	CS 535 Object-Oriented Programming & Design Fall Semester, 2000 An Example
	Previous Lecture Notes Index Next © 2000, All Rights Reserved, SDSU & Roger Whitney San Diego State University -- This page last updated 25-Sep-00

Contents of Doc 9, An Example

Reference
Java API documentation


Doc 9, An Example Slide # 2

An Example

Point

public class Point{
   private float x;
   private float y;
   
   public Point( float x, float y) {
      this.x = x;
      this.y = y;
   }
   public Point add( Point aPoint){
      return new Point( x + aPoint.x, y + aPoint.y);
   }
   public double distance( Point aPoint) {
      float deltaX = x - aPoint.x;
      float deltaY = y - aPoint.y;
      return Math.sqrt( deltaX* deltaX + deltaY* deltaY);
   }
      
   public boolean greaterThan(Point  aPoint) {
      return (x > aPoint.x ) && (y  > aPoint.y);
   }
   
   public String toString() {
      return "(" + x + ", " + y + ")";
   }
   
   public boolean equals( Point p) {
      return x == p.x && y == p.y;
   }
}
Doc 9, An Example Slide # 3

Using Point

public class Sample {
   public static void main(String args[])  {
      Point a = new Point( 1, 0);
      Point b = new Point( 3, -1);
      Point c = a.add( b);
      System.out.println( a + " + " + b + " = " + c);
      System.out.println( "The distance between " + a + " and " +
          b + " is " + a.distance( b ));
   }
}

Output (1.0, 0.0) + (3.0, -1.0) = (4.0, -1.0)
The distance between (1.0, 0.0) and (3.0, -1.0) is 2.23606797749979


Doc 9, An Example Slide # 4

toString() Standard
When an object is added to a string

Object is converted to a string by calling its toString() method

The two strings are then concatenated

println converts objects to strings via toString()

      Point a = new Point( 1, 0);
      System.out.println( a );


Doc 9, An Example Slide # 5

PointOperations

public class PointOperations {
   public String toString(float x, float y) {
      return "(" + x + ", " + y + ")";
   }
      
   public boolean greaterThan(float x, float y, float px, float py) {
      return (x > px ) && (y  > py);
   }
   public boolean equals(float x ,float y, float px , float py) {
      return x == px && y == py;
   }
   
   public double distance(float x, float y, float px ,float py) {
      float deltaX = x - px;
      float deltaY = y - py;
      return Math.sqrt( deltaX* deltaX + deltaY* deltaY);
   }
   public float[] add( float x, float y, float px, float py) {
      float[] answer = new float[2];
      answer[0] = x + px;
      answer[1] = y + px;
      return answer;
   }
}
Doc 9, An Example Slide # 6

Using PointOperations

public class TrivialApplication {
   public static void main(String args[])  {
      float x1 = 1;
      float y1 = 0;
      float x2 = 3;
      float y2 = -1;
      PointOperations a = new PointOperations();
      float[] answer = a.add( x1, y1, x2, y2);      
      System.out.println( a.toString( x1 , y1) + " + " + 
         a.toString( x1, y2) + " = " + a.toString( answer[0] , answer[1]));
      System.out.println( "The distance between " + a.toString( x1 , y1) +
          " and " + a.toString( x1, y2) + " is " + 
          a.distance( x1, y1, x2, y2 ));
      }
}

Output (1.0, 0.0) + (1.0, -1.0) = (4.0, 3.0)
The distance between (1.0, 0.0) and (1.0, -1.0) is 2.23606797749979


Doc 9, An Example Slide # 7

Some Problems with PointOperations
Harder to use
Error prone
Less flexible


Doc 9, An Example Slide # 8

A Drawing Program

Build a drawing program that knows about Points & Rectangles

import java.awt.Graphics;
public interface Drawable {
   public void drawOn(Graphics display);
}
import java.awt.Graphics;
 
public class Point implements Drawable {
   public void drawOn( Graphics display) {
      display.fillOval(  Math.round(x),  Math.round(y),  3,  3  );
   }
   //rest of class as before
}


