Why Can't I Add Elements to a List

Understanding List extends Number> Data Structures

In Java, understanding the limitations of using wildcards with generics is crucial. Let's delve into the specific issue of adding elements to a List declared as List extends Number>:

When you attempt to add an integer to foo3, you may encounter an error similar to:

Why You Can't Add to List extends Number>

The wildcard ? extends Number in List extends Number> foo3 implies that foo3 can hold values from a family of types that extend Number. This means that the following assignments are all valid:

However, given this flexibility, it becomes impossible to determine with certainty what kind of object you can add to foo3 without violating its integrity. Adding an Integer is forbidden because foo3 might be pointing to a List. Similarly, adding a Double is prohibited because foo3 might be referencing a List.

The Guarantee of Abstraction

In essence, List extends T> guarantees that you can only read from it and will get a T or subclass of T. Since you can't guarantee the specific type of the list, you can't safely add to it.

In contrast, List super T> allows you to add values of type T or its subclasses because you can be certain that the list can hold those values without violating its type safety.

Example: Understanding Collections.copy()

Consider the signature of Collections.copy():

Notice how the wildcard for src (? extends T) allows you to pass any list that holds values of type T or its subclasses, guaranteeing that the values produced by src will be of type T or its subclasses. This enables the method to process various list types, copying values safely.

Conclusion

Understanding the limitations of using wildcards with generics is essential for writing robust Java code. Remember that you can't add to a List extends T> because you can't guarantee the specific type of the list it's pointing to. Instead, you can only read from it and be assured of getting values of type T` or its subclasses.

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