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instanceof Operator

The instanceof operator is a binary operator that checks if a reference variable is an instance of a specific class or a subclass. It also works for interfaces, verifying whether an object implements a given interface.

Characteristics of instanceof

Returns true if the object belongs to the specified class, subclass, or implements the given interface.
Returns false if the object is null.
Prevents ClassCastException by ensuring safe type conversion.

Syntax

1	object instanceof ClassName

object – The reference variable to be tested.
ClassName – The class or interface to check against.
Returns true if object is an instance of ClassName, otherwise false.

Example

class Animal {}

class Dog extends Animal {}

public class InstanceofExample {

public static void main(String[] args) {

Dog myDog = new Dog();

System.out.println(myDog instanceof Dog); // true

System.out.println(myDog instanceof Animal); // true

System.out.println(myDog instanceof Object); // true

}

myDog instanceof Dog: true, because myDog is an instance of Dog.
myDog instanceof Animal: true, since Dog is a subclass of Animal.
myDog instanceof Object: true, because all Java classes implicitly extend Object.

Usage

Using instanceof with Interfaces

In Java, an interface defines a contract that classes can implement. The instanceof operator can check whether an object implements a particular interface.

interface Walkable {}

class Human implements Walkable {}

public class InterfaceInstanceofExample {

public static void main(String[] args) {

Human person = new Human();

System.out.println(person instanceof Human); // true

System.out.println(person instanceof Walkable); // true

}

person instanceof Human: true, since person is an instance of Human.
person instanceof Walkable: true, because Human implements the Walkable interface.

Handling Null References with instanceof

If a reference variable is null, instanceof never throws a NullPointerException, it simply returns false.

1 2	String text = null; System.out.println(text instanceof String); // false

Since text is null, it does not refer to any object, so instanceof safely returns false instead of throwing an exception.

Using instanceof Before Type Casting

The instanceof operator helps ensure safe downcasting, which prevents runtime exceptions.

class Animal {}

class Dog extends Animal {

void bark() {

System.out.println("Woof! Woof!");

}

public class SafeDowncastingExample {

public static void main(String[] args) {

Animal myAnimal = new Dog(); // Upcasting

if (myAnimal instanceof Dog) { // Check before downcasting

Dog myDog = (Dog) myAnimal; // Safe Downcasting

myDog.bark(); // Woof! Woof!

}

The instanceof check ensures myAnimal is actually a Dog before performing the downcast ((Dog) myAnimal).
Without instanceof, if myAnimal pointed to a different subclass of Animal, it could result in a ClassCastException.

instanceof Operator

Characteristics of instanceof

Syntax

Example

Usage

Using instanceof with Interfaces

Handling Null References with instanceof

Using instanceof Before Type Casting

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