Скачать или смотреть Understanding JAVA Generics Constructor Issue: Why B Works as A

Explore the intricacies of JAVA generics. Understand why using an object of type B as a parameter in the MyList constructor accepting A does not throw an error.
---
This video is based on the question https://stackoverflow.com/q/67846081/ asked by the user 'blueMountain' ( https://stackoverflow.com/u/15862648/ ) and on the answer https://stackoverflow.com/a/67846160/ provided by the user 'robymus' ( https://stackoverflow.com/u/16126413/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: JAVA generics constructor issue

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding JAVA Generics Constructor Issue: Why B Works as A

When delving into the world of JAVA generics, programmers often encounter situations that can lead to confusion. A common question arises around the type safety of generics, especially when dealing with class inheritance. This guide aims to clarify one such scenario that may leave you puzzled: why you can pass an object of type B (subclass) where an object of type A (superclass) is expected.

The Problem

You might find yourself asking:

Why can I pass an instance of class B into a constructor that expects an object of class A?

Here’s the relevant code snippet that illustrates this issue:

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

In this line, obj is an instance of class B, while MyList is defined to accept a type parameter TP1, which is set to A. This should, in theory, cause a compilation error. However, JAVA's rules around inheritance and polymorphism come into play, allowing this code to compile successfully.

Understanding Inheritance and Assignment

Class Hierarchy

Let’s take a closer look at the classes involved:

Class A: the base class.

Class B: the subclass of A, which inherits all behaviors (methods and fields) from class A.

In simple terms, class B is a specialized version of class A. Hence, an instance of B can be treated as an instance of A without any explicit casting. This intrinsic relationship is a cornerstone of object-oriented programming.

Why It Works

Inheritance: B extends A, meaning every instance of B is also an instance of A. This is called is-a relationship; therefore, JAVA allows B to be used wherever A is expected.

Generics Flexibility: The generic type parameter TP1 in MyList<TP1, TP2> doesn’t restrict the use of subclasses. When you define your MyList<A, B>, you are stating that TP1 can be of any type that is A or any of its subclasses.

Example Explained

In the provided code, when an instance of B (the argument obj) is passed into the MyList constructor, it can be assigned to data1, which is defined as type TP1 (or A in the MyList<A, B> instantiation). JAVA doesn’t balk at this because:

B is a subtype of A.

The JAVA compiler allows you to pass subclasses whenever a superclass reference is expected.

Conclusion

Understanding this fundamental concept of inheritance and polymorphism helps in clearing up confusion surrounding generics in JAVA. You can effortlessly work with generics knowing that subclasses can substitute their parent classes wherever applicable.

If you ever encounter similar situations in your coding journey, just remember: as long as a class is a subclass of the specified type, you should be able to use it where that type is expected. Keep exploring JAVA’s generics, and don't hesitate to experiment with different class hierarchies to see how they interact!

By embracing these principles, you can make the most out of JAVA's powerful features, helping you create more robust and flexible applications. Happy coding!