Question

class A{

      @Override
      public int hashCode() {
        return 10;
    }
}

public class SampleClass {

    public static void main(String[] args){
        Map map = new HashMap();

        map.put(new A(), "A");
        map.put(new A(), "B");

        System.out.println(map.size());

        System.out.println(new A().hashCode());
        System.out.println(new A().hashCode());
    }
}

Output :-

2

10

10

Why 2???. If we are implementing hashCode method which is returning same integer. Should not the size be 1???

Answer 1

You haven’t overriden equals(Object), so they don’t compare as equal.

Just because two objects have the same hash code doesn’t mean HashMap assumes they’re the same — indeed, if that was the case, that would be really, really bad.

If you want two A objects to be treated as equal by HashMap, you must override equals(Object) in A to define one A as equal to another.