Question

import java.util.*;

  class A extends HashSet<Integer> {
       public boolean add(Object obj){  //compiler error
        return true;
       }
    }
    or
   class Abc <T> {
    public void add(T t){}  //compiler error
    public void add(Object i){} //compiler error (can't overload?)
    }

Error:Name clash: The method add(Object) of type test2 has the same erasure as add(E) of type HashSet but does not override it

i do not know what is the concept behind above error can any one suggest where i can study this concept?

Answer 1

The concept at work here is called type erasure. HashSet defines a method add(T), and you define a method add(Object). At a glance one might think this is OK; that your method just overloads add. However, the erasure of T is Object and so the two have the same erased signature.

Now, that would be fine if your method properly overrode the method from HashSet. But to do so you should be using add(Integer) and not add(Object). You’re not properly overriding the parent method, so instead it is reported as a conflict since a class cannot provide two methods with the same signature.

Your Abc example follows the same reasoning. The two methods you declared have the same erased signature so they clash.

Further Reading

Angelika Langer Generics FAQ

• When does a method override its supertype’s method?
• Can a method of a non-generic subtype override a method of a generic supertype?