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

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