The instanceof keyword is used to check whether an object belongs to a specific class, subclass, or interface. It compares an object reference with a type and returns a boolean value (true or false). This keyword is commonly used to verify an object's type before performing typecasting, helping to avoid runtime errors such as ClassCastException.
- It can be used with classes, subclasses, and interfaces.
- If the reference variable contains null, instanceof always returns false.
Syntax
objectReference instanceof ClassName
// 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
Explanation: This program creates an object of the GFG class and checks whether it belongs to the same class using the instanceof keyword. Since the object is created from GFG, the condition returns true.
Examples of Java instance of keyword
Here are all the examples of instanceof keywords with their respective cases:
Example: Reating sample classes with a parent-child relationship.
// 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
Explanation: A Child object is created and tested against Child, Parent, and Object classes. Since a child object inherits from its parent and every class inherits from Object, all checks return true.
Example: Instanceof returning 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
Explanation: The reference variable tobj is assigned null and checked using instanceof. Since null does not refer to any object, the expression returns false
Example: Parent object is not an instance of Child
class Parent {
}
class Child extends Parent {
}
// Driver Class
class Test {
// main function
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
Explanation: A Parent object is created and checked whether it is an instance of Child. Since a parent object cannot be treated as a child object, the result is false.
Example: Parent reference referring to a Child is an instance of a Child
class Parent {
}
class Child extends Parent {
}
// Driver class
class Test {
// main function
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 instance of Child
Explanation: A parent reference variable stores a Child object. Although the reference type is Parent, the actual object is Child, so instanceof Child returns true.
Example: 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
Application of Java instance of keyword
- Safe Typecasting: Checks whether an object can be safely cast to another type before performing downcasting.
- Runtime Type Identification: Determines the actual type of an object during program execution.
- Inheritance Handling: Verifies relationships between parent and child classes.
- Polymorphism Support: Helps identify the real object type when a parent reference points to a child object.
- Avoiding Exceptions: Prevents
ClassCastExceptionby validating object types before casting. - Object Validation: Ensures that an object belongs to the expected class before accessing specific methods or fields.