HashSet contains method, strange behavior [duplicate]

Question

here is my code :

public class testGui {



    public static void main(String[] arg){
        class TESTS{
            String t;

            public TESTS(String t){
                this.t = t;
            }

            @Override
            public boolean equals(Object x){
                System.out.println("My method is called...");
                if(x instanceof TESTS){
                    TESTS zzz = (TESTS) x;
                    return zzz.t.compareTo(t)==0;
                }
                else return false;
            }
        }
        HashSet<TESTS> allItems = new HashSet<TESTS>();
        allItems.add(new TESTS("a"));
        allItems.add(new TESTS("a"));
        System.out.println(allItems.contains(new TESTS("a")));
    }

}

I do not get why the hashset contains method is not calling my equals method as mentionned in their specifications :

More formally, adds the specified element, o, to this set if this set contains no element e such that (o==null ? e==null : o.equals(e))

My code is returning false and not going into my equals method.

Thanks a lot for answering!

Answer 1

When you override equals, you must also override hashCode. Otherwise, equal objects will have different hash codes and be considered unequal.

It is also strongly recommended not to override only hashCode. But this is not essential, as unequal objects can have the same hash code.