equals and to string method

[Larry Lai]

Hi everyone,
How's everything? Busy in learning java?
A question is that I can use the equal and to string methods?
For example:

----------------------------------
|                                |   
|  public class Point            |
|                                |
|  private doulbe x,y            |    
|                                |
| public Point (double x,y)      |
|                                |
| public double x()              |
| public doulbe y()              |
|                                |
| public boolean equal(PointP)   |
|                                |
| public String toString()       |
|                                |
----------------------------------

1. How to write the 'equals() method?
2. How use the toString method that if the Point object had the coordinates(3,4) then the string "(3.0, 4.0)" would be returned.
3. if I need to use the equal() method, what would be the output of the following Java code fragement?

Point p = new Point (2, 3);
 
println("p.x() = " + p.x() + ", p.y()= " + p.y());
 
println ("p = " + p);
 
Point q = new Point (7, 4);
 
println ("q = " + q);
 
if (q.equals(p))  
    println ("q equals p");
else 
    println ("q does not equal p");
 
q = new Point (2, 3);
 
println ("q = " + q);
 
if (q.equals(p))  
    println ("q equals p");
else 
    println ("q does not equal p");
 
if (q == p)
    println ("q == p");
else
    println ("q != p");

I think it's a little bit hard ...anyone knows that how to solve that ?
Thanks
Larry
[ May 29, 2002: Message edited by: Dirk Schreckmann ]

[doug shumway]

easy answer - Point p will never equal Point q,
either equals() or ==
Point a = new Point();
Point b = a;
if (a.equals(b)) is true because they both reference the same object, the default requirement for the equals method.
I posted a answer for possible solution to the equals problem in the intermediate forum

[Pradyumna Hari]

Hi Larry,
I could not get your first question. Here is the reply for second and third questions.
(2) Here is the code that overrides the toString() method to get the string you want :

import java.awt.*;
class MyPoint extends Point{
    MyPoint(){
        super();
    }
    MyPoint(Point p){
        super(p);
    }
    MyPoint(int x, int y){
        super(x,y);
    }
    public String toString(){
        return("("+this.getX()+","+this.getY()+")");
    }
}
public class Ex3{
    public static void main(String args[]){
        MyPoint p = new MyPoint(2,3);
        Point q1 = new Point(4,6);
        MyPoint q = new MyPoint(q1);
        MyPoint r = new MyPoint();
        System.out.println("p = "+p);
        System.out.println("q = "+q);
        System.out.println("r = "+r);
    }
}

(3) To get the output of code fragment, you have to do some modifications. Here is the modified code :

import java.awt.*;
public class Ex1{
    public static void main(String args[]){
        Point p = new Point (2, 3);
        System.out.println("p.getX() = " + p.getX() + ", p.getY()= " + p.getY());
        System.out.println ("p = " + p);
        Point q = new Point (7, 4);
        System.out.println ("q = " + q);
        if (q.equals(p))
        System.out.println ("q equals p");
        else
        System.out.println ("q does not equal p");
        q = new Point (2, 3);
        System.out.println ("q = " + q);
        if (q.equals(p))
        System.out.println ("q equals p");
        else
        System.out.println ("q does not equal p");
        if (q == p)
        System.out.println ("q == p");
        else
        System.out.println ("q != p");

    }
}

The output is

p.getX() = 2.0, p.getY()= 3.0
p = java.awt.Point[x=2,y=3]
q = java.awt.Point[x=7,y=4]
q does not equal p
q = java.awt.Point[x=2,y=3]
q equals p
q != p

And if you see the javadocs for equals() method of point, it makes things very clear :

equals
public boolean equals(Object obj)
Determines whether two points are equal. Two instances of Point are equal if the values of their x and y member fields, representing their position in the coordinate space, are the same.
Overrides:
equals in class Point2D
Parameters:
obj - an object to be compared with this point.
Returns:
true if the object to be compared is an instance of Point and has the same values; false otherwise

[ May 29, 2002: Message edited by: Dirk Schreckmann ]

[Larry Lai]

Hi everyone,

Many many thanks your help....
I get it what you mean...tnaks a lot.
See you soon
Larry