ArrayList's custom Contains method

Question

I have some object

class A {   private Long id;    private String name;    public boolean equals(Long v) {      return this.id.equals(v);   } } 

and ArrayList of these objects. What I want is to be able to check if that list contains some object by object's field. For example:

ArrayList<A> list = new ArrayList<A>(); if (list.contains(0L)) {...} 

but overrided Equals method is not helps me. What I am doing wrong? Thank you

UPDATE And should I override a hashcode() method too?

Answer 1

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 :)