A key facet of the ``++equals++`` contract is that if ``++a.equals(b)++`` then ``++b.equals(a)++``, i.e. that the relationship is symmetric. 


Using ``++instanceof++`` breaks the contract when there are subclasses, because while the child is an ``++instanceof++`` the parent, the parent is not an ``++instanceof++`` the child. For instance, assume that ``++Raspberry extends Fruit++`` and adds some fields (requiring a new implementation of ``++equals++``):

----
Fruit fruit = new Fruit();
Raspberry raspberry = new Raspberry();

if (raspberry instanceof Fruit) { ... } // true
if (fruit instanceof Raspberry) { ... } // false
----

If similar ``++instanceof++`` checks were used in the classes' ``++equals++`` methods, the symmetry principle would be broken:

----
raspberry.equals(fruit); // false
fruit.equals(raspberry); //true
----
Additionally, non ``++final++`` classes shouldn't use a hardcoded class name in the ``++equals++`` method because doing so breaks the method for subclasses. Instead, make the comparison dynamic.


Further, comparing to an unrelated class type breaks the contract for that unrelated type, because while ``++thisClass.equals(unrelatedClass)++`` can return true, ``++unrelatedClass.equals(thisClass)++`` will not.


== Noncompliant Code Example

----
public class Fruit extends Food {
  private Season ripe;

  public boolean equals(Object obj) {
    if (obj == this) {
      return true;
    }
    if (obj == null) {
      return false;
    }
    if (Fruit.class == obj.getClass()) { // Noncompliant; broken for child classes
      return ripe.equals(((Fruit)obj).getRipe());
    }
    if (obj instanceof Fruit ) {  // Noncompliant; broken for child classes
      return ripe.equals(((Fruit)obj).getRipe());
    }
    else if (obj instanceof Season) { // Noncompliant; symmetry broken for Season class
      // ...
    }
    //...
----


== Compliant Solution

----
public class Fruit extends Food {
  private Season ripe;

  public boolean equals(Object obj) {
    if (obj == this) {
      return true;
    }
    if (obj == null) {
      return false;
    }
    if (this.getClass() == obj.getClass()) {
      return ripe.equals(((Fruit)obj).getRipe());
    }
    return false;
}
----


== See

* https://wiki.sei.cmu.edu/confluence/x/AzZGBQ[CERT, MET08-J.] - Preserve the equality contract when overriding the equals() method