Java instanceof operator

The java 'instanceof' operator is used to test whether an object is an instance of a specified type (class or sub - class or visual interface).

'instanceof' in Java is also known as the type comparison operator because it compares the instances with type. It returns true or false. If we apply the 'instanceof' operator with a variable value, the value returned is false.

Example of Java instanceof Operator

Let's look at a simple example of instanceof operator where it checks the current class.

student.java

{  
   public static void main( String args[ ] ) 
   {  
      // declaring an object 's' of the student class
      student s = new student( ) ;  
      // checking whether s is an instance of the student class using instanceof operator
      Boolean str = s instanceof student;
      // printing the string value
      System.out.println( str ) ;  
 }  
}  

Output:

true

An object of subclass type is also a type of parent class. For example, if a 'student' class extends the 'Teacher' class, then object of student class can be referred by either the student class itself or Teacher class. Let us consider the following example to understand this point more clearly.

Let's see another example.

Student.java

class Teacher { }  
public class Student extends Teacher
{  
    public static void main( String args[ ] )
    {  
      // declaring the object of the class 'Student'
      Student s = new Student( ) ;  
      
      // checking whether the object s is the instance of the parent class ' Teacher '
      Boolean str = s instanceof Teacher ;
      
      // printing the boolean value
      System.out.println( str ) ;  
    }  
}  

Output:

true

Using instanceof Operator with a Variable that has Null Value

Let's take some time here to think of the outcome if the instanceof operator is used with a variable that has null value. The answer is that it returns false. Consider the following example to gain better understanding of this point.

Student.java

public class Student
{  
  public static void main( String args[ ] ) 
  {  
    // initializing the object with null value
    Student s = null ;  
    
    // checking whether the s object with null value is the instance of the Student class
    Boolean str = s instanceof Student ;
    
    // printing the boolean value
    System.out.println( str ) ;
  }  
}  

Output:

false

Downcasting with Java instanceof Operator

In Downcasting the object of the parent class is assigned to the base class. On performing it directly, the compiler gives Compilation error. On the other hand, if we try to perform it by typecasting, ClassCastException is thrown at runtime. Downcasting is easily possible using 'instanceof' operator.

If we perform downcasting by typecasting, ClassCastException is thrown at runtime.

Compiles successfully but ClassCastException is thrown at runtime

Possibility of downcasting with instanceof Operator

Let's see the example, where downcasting is possible by instanceof operator.

Student.java

class Teacher { }  
  public class Student extends Teacher {  
  static void method( Teacher T ) 
  { 
    // checking if T (instance of Teacher class) is the instance of Student class
    if( T instanceof Student ) 
    {  
       // performing downcasting
       Student s = ( Student ) T ; 
       System.out.println( " Cool! Downcasting successfully performed! " ) ;  
    }  
  }  
   public static void main ( String [] args ) 
   {  
        // crating object of class 'Teacher'
        Teacher T = new Student( ) ; 
        
        Student.method( T ) ;  
  }      
 }  

Output:

Cool! Downcasting successfully performed!

Downcasting without the use of Java instanceof Operator

Let's see how we can perform downcasting without using the instanceof operator as shown in the following example:

Student.java

class Teacher { }  
public class Student extends Teacher 
{  
  static void method( Teacher T ) 
  {  
       // performing downcasting
       Student s = ( Student ) T ;
       
       System.out.println( " Cool! Downcasting successfully performed! " ) ;  
  }  
  
   public static void main ( String [ ] args ) 
   {  
    Teacher T = new Student( ) ;  
    Student.method( T ) ;  
  }  
}  

Output:

Cool! Downcasting successfully performed!

Understanding Real use of instanceof in java

Consider the example given below to understand what we have covered so far.

Temp1.java

// declaring an interface. An interface is a class that has no implementation of its own and is used as a parent class to be derived by the child class
interface School { }
// class Student implementing School
class Student implements School 
{  
    public void student( ) 
    { 
        System.out.println( " This is the method of Student class " ) ;    
    }  
}  
// class Teacher implementing School
class Teacher implements School 
{  
    public void teacher( ) 
    {
        System.out.println(" This is the method of Teacher class " ) ;
    }  
}  
class Temp
{  
    void call( School sc )
    { 
        // performing upcasting
        if( sc instanceof Student )
        {  
            // performing downcasting
            Student s = ( Student ) sc ;
            // calling the method student of 'Student' class
            s.student( ) ;  
    }  
    if( sc instanceof Teacher ) 
    {  
        // performing downcasting
        Teacher t = ( Teacher )sc ;
        // calling the method teacher of 'Teacher' class
        t.teacher( ) ;  
}    
}  
}  
public class Temp1 
{  
    public static void main(String[] args)
    {  
        School sc = new Teacher( ) ;  
        // creating object of temp class
        Temp t = new Temp();  
        t.call( sc ) ;  
}  
}

Output: