15.20

Equality Operators

EXPRESSIONS

At run time, the result of the

instanceof

 operator is

true

 if the value of the

RelationalExpression

 is not

null

 and the reference could be cast ( 15.15) to the

ReferenceType

 without raising a

ClassCastException

. Otherwise the result is

false

.

If a cast of the

RelationalExpression

 to the

ReferenceType

 would be rejected

as a compile time error, then the

instanceof

 relational expression likewise pro 

duces a compile time error. In such a situation, the result of the

instanceof

expression could never be

true

.

Consider the example program:

class Point { int x, y; }

class Element { int atomicNumber; }

class Test {

public static void main(String[] args) {

Point p = new Point();

Element e = new Element();

if (e instanceof Point) {

//

compile time error

System.out.println("I get your point!");

p = (Point)e;

//

compile time error

}

}

}

This example results in two compile time errors. The cast

(Point)e

 is incorrect

because no instance of

Element

 or any of its possible subclasses (none are shown

here) could possibly be an instance of any subclass of

Point

. The

instanceof

expression is incorrect for exactly the same reason. If, on the other hand, the class

Point

 were a subclass of

Element

 (an admittedly strange notion in this example):

class Point extends Element { int x, y; }

then the cast would be possible, though it would require a run time check, and the

instanceof

 expression would then be sensible and valid. The cast

(Point)e

would never raise an exception because it would not be executed if the value of

e

could not correctly be cast to type

Point

.

15.20   Equality Operators

The equality operators are syntactically left associative (they group left to right),

but this fact is essentially never useful; for example,

a==b==c

 parses as

(a==b)==c

. The result type of

a==b

 is always

boolean

, and

c

 must therefore be

of type

boolean

 or a compile time error occurs. Thus,

a==b==c

 does

not

 test to

see whether

a

,

b

, and

c

 are all equal.

362