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 :
pobj 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 :
cobj 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 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.
// 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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