Designing Java Classes (Part 1)

We wish to generate a class that embodies the characteristics of a two-dimensional point.

// Point.java
// Definition of class Point (version 1)
import java.awt.Graphics; 

/**
*  Point describes a two-dimensional point.
*/

public class Point
{
	static final double SCALE = 100;
	private double x, y; // coordinates of the Point

	// No-argument constructor
	public Point() { setPoint( 0, 0 ); }


	// Constructor
	public Point( double a, double b ) { setPoint( a, b ); }


	// Set x and y coordinates of Point
	public void setPoint( double a, double b )
	{
		x = a;
		y = b;
	}

	// get x coordinate
	public double getX() { return x; }  

	// get y coordinate
	public double getY() { return y; }

	public void draw(Graphics g)
	{
		int nx = (int) (x*SCALE);
		int ny = (int) (y*SCALE);
		g.fillOval(nx-5,ny-5,11,11);
	}   

   // convert the point into a String representation
   public String toString()
      { return " (" + x + ", " + y + ")"; }
}

Class Point contains

• instance variables (x, y).
• a static variable SCALE
• two (overloaded) constructors
• member access methods
• a rendering method, draw
• a replacement method for Object.toString()

Test Applet

// Test1.java
import java.awt.*;
import javax.swing.*;

public class Test extends JApplet {
	Point pt[];

	public void init()
	{
		pt = new Point[3];
		pt[0] = new Point(0.1,0.1);
		pt[1] = new Point(0.5, 0.8);
		pt[2] = new Point(0.8,0.1);
		System.out.println("Points:" + pt[0] + pt[1] + pt[2]);
	}
	public void paint(Graphics g)
	{
		for (int i=0; i<pt.length; i++) pt[i].draw(g);
	}
}

The test applet

• instantiates an array of points.
• prints their String descriptions on the console
• renders them on the screen.

Maintained by John Loomis, last updated 6 June 2000