instanceof Keyword in Java

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. Henceforth it is known as a comparison operator where the instance is getting compared to type returning boolean true or false as in java we do not have 0 and 1 boolean return types.

Example:

Java

// Java Program to Illustrate instanceof Keyword

// Importing required I/O classes
import java.io.*;

// Main class
class GFG {
    public static void main(String[] args)
    {

        // Creating object of class inside main()
        GFG object = new GFG();

        // Returning instanceof
        System.out.println(object instanceof GFG);
    }
}

Output

true

Implementation: Here we will be creating sample classes with a parent-child relationship.

Example 1:

Java

// Java program to demonstrate working of instanceof Keyword

// Class 1
// Parent class
class Parent {}

// Class 2
// Child class
class Child extends Parent {}

// Class 3
// Main class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating object of child class
        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 returning 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

Example 2: instanceof returning false for null 

Java

// 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

Example 3: Parent object is not an instance of Child  

Java

// 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

Example 4: Parent reference referring to a Child is an instance of Child

Java

// 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

Now, the application of instanceof keyword is as follows:

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 members using typecasting. 

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.

Example

Java

// 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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