In this article, we will learn instanceof operator or keyword in Java and understand how it’s help to avoid ClassCastException during type casting rather to be precise down-casting
1. instanceof keyword:
- instanceof operator is used to check, if an object belongs to any particular class or type during execution
1.1 Syntax:
(referenceObject instanceof classType)
In above expression,
- There are 2 operands between instanceof operator
- L.H.S operand is referenceObject and it is an object in question to test
- R.H.S operand is class-type and it is the class or interface against which above object will be evaluated
- Note: R.H.S represents class or interface, anything else will raise a compile-time error
- Above expression evaluates either true or false; let see the results,
1.2 Results:
- Returns true, if reference object is an instance of type class
- Returns false, if reference object is not an instance of type class
- Always returns false for null object in comparison
- Alternatively, instanceof operator referred as comparison operator because it compares an object is an instance of type class or NOT
2. Examples on instanceof operator
- instanceof operator example
- instanceof operator with Inheritance
- instanceof operator with interface example in Inheritance
- instanceof operator with null object
- Down-casting and instanceof operator
2.1 instanceof operator example:
- A simple Employee class, checking Employee object using instanceof operator/keyword
Employee.java
package in.bench.resources.instance.of.operator;
public class Employee {
public static void main(String[] args) {
// creating an object of type Employee
Employee emp = new Employee();
// checking whether emp is an instance of type Employee
if(emp instanceof Employee) {
System.out.println("emp object is "
+ "an instance of type Employee");
}
else{
System.out.println("emp object is "
+ "NOT an instance of type Employee");
}
}
}
Output:
emp object is an instance of type Employee
Explanation:
- Here, we have created an object of type Employee
- Later, checking or comparing this object with type Employee using instanceof operator
- It returns true and prints the message in console
2.2 instanceof operator in Inheritance
- Let us understand an example with Inheritance concept
Car.java and Vehicle.java
package in.bench.resources.instance.of.operator;
class Vehicle {
// super class Vehicle
}
public class Car extends Vehicle {
// sub class Car which extends Vehicle
public static void main(String[] args) {
// creating an object of type Car
Car newCar = new Car();
// checking whether newCar object is an
// instance of type Vehicle or NOT
if(newCar instanceof Vehicle) {
System.out.println("newCar object is "
+ "an instance of type Vehicle");
}
else{
System.out.println("newCar object is "
+ "NOT an instance of type Vehicle");
}
}
}
Output:
newCar object is an instance of type Vehicle
Explanation:
- There are 2 classes viz.; Vehicle and Car
- Relationship between them is, Car IS-A Vehicle (extends)
- Actually, we have created an object of type Car
- But when compared with its super type class Vehicle, still it evaluates to true and corresponding message prints in the console
- It means, if we create object of sub-class type and compare with super-class type using instanceof operator, evaluates true considering there exists a IS-A relationship between them
2.3 instanceof operator with interface example in Inheritance
- Let us tweak above example to understand how it behaves with interface in Inheritance
Car.java and Vehicle.java
package in.bench.resources.instance.of.operator;
interface Vehicle {
// interface Vehicle
}
public class Car implements Vehicle {
// sub class Car which extends Vehicle
public static void main(String[] args) {
// creating an object of type Car
Car newCar = new Car();
// checking whether newCar object is an
// instance of type Vehicle or NOT
if(newCar instanceof Vehicle) {
System.out.println("newCar object is "
+ "an instance of type Vehicle");
}
else{
System.out.println("newCar object is "
+ "NOT an instance of type Vehicle");
}
}
}
Output:
newCar object is an instance of type Vehicle
Explanation:
- One class and another interface viz.; interface Vehicle and class Car
- Relationship between them is, Car IS-A Vehicle (implements)
- Actually, we have created an object of type Car
- But when compared with its interface-type Vehicle, still it evaluates to true and corresponding message prints in the console
- It means, if we create object of sub-class type and compare with interface-type using instanceof operator, evaluates true considering there exists a IS-A relationship between them
2.4 instanceof operator with null object
- Comparing with null object using instanceof operator –> always returns false
- Let us see an example using null object
Employee.java
package in.bench.resources.instance.of.operator;
public class Employee {
public static void main(String[] args) {
// creating reference type of Employee
Employee emp = null; // explicitly assigning null
// checking whether emp object is an
// instance of type Employee or NOT
if(emp instanceof Employee) {
System.out.println("emp object is "
+ "an instance of type Employee");
}
else{
System.out.println("emp object is "
+ "NOT an instance of type Employee");
}
}
}
Output:
emp object is NOT an instance of type Employee
Explanation:
- A reference type emp is created and explicitly assigned with null value
- And when compared with class type Employee using instanceof operator, returns false
- Reason: whenever null is compared using instanceof operator –> will always returns false
2.5 Down-casting and instanceof operator
- Whenever super-class’ object is assigned to sub-class reference-type, then compiler throws a compile-time error stating reason
- Compile-time error :- “Type mismatch: cannot convert from <super-class-type> to <sub-class-type>”
Car newCar = new Vehicle();
- But explicit type-casting i.e.; down-casting in this example, leads to successful compilation but ClassCastException exception is thrown during execution/runtime
Car newCar = (Car) new Vehicle();
- Runtime-time error :- java.lang.ClassCastException: Vehicle cannot be cast to Car
- Solution :- But cautiously checking/comparing, before assigning or down-casting using instanceof operator, neither raises compile-time error nor exception is thrown at execution/runtime
Car.java and Vehicle.java
package in.bench.resources.instance.of.operator;
class Vehicle {
// super class &gt;&gt; Vehicle
}
public class Car extends Vehicle {
// sub class &gt;&gt; Car which extends Vehicle
public static void main(String[] args) {
// creating an object of type Car and
// assigning with super class type Vehicle
Vehicle newCar = new Car();
// checking whether newCar object is an
// instance of type Vehicle or NOT
if(newCar instanceof Car) {
Car car1 = (Car) newCar; // down-casting
System.out.println("down-casting done !!");
}
else{
System.out.println("down-casting NOT possible !!");
}
}
}
Output:
down-casting done !!
Explanation:
- In this example, we have created an object of sub-class type and assigned it to super-class type
- Later, when we want to down-cast –> didn’t performed directly/explicitly using type-casting
- Rather we used instanceof operator to check whether down-casting is possible
- Then, down-casted the object after instanceof operator evaluation returns true
- This way, we can get rid of compile-time error during compilation and class-cast exception during execution/runtime
3. Point to be remember about instanceof operator:
- instanceof operator in Java is used to test or check, if an object belongs to any class/interface or NOT
- Alternatively it is referred as type-comparison operator
- Returns true, if object in question is an instance of that type (class/interface)
- Returns false, if object in question is NOT an instance of that type (class/interface)
- Always returns false when comparing with null object
- It’s always a good practice to compare object with type (class/interface) before type-casting
- To get rid of ClassCastException during execution/runtime, always check type (class/interface) using instanceof operator
- Very useful operator while performing up-casting & down-casting in Inheritance
That’s all about instanceof operator in Java
Related Articles:
- Java – instanceof operator or keyword
- Java – final keyword
- Java – static keyword
- Java – this keyword
- Java – super keyword
- Java – Interview Question and Answers on final keyword
References:
Happy Coding !!
Happy Learning !!
