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 🙂