equals() & hashCode()

[Santosh Pasupuleti]

What happens if I override the equals() method but do not override the hashCode() if I am using the following class as a key in a hashmap? What will be the impact on performance while I perform a searching assuing that the HashMap has a milion entrie?

class MyKey{
int i;
boolean equals(Object obj){
if (obj == null)
return false;
if (obj == this)
return true;
return (((MyKey)obj).i == this.i);
}

[Ernest Friedman-Hill]

The impact on performance will be that it won't work. If hashCode() and equals() don't agree, it will be impossible to find many keys in the map.

[Jim Yingst]

For this code, an appropriate hashCode() is very simple anyway, since the equals depends only on i:

public boolean equals(Object obj) { 
    return (obj instanceof MyKey) && ((MyKey)obj).i == this.i);
}
 
public int hashCode() {
    return i; 
}

For discussion of hashCode() for more complex classes (and many other useful issues), I recommend Effective Java.

[Chetan Raju]

When you override equals method, you are deciding as to in what cases you say two objects of your class as equal. If you decide that two objects are equal then hashcode must be equal. we cannot have the same guy at different places simultaneously right.