ArrayList's custom Contains method

here's some code that might demonstrate how it works out:

import java.util.ArrayList;

class A {
  private Long id; 
  private String name; 

  A(Long id){
      this.id = id;
  }

    @Override
  public boolean equals(Object v) {
        boolean retVal = false;

        if (v instanceof A){
            A ptr = (A) v;
            retVal = ptr.id.longValue() == this.id;
        }

     return retVal;
  }

    @Override
    public int hashCode() {
        int hash = 7;
        hash = 17 * hash + (this.id != null ? this.id.hashCode() : 0);
        return hash;
    }
}

public class ArrayList_recap {
    public static void main(String[] args) {
        ArrayList<A> list = new ArrayList<A>(); 

        list.add(new A(0L));
        list.add(new A(1L));

        if (list.contains(new A(0L)))
        {
            System.out.println("Equal");
        }
        else
        {
            System.out.println("Nah.");
        }    
    }

}

First, there is an override of the equals(Object o) method. Then there is the override of the hashCode() as well. Also note that the instanceof A check in the equals will ensure that you're not trying to compare different objects.

That should do the trick! Hope it helped! Cheers :)

What I am doing wrong?

You are not overriding. You are overloading.

The contains method calls the equals method with signature equals(Object), so this (new) method that you've added won't be called.

The other problem is that your equals method has the wrong semantics for contains. The contains method needs to compare this against an object that could be a member of the list. Your list does not contain Long objects. It contains objects of type A.

Now you might get away with this ... if you use raw list types ... but what you are trying to do is a violation of the API contract, and bad practice. A better solution is to iterate and test the elements of the list explicitly.

And should I override a hashcode() method too?

If you override equals(Object) then you should also override hashcode().

It won't make any difference here, but it is essential if you ever put your A objects into hashed data structures. And since you don't know what the next guy is going to do with your code, it is good practice to make sure that equals(Object) and hashCode() have compatible semantics.