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instanceof Keyword in Java

  • Difficulty Level : Easy
  • Last Updated : 17 Oct, 2021
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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 Illusttrate instanceof Keyword
  
// Importing rquired 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

This article is contributed by Twinkle Tyagi. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@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. 

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