.equals method

[Gabriel White]

Hi all, I am having some trouble with the .equals method. I have a solid interface with 3 shape classes (Sphere, Cube, and a Cylinder). I have been asked to use the .equals method to determine if they are equal or not equal by creating new objects and then testing them. The problem that I am having is the actual implmentation of the .equals method of the Object class. Here is my main:

public class TestShape
{
public static void main(String args[])
{
Solid shapes[] = new Solid[3];
shapes[0] = new Cube(30.0);
shapes[1] = new Cylinder(30.0, 10.0);
shapes[2] = new Sphere(10.0);
System.out.println("These are the three shapes and their dimensions" +
                       "(Area and Volume)");
String out = "";
for (int i = 0; i < 3; i++)
{
out += "\n\n" + shapes[i].getClass() + ":" + "\n"+
       shapes[i].toString();
}
System.out.println(out);
 
for (int i = 0; i < 3; i++)
{
if (i == 0)
System.out.println("Length of Cube: " + ((Cube)shapes[i]).getLength());
if (i == 1)
System.out.println("Height and Radius of Cylinder: "
+ ((Cylinder)shapes[i]).getHeight()
+ " " + ((Cylinder)shapes[i]).getRadius());
if (i == 2)
System.out.println("Radius of Sphere: " + ((Sphere)shapes[i]).getRadius());
}
 
Object obj1 = new Cube(30.0);
Object obj2 = new Cube(50.0);
Object obj3 = new Sphere(30.0);
Object obj4 = new String("Hello");
 

if (((Cube)shapes[0]).equals(obj1) == false)
{
System.out.println("Cube[length=" + ((Cube)shapes[0]).getLength() + "] equals [" + ((Cube)shapes[0]).getLength() + "]");
}
else
{
System.out.println("test");
}



}
}    

I am supposed to print out the statement if they are equal when they are true, but it only works if they are false. I know for a fact that the Cube object has a lenght of 30 and the new object has a lenght of 30. I am supposed to compare it to a BIG O object so that is why I declared the objects like that.

Thanks in advance.

[Keith Lynn]

One point of confusion I think you have is with where the equals method is implemented. It should be placed inside your class definitions.

[Gabriel White]

Thanks Keith, but why? It will work if I call the correct object won't it?

[Jeff Albertson]

Hmmm... you've showed us some testing code, but the most important code of all is the code for equals. Do you realize that Object's implementation of equals is:

public boolean equals(Object x) {
    return this == x;
}

Should your Shape classes inherit this version of equals or should they define their own? What did you do?

[Gabriel White]

Got it. I will definately define my own, as the Object implementation of this object is inadequate to say the least.

Thanks for the heads up.

[Layne Lund]

Originally posted by Keith Lynn:
One point of confusion I think you have is with where the equals method is implemented. It should be placed inside your class definitions.

Originally posted by Gabriel White:
Thanks Keith, but why? It will work if I call the correct object won't it?

Short answer: because this is the way it works in Java.

Slightly longer answer: Ever class inherits the equals() method from Object, as already mentioned. Often the default implementation is not what you need, so you have to override it.

In order to override the equals() method, you first need to clearly define what it means for two objects to be "equal". So what does it mean for two Sphere objects to be equal? You might want to see what data you are using to represent the sphere. This should help in figuring this part out.

Please post your ideas. For now a (mostly) English description is desirable. From there we will be glad to help you with the syntax to implement it.

Layne