In Java, instanceof is a keyword used for checking if a reference variable contains 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 of the Java instanceof Keyword
// 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);
}
} |
true
Examples of Java instaceof keyword
Here are all the examples of instanceof keywords with their respective cases:
1. Here we will be creating sample classes with a parent-child relationship.
// 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" );
}
} |
cobj is instance of Child cobj is instance of Parent cobj is instance of Object
2. instanceof returning 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" );
}
} |
tobj is NOT instance of Test
3. 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 {
} // 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" );
}
} |
pobj is NOT instance of Child
4. Parent reference referring to a Child is an instance of a 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 {
} // 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" );
}
} |
cobj is instance of Child
Application of Java instanceof keyword
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 typecasting, 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 instanceof, then do typecasting.
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);
}
}
} |
Value accessed through parent reference with typecasting is 10