Question

I tried to override equals and hashcode methods in a class. It is a subclass of another class which does not implement the equals method and hashCode methods.

Eclipse gave the below warning .

  The super class ABC does not implement equals() and hashCode() methods.
  The resulting code may not work correctly. 

Why is the above warning given ? Under what circumstances it may not work correctly ?

Answer 1

If you say a.equals(b) versus b.equals(a) it is reasonable to expect the behaviour to be the same. But if they are of corresponding types B and A related by inheritance and only one of them properly implements equals then the behaviour will be different in those two examples.

Here, A is the superclass and does not implement equals at all (so it inherits java.lang.Object.equals). Subclass B overrides equals to depend on the name field.

class A {

  String name;

  public A() {
    this.name = "Fred";
  }

}

class B extends A {

  public boolean equals(Object o) {
    A a = (A)o;
    return a != null && a.name.equals(this.name);
  }
}

public class Test {

  public static void main(String[] args) {

    A a = new A();
    B b = new B();

    System.out.println(a.equals(b) == b.equals(a));
  }
} 

Unsurprisingly, the output is false, thus breaking symmetry.