instanceof operator and isInstance() method both are used for checking the class of the object. But main difference comes when we want to check the class of object dynamically. In this case isInstance() method will work. There is no way we can do this by instanceof operator.
instanceof operator and isInstance() method both return a boolean value. isInstance() method is a method of class Class in java while instanceof is a operator.
Consider an example:
// Java program to demonstrate working of
// instanceof operator
public class Test
{
public static void main(String[] args)
{
Integer i = new Integer(5);
// prints true as i is instance of class
// Integer
System.out.println(i instanceof Integer);
}
}
result:
true
Now if we want to check the class of the object at run time, then we must use isInstance() method.
// Java program to demonstrate working of isInstance()
// method
public class Test
{
// This method tells us whether the object is an
// instance of class whose name is passed as a
// string 'c'.
public static boolean fun(Object obj, String c)
throws ClassNotFoundException
{
return Class.forName(c).isInstance(obj);
}
// Driver code that calls func()
public static void main(String[] args)
throws ClassNotFoundException
{
Integer i = new Integer(5);
// print true as i is instance of class
// Integer
boolean b = fun(i, "java.lang.Integer");
// print false as i is not instance of class
// String
boolean b1 = fun(i, "java.lang.String");
/* print true as i is also instance of class
Number as Integer class extends Number
class*/
boolean b2 = fun(i, "java.lang.Number");
System.out.println(b);
System.out.println(b1);
System.out.println(b2);
}
}
result:
true
false
true
NOTE: instanceof operator throws compile time error(Incompatible conditional operand types) if we check object with other classes which it doesn’t instantiate.
public class Test
{
public static void main(String[] args)
{
Integer i = new Integer(5);
// Below line causes compile time error:-
// Incompatible conditional operand types
// Integer and String
System.out.println(i instanceof String);
}
}
Result :
13: error: incompatible types: Integer cannot be converted to String
System.out.println(i instanceof String);
^