hashcode and equals()

[Shiva Mohan]

    
class Objective2425 extends Object{
    private int iKey;
    private String sValue;
        
    public Objective2425(int iKey, String sValue){
        this.iKey=iKey;   
this.sValue=sValue;
    }
    
    public int hashCode(){
        System.out.println(super.hashCode());
return super.hashCode();
    }
    
    public boolean equals(Object o){
        
if(o instanceof Objective2425){//true
            
    if(this.hashCode() == o.hashCode()){//FALSE WHY IT COMES AS FALSE ALWAYS AND FINALLY THE EQULAS() IS FALSE.
return true;
    }
}
return false;
    } 
    public static void main(String[] args) {
    
      
      Objective2425 mn=new Objective2425(5, "priyua"); 
      Objective2425 m=new Objective2425(5, "priyua"); 
     System.out.println("equals"+(m.equals(mn)));//-----------------------------line1 
      System.out.println("m.hashcode"+m.hashCode());//distinct value
      System.out.println("mn.hashcode"+mn.hashCode());//distinct value
      
     
    }
}

My Understading
----------------
this.hashCode() == o.hashCode()----->Both called Objective2425.hashcode().inside the overidden method,super.hashcode calls the Object.hashcode(retrurning one distinct value for this.hashcode and returning one distinct valus for o.hashcode.so two distinct number comparision of ==false,finally the execution of equals() returns false.

Questions
----------
1.//m.equals(mn) is true if and only if mn.equals(m) is true.(where is the use of rule no 2 of equals() contract).FALSE IS RETUERNING AFTER EXCEUTION OF OBJECTive2425.EQULAS().but objective2425.equlas() means look for same object and same values.but both are here in m and mn.
2.when m.equals(mn at line 3 is false,the hashcode contract is no requirements.right?but when i execute statemnts (m.hashcode() and mn.hashcode()),both ARE ALWAYS RETURNING FALSE.
3.does this program of hashcode() and equals() are correctly implemented.If so,how do i know that when executing coding.

Please help me on the above three questions of mine.

[Shiva Mohan]

can anyone please answer this.i would be thankful.

[Aniket Patil]

FALSE WHY IT COMES AS FALSE ALWAYS AND FINALLY THE EQULAS() IS FALSE.

You need to override hashCode() yourself, since the hashCode() of Object returns a different value even for "meaningfully same" objects.

Test it through this:

  
class TestHashCode
{
    int i;
 
    TestHashCode(int i){ this.i = i;}

    public int hashCode()
    {
       return super.hashCode();
    }
 
    public static void main(String[] args)
    {
TestHashCode o1 = new TestHashCode(5);
TestHashCode o2 = new TestHashCode(5);
System.out.println("o1's hashcode is: "+o1.hashCode());
System.out.println("o2's hashcode is: "+o2.hashCode());
    }
}  

.//m.equals(mn) is true if and only if mn.equals(m) is true.(where is the use of rule no 2 of equals() contract).FALSE IS RETUERNING AFTER EXCEUTION OF OBJECTive2425.EQULAS().but objective2425.equlas() means look for same object and same values.but both are here in m and mn.

Please note that the equals contract is for YOU to implement when you create your own classes. How do you expect the Java compiler to be intelligent enough to figure out what "equal" means for your classes?
Test this:

 
class TestHashCode
{
    String i;
 
    TestHashCode(String i){ this.i = i;}

    public int hashCode()
    {
       return i.hashCode();
    }
 
    public static void main(String[] args)
    {
TestHashCode o1 = new TestHashCode("Aniket");
TestHashCode o2 = new TestHashCode("Aniket");
System.out.println("o1's hashcode is: "+o1.hashCode());
System.out.println("o2's hashcode is: "+o2.hashCode());
    }
}  

For API classes, the equals() method is already overridden to folllow the "equals" contract (This doesent apply to StringBuffer equals()).

when m.equals(mn at line 3 is false,the hashcode contract is no requirements.right?but when i execute statemnts (m.hashcode() and mn.hashcode()),both ARE ALWAYS RETURNING FALSE.

If 2 objects are meaningfully equal, their hashcode MUST be same. Of course, you may choose not to override hashCode() and have the hashcode different for 2 equal objects, but then you are violating the hashCode() contract.
2 different objects MAY return the same hashcode, or it could be different.

does this program of hashcode() and equals() are correctly implemented

Obviously not!

And finally, Patience is a Virtue
[ January 18, 2007: Message edited by: Aniket Patil ]

[Shiva Mohan]

Thank you so much for taking the time and sharing your knowledge with me Aniket.I really appreciate it.One more doubt......

    
public int hashCode(){
System.out.println(super.hashCode());
return super.hashCode();
}
 
public boolean equals(Object o){
 
if(o instanceof Objective2425){//true
 
if(this.hashCode() == o.hashCode()){//FALSE WHY IT COMES AS FALSE ALWAYS AND FINALLY THE EQULAS() IS FALSE.
return true;
}
}
return false;
} 

objects given are
------------------
Objective2425 mn=new Objective2425(5, "priyua");
Objective2425 m=new Objective2425(5, "priyua");

In that program,.hashcode is implemented correct and equals() is implemented incorrect since the equlas() is retruning false,there is no hashcode requirement.Am i right?

[Shiva Mohan]

Hello any answer please

[Henry Wong]

In that program,.hashcode is implemented correct and equals() is implemented incorrect since the equlas() is retruning false,there is no hashcode requirement.Am i right?

Whether the equals() and hashcode() method is implemented correctly or not is ... your call. You are the designer of the class and it is your decision on whether two instances of your class, are equal or not. And if they are equal, then the hashcode() method must return the same hashcode.

Your hashcode() method simply calls the Object class' hashcode() method, which returns a different hashcode for each instance. Is this what you want?

Henry