Doc 9, An Example Slide # 9

The Program

import  java.awt.*;
import java.util.*;
class  Drawing  extends  Frame {
   Vector pictureElements = new Vector();
   public void show( int  WidthInPixels, int heightInPixels ) {
      setTitle( "Drawing" );
      setSize( WidthInPixels, heightInPixels );
      setLocation( 40, 40);
      setBackground( Color.green);
      show();
   }
   
   public void add(Drawable element ){
      pictureElements.addElement( element );
   }
      
   public  void  paint(  Graphics  display  )  {
      Enumeration elements = pictureElements.elements();
      while (elements.hasMoreElements() ){
         Drawable x = (Drawable) elements.nextElement();
         x.drawOn( display);
      }
   }
   public  static  void  main( String  args[] ){
      Drawing example = new  Drawing(  );
      example.add( new Point( 10, 10));
      example.add( new Point( 20 , 20));
      example.add( new Rectangle( new Point( 30, 30), 20, 55));
      example.show( 150, 100);
   }
}
Doc 9, An Example Slide # 10

Program Output

In graphics the upper left corner is (0, 0) with positive y going down the screen

How could we write the program with PointOperations instead of Point?


Doc 9, An Example Slide # 11

Circle

import java.awt.Graphics;
public class Circle implements Drawable{
   Point center;
   float radius;
   
   public Circle( Point center, float radius){
      this.center = center;
      this.radius = radius;
   }
   
   public boolean includes(Point p ){
      return center.distance( p ) < radius;
   }
   
   public boolean equals( Circle aCircle ) {
      return center.equals(aCircle.center) && radius == aCircle.radius;
   }
   public void drawOn(Graphics display){
      display.drawOval( Math.round(center.x()), Math.round(center.y()), 
         Math.round(radius), Math.round(radius));
      }
   public String toString(){
      return "Circle( center" + center + " raduis " + radius + " )";
      }
   }
Note requires accessor methods in point class.
Doc 9, An Example Slide # 12

Testing using text based TestRunner

import junit.framework.TestCase;
import junit.textui.TestRunner;
public class TestCircle extends TestCase {
   public static void main(String[] arg) {
      TestRunner.run( TestCircle.class);
   }
   public TestCircle(String name) {
      super( name);
   }
   
   public void testIncludes() {
      // {x , y}
      float[][] testData = { { 5, 5}, {6 , 6} , {7, 5} , {8, 8}};
      boolean[] answers = { true, true, false, false};
      Circle tester = new Circle( new Point( 5, 5), 2);
      for (int k = 0; k < testData.length; k++)
         {
         float x = testData[k][0];
         float y = testData[k][1];
         Point test = new Point( x, y);
         assert( "Data point " + k , answers[k] == tester.includes( test));
      }
   }
Doc 9, An Example Slide # 13

Testing Continued

   public void testEquals()
      {
      // {centerX, centerY, radius}
      float[][] testData = { { 5.5f, 4.3f, 2}, 
         { 5.5f, 4.3f, 2.001f} , 
         { 5.5f, 5, 2} , 
         { 2, 1, 8}};
      boolean[] answers = { true, false, false, false};
      Circle tester = new Circle( new Point( 5.5f, 4.3f), 2);
      assert( "self " , tester.equals( tester));
      for (int k = 0; k < testData.length; k++)
         {
         Point center = new Point( testData[k][0], testData[k][1]);
         float radius = testData[k][2];
         Circle test = new Circle( center, radius);
         assert( "Data point " + k , answers[k] == tester.equals( test));
         }
      }

Running the Tests
java TestCircle

Output ..
Time: 0.485

OK (2 tests)


Doc 9, An Example Slide # 14

Some Java Info

Names
Use full words for names

origin
radius

Classes start with capital letter

Circle, Point

Methods, fields, & variables start with lowercase letter


Doc 9, An Example Slide # 15

Compiler Option
java -Djava.compiler=NONE className

Program runs slower, but provides better error messages

Use
setenv JAVA_COMPILER NONE
to set this option in your environment

Example

public class CompileExample {
   public static void main(String args[]) 
      {
      int a = 1;
      int b = 0;
      float c = a/b;
      }
   }

rohan 14-> javac CompileExample.java 
rohan 15-> java CompileExample
Exception in thread "main" java.lang.ArithmeticException: / by zero
        at CompileExample.main(Compiled Code)

rohan 16-> java -Djava.compiler=NONE  CompileExample
Exception in thread "main" java.lang.ArithmeticException: / by zero
        at CompileExample.main(CompileExample.java:7)
Doc 9, An Example Slide # 16

printStackTrace
Prints the stack trace of an exception

public class StackTraceExample {
   public static void main(String args[]) 
      {
      try
         {
         Drawing.main( args);
         TestCircle.main(args);
         int a = 1;
         int b = 0;
         float c = a/b;
         }
      catch (Exception mathError)
         {
         mathError.printStackTrace( System.err);
         }
      }
   }

Previous visitors since 25-Sep-00 Next