Discovering Classes and Objects - Beginning Java 7 - page 124

return p.x == x && p.y == y;

}
public static void main(String[] args)
{

Point p1 = new Point(10, 20);
Point p2 = new Point(20, 30);
Point p3 = new Point(10, 20);
// Test reflexivity
System.out.println(p1.equals(p1)); // Output: true
// Test symmetry
System.out.println(p1.equals(p2)); // Output: false
System.out.println(p2.equals(p1)); // Output: false
// Test transitivity
System.out.println(p2.equals(p3)); // Output: false
System.out.println(p1.equals(p3)); // Output: true
// Test nullability
System.out.println(p1.equals(null)); // Output:

false

// Extra test to further prove the instanceof oper-
ator's usefulness.
System.out.println(p1.equals("abc")); // Output:

false
}

}

Listing 2-27 ' s overriding equals() method begins with an if statement that uses
the instanceof operatortodeterminewhethertheargumentpassedtoparameter o is
an instance of the Point class. If not, the if statement executes return false; .
The o instanceof Point expression satisfies the last portion of the contract:
Foranynonnullreferencevalue x , x .equals(null) returnsfalse.Becausethenull
reference is not an instance of any class, passing this value to equals() causes the
expression to evaluate to false.
The o instanceof Point expression also prevents a
java.lang.ClassCastException instance from being thrown via expression
(Point) o in the event that you pass an object other than a Point object to
equals() . (I will discuss exceptions in the next chapter.)