Using instanceof
Sometimes, knowing the type of an object during run time is useful. For example, you might
have one thread of execution that generates various types of objects, and another thread
that processes these objects. In this situation, it might be useful for the processing thread to
know the type of each object when it receives it. Another situation in which knowledge of
an object's type at run time is important involves casting. In Java, an invalid cast causes a
run-time error. Many invalid casts can be caught at compile time. However, casts involving
class hierarchies can produce invalid casts that can be detected only at run time. For example,
a superclass called A can produce two subclasses, called B and C. Thus, casting a B object
into type A or casting a C object into type A is legal, but casting a B object into type C (or
vice versa) isn't legal. Because an object of type A can refer to objects of either B or C, how
can you know, at run time, what type of object is actually being referred to before attempting
the cast to type C? It could be an object of type A, B, or C. If it is an object of type B, a run-
time exception will be thrown. Java provides the run-time operator instanceof to answer
this question.
The instanceof operator has this general form:
objref instanceof type
Here, objref is a reference to an instance of a class, and type is a class type. If objref is of the
specified type or can be cast into the specified type, then the instanceof operator evaluates to
true. Otherwise, its result is false. Thus, instanceof is the means by which your program can
obtain run-time type information about an object.
The following program demonstrates instanceof:
// Demonstrate instanceof operator.
class A {
int i, j;
}
class B {
int i, j;
}
class C extends A {
int k;
}
class D extends A {
int k;
}
class InstanceOf {
public static void main(String args[]) {
A a = new A();
B b = new B();
C c = new C();
D d = new D();
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