for all releases you use bookmarked in your browser.

API documents for Java 2 Platform, Standard Edition, v1.4

To learn more about all the classes and interfaces from the Java platform used by the ClickMe applet, you can look at the API documentation for these classes:

Summary

This discussion glossed over many details and left some things unexplained, but you should have some understanding now of what object-oriented concepts look like in code. You should now have a general understanding of the following:

  • That a class is a prototype for objects.
  • That objects are created from classes.
  • That an object's class is its type.
  • How to create an object from a class?
  • What constructors are?
  • How to initialise objects?
  • What the code for a class looks like?
  • What class variables and methods are?
  • What instance variables and methods are?
  • How to find out what a class's superclass is?
  • That an interface is a protocol of behaviour.
  • What it means to implement an interface?

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