How Does Java's `instanceof` Operator Work for Class and Interface Checking?

What is the 'instanceof' Operator Used For in Java?

The Java instanceof operator checks whether an object belongs to a specific class or implements a particular interface. It's a binary operator that returns a boolean value: true if the object is an instance of the specified type and false otherwise.

Understanding the Behavior of instanceof

Consider the following code snippet:

Object source = new Button();

if (source instanceof Button) {
    // ...
} else {
    // ...
}

In this example, the source variable holds an object that was created using the Button class. The instanceof operator is used to check if source is actually a Button object. If the check returns true (because source is a Button), the code inside the if block will be executed. Otherwise (if source is not a Button), the code in the else block will be executed.

Inheritance and instanceof

The instanceof operator considers inheritance relationships. Let's say we have the following class hierarchy:

interface Domestic {}
class Animal {}
class Dog extends Animal implements Domestic {}

If we have a Dog object dog, we can use instanceof to check if it belongs to different types:

dog instanceof Domestic // true
dog instanceof Animal // true
dog instanceof Dog // true

However, instanceof cannot check if an object is an instance of a supertype that is not in the same inheritance hierarchy. For example, the following check would not compile:

dog instanceof Cat // does not compile

Runtime Type Checking

instanceof is a runtime operation, which means it's performed during program execution. This allows programmers to determine the type of an object and handle it accordingly. It's commonly used in situations where an object's type is not known at compile time.

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