Java instanceof and its applications

instanceof is a keyword that is used for checking if a reference variable is containing a given type of object reference or not.

Following is a Java program to show different behaviors of instanceof.

// Java program to demonstrate working of instanceof 
  
// Creating sample classes with parent Child 
// relationship 
class Parent {  } 
class Child extends Parent { } 
  
class Test 
{ 
    public static void main(String[] args) 
    { 
        Child cobj = new Child(); 
  
        // A simple case 
        if (cobj instanceof Child) 
           System.out.println("cobj is instance of Child"); 
        else
           System.out.println("cobj is NOT instance of Child"); 
  
        // instanceof returns true for Parent class also  
        if (cobj instanceof Parent) 
           System.out.println("cobj is instance of Parent"); 
        else
           System.out.println("cobj is NOT instance of Parent"); 
  
        // instanceof returns true for all ancestors (Note : Object 
        // is ancestor of all classes in Java)  
        if (cobj instanceof Object) 
           System.out.println("cobj is instance of Object"); 
        else
           System.out.println("cobj is NOT instance of Object");            
    } 
} 

Output :

cobj is instance of Child
cobj is instance of Parent
cobj is instance of Object

instanceof returns false for null

// Java program to demonstrate that instanceof 
// returns false for null 
  
class Test {  } 
  
class Main 
{ 
    public static void main(String[] args) 
    { 
        Test tobj = null; 
  
        // A simple case 
        if (tobj instanceof Test) 
           System.out.println("tobj is instance of Test"); 
        else
           System.out.println("tobj is NOT instance of Test"); 
    } 
} 

Output :

tobj is NOT instance of Test

A parent object is not an instance of Child

// A Java program to show that a parent object is 
// not an instance of Child 
  
class Parent {  } 
class Child extends Parent { } 
  
class Test 
{ 
    public static void main(String[] args) 
    { 
        Parent pobj = new Parent(); 
        if (pobj instanceof Child) 
           System.out.println("pobj is instance of Child"); 
        else
           System.out.println("pobj is NOT instance of Child"); 
    } 
} 

Output :

pobj is NOT instance of Child

A parent reference referring to a Child is an instance of Child

// A Java program to show that a parent reference 
// referring to a Child is an instance of Child 
  
class Parent {  } 
class Child extends Parent { } 
  
class Test 
{ 
    public static void main(String[] args) 
    { 
        // Reference is Parent type but object is 
        // of child type. 
        Parent cobj = new Child(); 
        if (cobj instanceof Child) 
           System.out.println("cobj is instance of Child"); 
        else
           System.out.println("cobj is NOT instance of Child"); 
    } 
} 

Output :

cobj is an instance of Child

Application:
We have seen here that a parent class data member is accessed when a reference of parent type refers to a child object. We can access child data member using type casting.

Syntax: (child_class_name) Parent_Reference_variable).func.name().

When we do typecast, it is always a good idea to check if the typecasting is valid or not. instanceof helps us here. We can always first check for validity using instancef, then do typecasting.

// A Java program to demonstrate that non-method 
// members are accessed according to reference 
// type (Unlike methods which are accessed according 
// to the referred object) 
  
class Parent 
{ 
    int value = 1000; 
} 
  
class Child extends Parent 
{ 
    int value = 10; 
} 
  
// Driver class 
class Test 
{ 
    public static void main(String[] args) 
    { 
        Parent cobj = new Child(); 
        Parent par = cobj; 
  
        // Using instanceof to make sure that par 
        // is a valid reference before typecasting 
        if (par instanceof Child) 
        { 
            System.out.println("Value accessed through " + 
                "parent reference with typecasting is " + 
                                     ((Child)par).value); 
        } 
    } 
} 

Output:

Value accessed through parent reference with typecasting is 10

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Java instanceof and its applications

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