Скачать или смотреть Understanding Why You Can’t Add an Integer to an ArrayList of ? extends Number

Explore the intricacies of Java generics and learn why it’s impossible to add an `Integer` to an ArrayList declared with a wildcard that extends the `Number` class.
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Understanding Why You Can’t Add an Integer to an ArrayList of ? extends Number

When working with Java's generics, you may encounter situations that can be puzzling, particularly when it comes to wildcards. A common question is: How do I add an Integer to an ArrayList that uses the wildcard ? extends Number? If you've found yourself stumped by this, you're not alone. In this guide, we will break down the problem and explore why the answer may be simpler than you think - it's just not possible.

The Scenario

You might be in a situation where you have an ArrayList defined to accept a wildcard that extends the Number class. The following is a snippet of code that clearly illustrates the issue:

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

The Problem Explained

What Does ? extends Number Mean?

The wildcard ? extends Number signifies that the ArrayList can hold objects of any type that is a subclass of Number—including Integer, Double, Float, etc. However, it doesn’t mean you can add to this list.

The Immutable Nature of Wildcards

When you declare your ArrayList with ? extends Number, you are essentially saying that you don't know the specific type of Number it will hold. This makes it impossible to add new elements to the list because the compiler has no way of ensuring that the type you're attempting to add is compatible with the declared wildcard. Here's why:

Any attempt to add an element—whether it’s an Integer, Double, or any other subclass of Number—could disrupt the integrity of the list.

For example, if you managed to add an Integer to an ArrayList<Double>, this would lead to type errors and inconsistencies, as the list is fundamentally intended to hold Double values only.

Illustrative Example

Consider the following code snippet:

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

In this case, whoknows and doubles reference the same list. If the code allowed adding an Integer to whoknows, you would essentially add it to the list of Double, which is not valid. Hence, the compiler restricts this action to prevent type safety violations.

Conclusion

Understanding generics and wildcards in Java is crucial for developing robust applications. While it might seem limiting that you can’t add an Integer to an ArrayList<?>, this restriction exists to maintain type safety throughout your code.

In summary, the answer to the question of how to add an Integer to an ArrayList<? extends Number> is quite straightforward: it simply can’t be done. The concept of wildcards requires that a level of restraint ensures you don’t violate the integrity of the data structure.

By mastering these fundamental principles of Java generics, you'll enhance your capabilities as a proficient Java developer.