equals() fails here

[Nisheeth Kaushal]

In the following program why the output is of else statement ie.
"Objects have different values"
1: class MyClass
2: {
3: static int maxElements;
4:
5: MyClass(int maxElements)
6: {
7: this.maxElements = maxElements;
8: }
9:
10: }
11:
12: public class Q19
13: {
14: public static void main(String[] args)
15: {
16:
17: MyClass a = new MyClass(100);
18: MyClass b = new MyClass(100);
19:
20: if(a.equals(b))
21: System.out.println("Objects have the same values");
22: else
23: System.out.println("Objects have different values");
24: }
25: }
A) Compilation error at line 20. equals() method was not defined.
B) Compiles fine, runtime exception at line 20.
C) Prints "Objects have the same values".
D) Prints "Objects have different values".
Correct awr is D
HIHI

[leena rane]

Since Myclass class does not hav any equals method,
the equals method of Object is used.This only check whether the two objects have the same reference.This is only possible if it's the same object.Since you hav used new to create 2 objects naturally 2 objects are created.So it returns false.
HIH

[Nisheeth Kaushal]

Thanx Leena
Please solve my one more prob -
1: public void check()
2: {
3: System.out.println(Math.min(-0.0,+0.0));
4: System.out.println(Math.max(-0.0,+0.0));
5: System.out.println(Math.min(-0.0,+0.0) == Math.max(0.0,+0.0));
6: }

The order of floating/double values is -Infinity --> Negative Numbers/Fractions --> -0.0 --> +0.0 --> Positive Numbers/Fractions --> Infinity
then why the fifth line returns true.
this means -0.0 and 0.0 are equal.
Please reply urgently.
Nisheeth.
HIHI

[Anonymous]

Nisheeth,
I had written a small program to understand the "equals and == " bit...see if it helps...

<font size = 2>
class pizza{
   String msg;
   pizza(String s){
      msg = s;
   }
}
public class pizzatest{
   public static void main(String[] args){
       boolean ptest;
       System.out.println();
       String s1 = new String("HI");
       String s2 = new String("HI"); 
       
       pizza pizza1 = new pizza("VeggiesDelight");
       pizza pizza2 = new pizza("VeggiesDelight");
       pizza pizza3 = new pizza("Tomatopizza");       
       ptest = pizza1 == pizza2;
       System.out.println("pizza1.msg : " + pizza1.msg);
       System.out.println();

       System.out.println("pizza2.msg : " + pizza2.msg);
       System.out.println();

       System.out.println("pizza3.msg : " + pizza3.msg);
       System.out.println();

       System.out.println("s1 : " + s1);
       System.out.println();

       System.out.println("s2 : " + s2);
       System.out.println();

       System.out.println("s1.equals(s2)" + ": " + s1.equals(s2));
       System.out.println();

       System.out.print("s1 == s2: "); 
       System.out.println(s1 == s2);
       System.out.println();

       System.out.println("pizza1.equals(pizza2)" + ": " + pizza1.equals(pizza2));
       System.out.println();
       System.out.println("pizza1 == pizza2" + ": " + ptest);
 //System.out.println(s1 == pizza1); //[B] This will give a compiler error.[/B]
   }
}
</font>

The output is:

<font size=2>
C:\Temp>java pizzatest
pizza1.msg : VeggiesDelight
pizza2.msg : VeggiesDelight
pizza3.msg : Tomatopizza
s1 : HI
s2 : HI
s1.equals(s2): true
s1 == s2: false
pizza1.equals(pizza2): false
pizza1 == pizza2: false
</font>

The lowdown on equals and == is that if it is used by any class that does not override it...it impartially returns false. Also, it is a compile time error if one of the operands cannot be cast into the other's type.
Some of the classes that overrides the equals() method are BitSet, Date, File and the Wrapper Classes.
Hope this helps
Shyam

[This message has been edited by Shyamsundar Gururaj (edited October 29, 2001).]

[Nisheeth Kaushal]

Thanx Gururaj,
Your program is sufficient enough to understand equals() and ==.
And will u please explain me that then how can we test for the equality of two objects and their object references.
Please reply fast.
Nisheeth.
Thanx in advance.
HIHI.

[Anonymous]

Nisheeth,
If your class needs to use equals(), it must override the method defined in the object class. For more information, Visit

• http://www.cs.rice.edu/~cork/teachjava/2001/revised/rev/node44.html
  
• http://www.jfind.com/articles/glass01102000.shtml
  

Hope this helps
Shyam
[This message has been edited by Shyamsundar Gururaj (edited October 28, 2001).]

[Nisheeth Kaushal]

Thanx Gururaj,
The point is clear can u also suggest me some good site for Threads and awt like the above one.
Thanx in advance.
NISHEETH