Chapter: Java The Complete Reference - The Java Language - I/O, Applets, and Other Topics

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Using instanceof - Java

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.

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.

 

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