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 :

tobj 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 :

tobj 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 typecasting, it is always a good idea to check if the typecasting is valid or not. instanceof helps use 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

This article is contributed by Twinkle Tyagi .If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.