Regarding instanceof operator

[Sneha Kulkarni]

Hi,

Can anybody please explain me about instanceof operator,how it is used etc?

Thanks in advance

Sneha

[Raghavan Muthu]

Originally posted by Sneha Kulkarni:
Can anybody please explain me about instanceof operator,how it is used etc?

"instanceof" operator is used to determine the object type at runtime.

You can take a decision based on the actual object so as to invoke its functionality.

A sample code would be:

String s = "test";
 
if(s instanceof String)
{
  System.out.println("s is of type String");
}
 

Note:

You can use only OBJECTS to check using instanceof operators as the term 'instance' indicates

you can use the fully qualified classname in the checking as 'java.lang.String' instead of 'String'. This will be helpful to use the classes-happen-to-have-same-name (for example, java.awt.List and java.util.List)

the instanceof operator returns true for the object when compared against any of its compatible and/or super class(es). Any object when compared against java.lang.Object would return true always

Does that help you?
[ September 10, 2007: Message edited by: Raghavan Muthu ]

[Sneha Kulkarni]

Originally posted by Raghavan Muthu:


"instanceof" operator is used to determine the object type at runtime.

You can take a decision based on the actual object so as to invoke its functionality.

A sample code would be:

String s = "test";
 
if(s instanceof String)
{
  System.out.println("s is of type String");
}
 

Note:

You can use only OBJECTS to check using instanceof operators as the term 'instance' indicates

you can use the fully qualified classname in the checking as 'java.lang.String' instead of 'String'. This will be helpful to use the classes-happen-to-have-same-name (for example, java.awt.List and java.util.List)

the instanceof operator returns true for the object when compared against any of its compatible and/or super class(es). Any object when compared against java.lang.Object would return true always

Does that help you?

[ September 10, 2007: Message edited by: Raghavan Muthu ]

[Sneha Kulkarni]

Hi Raghav

If you dont mind,can you please explain me in brief?

Thanks
Sneha

[Raghavan Muthu]

Yeah sure.

Please try to look at the sample code given below.

Tell me whether this code helps you to get the information?

interface AnimalBehavior
{
public abstract void shout();
}
 
class AnimalImpl implements AnimalBehavior
{
Animal a;
public void setAnimal(Animal a)
{
this.a = a;
}
public void shout()
{
System.out.println("actual instance type of 'a' is : "+
                              a.getClass().getName());

        if([B]a instanceof Dog[/B])
((Dog)a).shout();
else if([B]a instanceof Frog[/B])
((Frog)a).shout();
else
a.shout();
}
}
 
class Animal
{
public void shout()
{
System.out.println("shout() in Animal");
}
}
 
class Dog extends Animal
{
public void shout()
{
System.out.println("shout() in Dog");
}
}
 
class Frog extends Animal
{
public void shout()
{
System.out.println("shout() in Frog");
}
}
 
class InstanceOfDemo
{
public static void main(String[] args) 
{
System.out.println("Testing for using instanceof inside");
[B]testInstanceOfUsage();[/B]
System.out.println("==============================================");
System.out.println("With polymorphism, no need of instanceof!" +
                              "JVM takes care of it ");
[B]testWithPolymorphism();[/B]
}
 
/* testing the instanceof code */
public static void testInstanceOfUsage()
{
AnimalImpl aImpl = new AnimalImpl();
        Animal animalObj = new Animal();
aImpl.setAnimal(animalObj);
aImpl.shout();
animalObj = new Dog();
aImpl.setAnimal(animalObj);
aImpl.shout();
animalObj = new Frog();
aImpl.setAnimal(animalObj);
aImpl.shout();
}
 
/* pretty usual; you might have seen this everywhere */
public static void testWithPolymorphism()
{
Animal a = new Animal();
System.out.println("calling via Animal Reference " + 
                      "and Animal Object");
a.shout();
a = new Dog();
System.out.println("calling Animal Reference," + 
                       " but Dog Object");
a.shout();
a = new Frog();
System.out.println("calling via Animal Reference," +                    
                      " but Frog Object");
a.shout();
}
}

The output it gives is as follows:

Testing for using instanceof inside
[B]actual instance type of reference variable 'a' is : Animal[/B]
shout() in Animal
[B]actual instance type of reference variable 'a' is : Dog[/B]
shout() in Dog
[B]actual instance type of reference variable 'a' is : Frog[/B]
shout() in Frog
==============================================
With polymorphism, no need of instanceof! JVM takes care of it 
calling via Animal Reference and Animal Object
shout() in Animal
calling via Animal Reference, but Dog Object
shout() in Dog
calling via Animal Reference,  but Frog Object
shout() in Frog

If you look at the shout() method implementation code in the AnimalImpl class,
you may get the practical usage of "instanceof" operator.

Hope it helps to clear the doubt.
[ September 11, 2007: Message edited by: Raghavan Muthu ]

[Raghavan Muthu]

Perhaps, the following links may help you to get further ideas.

Prefer polymorphism over instanceof and downcasting, a good article explains about Polymorphism Vs instanceof.

Have a look at "instanceof definition" and "instanceof examples" section in this page

Using the instanceof keyword in Java, an article from devx.com.

[Raghavan Muthu]

Another simple example is here

class DealPrimitive
{
public void getSum(Object o1, Object o2)
{
if([B]o1 instanceof Integer && o2 instanceof Integer[/B])
{
int sum = ((Integer)o1).intValue() + ((Integer)o2).intValue();
System.out.println("[Integer] sum is : "+sum);
}
if([B]o1 instanceof Float && o2 instanceof Float[/B])
{
float sum = ((Float)o1).floatValue() + ((Float)o2).floatValue();
System.out.println("[Float] sum is : "+sum);
}
}
}
 
class InstanceOfDemo2
{
public static void main(String[] args) 
{
DealPrimitive obj = new DealPrimitive();
obj.getSum(new Integer(1), new Integer(2));
obj.getSum(new Float(2.0F), new Float(3.5F));
}
}

The output is:

[Integer] sum is : 3
[Float] sum is : 5.5

[Sneha Kulkarni]

Hi Raghavan

Thank you!,i understood the instanceof now.

Regards
Sneha