`InstanceOf`

Is a variable an instance of a class? If we have a variable of a base class like Object, we can determine what the derived class is with instanceof.

And with instanceof, we can get avoid casting directly and simply access a variable of the derived class we tested for. This can lead to cleaner Java code.

Example

We have a class Program that is declared in this Java program. Because Program is a class, it is derived automatically from Object, the ultimate base class.

Part 1 We create an instance of Program with the new operator, but refer to this as an Object. The instanceof operator then returns true.

Part 2 Here we refer to an ArrayList of Strings as an object, and the instanceof operator also matches the ArrayList class.

Part 3 It is possible to get an instance of the type we tested for—here we have a String, and we get a String from instanceof.

public class Program {
    public static void main(String[] args) {

        // Part 1: create an instance of a class and refer to it by a base class Object.
        Object value = new Program();
        if (value instanceof Program) {
            System.out.println("Object is instanceof Program");
        }

        // Part 2: use instanceof with a type parameter.
        Object value2 = new java.util.ArrayList<String>();
        if (value2 instanceof java.util.ArrayList) {
            System.out.println("ArrayList");
        }

        // Part 3: use instanceof and declare a name for the result of the cast.
        Object value3 = new String("bird");
        if (value3 instanceof String s) {
            System.out.println("String = " + s);
        }
    }
}Object is instanceof Program
ArrayList
String = bird

Summary

With instanceof in Java, we can test is a variable can be cast to another type. And we can even store the cast variable and use it later in our code.