Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 571

to the object that was referred to by p2 . This means that a change to p2 , such as a call
of its translate method, would also be reflected in p3 :

p1 x 3 x 7 y 2

p2 x 3 x 7 y 2

p3

In the case of the preceding Point objects, the expression p1 == p2 would evalu-
ate to false because p1 and p2 do not refer to the same object. The object referred to
by p1 has the same state as p2 's object, but they have different identities. The expres-
sion p2 == p3 would evaluate to true , though, because p2 does refer to the same
object as p3 .
Often, when comparing two objects, we want to know whether the objects have
the same state. To perform such a comparison, we use a special method called
equals . Every Java object contains an equals method that it uses to compare itself
to other objects.
The previous section mentioned that a class without a toString method receives
a default version of the method. Similarly, a class without an equals method
receives a default version that uses the most conservative definition of equality, con-
sidering two objects to be equal only if they have the same identity. This means that
the default equals method behaves identically to the == operator. If you want a
method that will behave differently, you must write your own equals method to
replace the default behavior.
A proper equals method performs a comparison of two objects' states and returns
true if the states are the same. With the preceding Point objects, we'd like the
expressions p1.equals(p2) , p1.equals(p3) , and p2.equals(p3) to evaluate to
true because the Point s all have the same ( x , y ) coordinates.
You can imagine a piece of client code that examines two Point objects to see
whether they have the same x and y field values:

if (p1.getX() == p2.getX() && p1.getY() == p2.getY()) {
// the objects have equal state
...

}

But the equals functionality should actually be implemented in the Point class
itself, not in the client code. Rather than having two Point s p1 and p2 , the equals
method considers the first object to be the “implicit parameter” and accepts the sec-
ond object as a parameter. It returns true if the two objects are equal.