How Do These Concepts Translate into Code?

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Trail: Learning the Java Language
Lesson: Object-Oriented Programming Concepts

How Do These Concepts Translate into Code?

Now that you have a conceptual understanding of object-oriented programming let's look at how these concepts get translated into code.
Here is an applet named ClickMe. A red spot appears when you click the mouse within the applet's bounds.

Note: If you don't see the applet running above, you need to install Java Plug-in, which happens automatically when you install the J2SE JRE or SDK. We strongly recommend that you install the latest version; at least 1.3.1 is required for all our applets. You can find more information in the Java Plug-in home page.

The ClickMe applet is a relatively simple program and the code for it is short. However, if you don't have much experience with programming, you might find the code daunting. We don't expect you to understand everything in this program right away, and this section won't explain every detail. The intent is to expose you to some source code and to associate it with the concepts and terminology you just learned. You will learn the details in later trails.

The Source Code and the Applet Tag for ClickMe

To compile this applet you need two source files: ClickMe.java and Spot.java. To run the applet you need to create an html file with this applet tag in it:
<applet code="ClickMe.class"
        width="300" height="150">
</applet>
Then load the page into your browser or the appletviewer tool. Make sure all the necessary files are in the same directory.

Note: If you're having problems running this example:
Make sure you download both Spot.java and ClickMe.java and place them in the same directory.
If you can compile one file but not the other, you probably have a CLASSPATH problem. If "." (indicating your current directory) isn't in your class path, the ClickMe class can't find the Spot class, even though they are in the same directory. Try unsetting CLASSPATH and recompiling, like this:
set CLASSPATH=
javac ClickMe.java
If your problem is fixed, then you should modify your global CLASSPATH variable to add "." to it. See Update the PATH variable (for Microsoft Windows) (or the appropriate documentation for your platform).

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, nonvisible objects play a part in this applet. Three objects represent the three colors 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:
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 initialize new objects created from the class. You can recognize a constructor because it has the same name as the class. The constructor initializes 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 initialized. Here's the relevant code from the applet class:
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 initializes 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 two lines of code from the ClickMe applet:
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 color 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:
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 initialized, 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 initialization code must be in a method called init, and so on.
public void init() {
    ... // ClickMe's initialization 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. The code that participates in mouse event handling is red:
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.

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 initialize 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 behavior
What it means to implement an interface

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