hashCode() and equals()

[Ravissant Markandey]

Hi Everyone,
I have this doubt regarding hashcodes from KATHY SEIRRA(pg.no. 534).It has a table stating the following conditions/requirements:

Condition Required
(1) x.equals(y)
== true x.hashCode() ==y.hashCode()



(2) x.hashCode() != x.equals(y) == false
y.hashCode()


Now, I have the following code:

class HashCodeClass {
 
String value;
 
HashCodeClass(String v) {
value = v; 
}
 
public int hashCode() {
return 1;
}
 
public boolean equals(Object o) {
if ((o instanceof HashCodeClass)) {
if (value.charAt(0) == ((HashCodeClass) o).value.charAt(0)) return true;
}
return false;
} 
 
public static void main(String[] sss){
HashCodeClass n1 = new HashCodeClass("aaa");
HashCodeClass n2 = new HashCodeClass("aaa"); 
String s ="-";
if (n1.equals(n2)) s += "1";
if (n1 == n2) s+= "2";
Set <HashCodeClass> set = new HashSet<HashCodeClass>();
set.add(n1);
set.add(n2);
System.out.println(s + " " + set.size());
}
} 

here, the equals test for n1 and n2 holds true even when i comment the overridden hashCode() method which contradicts (1) above and hence automatically contradicts (2) as well.
Please help.

Regards,
Ravissant Markenday

[Ankit Garg]

Well if you are saying that the contract is being broken, then yes it is. But what do you expect. Do you think that it will be a compiler error. If you think so than you are wrong.

The equals and hashCode contract is just a suggestion so that you can use your classes in Collections. If you don't follow the contract, you will not get any errors but will get unexpected outputs like you won't be able to find objects is hash based collections etc....

[Campbell Ritchie]

The "contract" about equals and hashCode is more than a "suggestion." What you are doing by implementing an equals() method which overrides the Object#equals(java.lang.Object) method is telling users (implicitly) that you are fulfilling the requirements stated there. And you don't want your classes going round telling fibs to their users, do you?

But you are correct that it is specifically used in Maps and Sets.

[Djonatah Stiegler]

Keep in mind that legal hashCode() override is different than an efficient override

There's an "exam watch" in that section that talks about legal and appropriate hashCode override.

Thanks
DJ.