.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(); }