a == b; what does it means?

a and b are reference variables of the same type and if we test equality between these two variables we are testing whether they have the same reference.

Some programing languages gives us implementations for standard types like integers and strings. In Java we can test the equality of two Strings, a and b, with a.equals(b).

When we want to test equality on our own types like Person, we need to override equals(). In order to correctly override the equals() method we must follow a few rules; the relation between the objects must be:

• Reflexive: a.equals(a) is true.
• Symmetric: a.equals(b) is true if and only if b.equals(a).
• Transitive: if a.equals(b) and b.equals(c) are true, then a.equals(c) is also true.
• Consistent: multiple invocations of a.equals(b) consistently return the same value.
• Not null: a.equals(null) is false.

In order to follow these natural definitions we need to implement them:

• If the reference to this object is the same as the reference to the argument object, return true.
• If the argument is null, return false.
• If the objects are not from the same class, return false.
• If any instance variables do not match, return false.

public boolean equals(Object b) {
  if (this == b) return true;
  if (b == null) return false;
  if (this.getClass() != b.getClass()) return false;

  Person other = (Person) b;
  if (this.firstName != other.firstName) return false;
  if (this.lastName != other.lastName) return false;

  return true;
}

This implementation can be used for any custom type.