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();
if(a instanceof A)
System.out.println("a is instance of
A"); if(b instanceof B)
System.out.println("b is instance of
B"); if(c instanceof C)
System.out.println("c is instance of
C"); if(c instanceof A)
System.out.println("c can be cast to
A");
if(a instanceof C)
System.out.println("a can be cast to
C"); System.out.println();
// compare types of derived types A ob;
ob = d; // A reference to d
System.out.println("ob now refers to
d");
if(ob instanceof D)
System.out.println("ob is instance of
D");
System.out.println();
ob = c; // A reference to c
System.out.println("ob now refers to
c");
if(ob instanceof D)
System.out.println("ob can be cast to
D"); else
System.out.println("ob cannot be cast to
D");
if(ob instanceof A)
System.out.println("ob can be cast to
A");
System.out.println();
// all objects can be cast to Object
if(a instanceof Object)
System.out.println("a may be cast to
Object");
if(b instanceof Object)
System.out.println("b may be cast to
Object");
if(c instanceof Object)
System.out.println("c may be cast to
Object");
if(d instanceof Object)
System.out.println("d may be cast to
Object");
}
}
The output from this program
is shown here:
a is instance of A
b is instance of B
c is instance of C
c can be cast to A
ob now refers to d
ob is instance of D
ob now refers to c
ob cannot be cast to D
ob can be cast to A
a may be cast to Object
b may be cast to Object
c may be cast to Object
d may be cast to Object
The instanceof operator isn’t needed by most programs, because,
generally, you know the type of object with which you are working. However, it
can be very useful when you’re writing generalized routines that operate on
objects of a complex class hierarchy.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2023 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.