Questions tagged [equals]

General Introduction:

equals() method is defined in the java.lang.Object class, from which all other classes are derived, hence it's automatically defined for every class. However, it doesn't perform an intelligent comparison for user defined classes unless they override it. If it's not defined for a (user) class, it behaves the same as equality operator ==, which compares equality of references in Java.

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

1. It is reflexive: for any non-null reference value x, x.equals(x) should return true.
2. 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.
3. It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
4. It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
5. For any non-null reference value x, x.equals(null) should return false.

Hence, if any user defined class overrides equals() method then it should follow the above criteria for proper implementation.

Note that it is generally necessary to override the hashCode() method whenever this method is overridden, so as to maintain the general contract for the hashCode() method, which states that equal objects must have equal hash codes.

Further Reading:

1. Javadoc for equals().

2. Stackoverflow answers: Overriding equals and hashCode in Java , Java equals for a Class. Is == same as .equals,Java: equals and == and Should equals(Object) method be overridden when overriding hashCode() in java.