Understanding instanceof in Java: Ensuring the Right Return Value

Описание: Learn how to use the `instanceof` operator in Java correctly, especially when dealing with interfaces and implementing classes.
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This video is based on the question https://stackoverflow.com/q/70345689/ asked by the user 'Võ Khắc Bảo' ( https://stackoverflow.com/u/16443791/ ) and on the answer https://stackoverflow.com/a/70345817/ provided by the user 'Marcello Zago' ( https://stackoverflow.com/u/16872314/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding instanceof in Java: Ensuring the Right Return Value

As a self-taught Java programmer, it's crucial to understand how the instanceof operator operates, especially when working with interfaces and their implementing classes. This post will help clarify a common point of confusion regarding the return values of instanceof, specifically when checking if an interface instance belongs to its implementing class.

The Problem: Confusion with instanceof

A newbie Java developer posed an interesting question: When using instanceof to check if an interface instance is of an implementing class, why did the return value come out as false instead of true? They were struggling with how to set this up correctly in their code. Let's break down the original code snippet for clarity:

[[See Video to Reveal this Text or Code Snippet]]

In the code above, the variable demo is initialized as an anonymous class implementing the Demo interface, not as an instance of class A. This is the root of the confusion.

The Solution: Correcting the Implementation

To achieve the expected behavior and to have result return true, we need to ensure that demo is assigned an instance of class A. Here’s how to correct the code:

Step 1: Create an Instance of the Implementing Class

Instead of creating an anonymous class, we assign demo as an instance of class A directly:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Use instanceof to Check the Type

Now that demo correctly references an instance of A, we can check if it is an instance of the Demo interface:

[[See Video to Reveal this Text or Code Snippet]]

Final Corrected Code

Here's the complete and corrected code that ensures the expected return value:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

It's essential to remember that with instanceof, the object type must directly match what you are comparing against. In this case, having demo as an instance of A led to a return value of true when checked against the Demo interface. Understanding these principles is crucial for mastering Java and object-oriented programming.

Feel free to reach out if you have more questions or need further explanations on Java concepts. Happy coding!