.Contains() method not calling Overridden equals method

Question

I am having an issue where I make an ArrayList of Foo objects, I override the equals method, and I cannot get the contains method to call the equals method. I have tried overriding equals and hashcode together, but it still doesn't work. I'm sure there is a logical explanation to why this is, but I cannot figure it out at the moment on my own lol. I just want a way to see if the list contains the specified id.

Here's some code:

import java.util.ArrayList;
import java.util.List;

public class Foo {

    private String id;


    public static void main(String... args){
        Foo a = new Foo("ID1");
        Foo b = new Foo("ID2");
        Foo c = new Foo("ID3");
        List<Foo> fooList = new ArrayList<Foo>();
        fooList.add(a);
        fooList.add(b);
        fooList.add(c);
        System.out.println(fooList.contains("ID1"));
        System.out.println(fooList.contains("ID2"));
        System.out.println(fooList.contains("ID5"));
    }   

    public Foo(String id){
        this.id = id;
    }

    @Override
    public boolean equals(Object o){
        if(o instanceof String){
            String toCompare = (String) o;
            return id.equals(toCompare);
        }
        return false;
    }



    @Override
    public int hashCode(){
        return 1;
    }
}

OUTPUT: false false false

Answer 1

This is because your equals() is not symmetric:

new Foo("ID1").equals("ID1");

but

"ID1".equals(new Foo("ID1"));

is not true. This violates the equals() contract:

The equals method implements an equivalence relation on non-null object references:

• [...]

• It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.

It is not reflexive either:

• It is reflexive: for any non-null reference value x, x.equals(x) should return true.

Foo foo = new Foo("ID1");
foo.equals(foo)  //false!

@mbockus provides correct implementation of equals():

public boolean equals(Object o){
  if(o instanceof Foo){
    Foo toCompare = (Foo) o;
    return this.id.equals(toCompare.id);
  }
  return false;
}

but now you must pass instance of Foo to contains():

System.out.println(fooList.contains(new Foo("ID1")));
System.out.println(fooList.contains(new Foo("ID2")));
System.out.println(fooList.contains(new Foo("ID5")));

Finally you should implement hashCode() to provide consistent results (if two objects are equal, they must have equal hashCode()):

@Override
public int hashCode() {
    return id.hashCode();
}