Java instanceof

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In Java, instanceof operator is used to check the type of an object at runtime. It is the means by which your program can obtain run-time type information about an object. instanceof operator is also important in case of casting object at runtime. instanceof operator return boolean value, if an object reference is of specified type then it return true otherwise false.

Example

public class Test
{
    public static void main(String[] args)
    {
      Test t= new Test();
      System.out.println(t instanceof Test);
      }
}


Output:


true
downcasting in java
Example

class Parent{ }

public class Child extends Parent
{
    public void check()
    {
        System.out.println("Sucessfull Casting");
    }

    public static void show(Parent p)
    {
       if(p instanceof Child)
       {
           Child b1=(Child)p;
           b1.check();
       }
    }

    public static void main(String[] args)
    {

      Parent p=new Child();

      Child.show(p);

      }
}


Output:


Sucessfull Casting
More example of instanceof operator

class Parent{}

class Child1 extends Parent{}

class Child2 extends Parent{}

class Test
{
  public static void main(String[] args)
  {
      Parent p =new Parent();
      Child1 c1 = new Child1();
      Child2 c2 = new Child2();

      System.out.println(c1 instanceof Parent);  //true
      System.out.println(c2 instanceof Parent);  //true
      System.out.println(p instanceof Child1);  //false
      System.out.println(p instanceof Child2);  //false

      p = c1;
      System.out.println(p instanceof Child1);  //true
      System.out.println(p instanceof Child2);  //false

      p = c2;
      System.out.println(p instanceof Child1);  //false
      System.out.println(p instanceof Child2);  //true

   }

}


Output:

true
true
false
false
true
false
false
true
Another example of java instanceof operator

class Animal{}  
class Dog1 extends Animal{//Dog inherits Animal  
  
 public static void main(String args[]){  
 Dog1 d=new Dog1();  
 System.out.println(d instanceof Animal);//true  
 }  
}


Output:


Output:true
instanceof in java with a variable that have null value

If we apply instanceof operator with a variable that have null value, it returns false. Let's see the example given below where we apply instanceof operator with the variable that have null value.


class Dog2{  
 public static void main(String args[]){  
  Dog2 d=null;  
  System.out.println(d instanceof Dog2);//false  
 }  
}


Output:


false
Downcasting with java instanceof operator

When Subclass type refers to the object of Parent class, it is known as downcasting. If we perform it directly, compiler gives Compilation error. If you perform it by typecasting, ClassCastException is thrown at runtime. But if we use instanceof operator, downcasting is possible.


Dog d=new Animal();//Compilation error

If we perform downcasting by typecasting, ClassCastException is thrown at runtime.


Dog d=(Dog)new Animal();  
//Compiles successfully but ClassCastException is thrown at runtime
Possibility of downcasting with instanceof

class Animal { }  
  
class Dog3 extends Animal {  
  static void method(Animal a) {  
    if(a instanceof Dog3){  
       Dog3 d=(Dog3)a;//downcasting  
       System.out.println("ok downcasting performed");  
    }  
  }  
   
  public static void main (String [] args) {  
    Animal a=new Dog3();  
    Dog3.method(a);  
  }  
    
 }


Output:


ok downcasting performed
Downcasting without the use of java instanceof

class Animal { }  
class Dog4 extends Animal {  
  static void method(Animal a) {  
       Dog4 d=(Dog4)a;//downcasting  
       System.out.println("ok downcasting performed");  
  }  
   public static void main (String [] args) {  
    Animal a=new Dog4();  
    Dog4.method(a);  
  }  
}


Output:

ok downcasting performed

Let's take closer look at this, actual object that is referred by a, is an object of Dog class. So if we downcast it, it is fine. But what will happen if we write:


Animal a=new Animal();  
Dog.method(a);  
//Now ClassCastException but not in case of instanceof operator
Understanding Real use of instanceof in java

interface Printable{}  
class A implements Printable{  
public void a(){System.out.println("a method");}  
}  
class B implements Printable{  
public void b(){System.out.println("b method");}  
}  
  
class Call{  
void invoke(Printable p){//upcasting  
if(p instanceof A){  
A a=(A)p;//Downcasting   
a.a();  
}  
if(p instanceof B){  
B b=(B)p;//Downcasting   
b.b();  
}  
  
}  
}//end of Call class  
  
class Test4{  
public static void main(String args[]){  
Printable p=new B();  
Call c=new Call();  
c.invoke(p);  
}  
}


Output:


b method



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