CODEWITHSUNDEEP
javagenericscollections
Vasu
Vasu

Reputation: 4982

Why there is <? extends T> instead of <T> in java.util.Collections.unmodifiableList API

Why there is <?e extends T> instead of <T> in unmodifiable APIs of java.util.Collections e.g. consider unmodifiableLis API

public static <T> List<T> unmodifiableList(List<? extends T> list)

Why not, just

public static <T> List<T> unmodifiableList(List<T> list)

Edit: This is not duplicate of When do Java generics require instead of and is there any downside of switching. That question intend to find the importance of <? extends T> over <T> and what would be the impact if change is done from one to other. I understand the difference in both syntax (<? extends T> and <T>), however in case of unmodifiable collection APIs, context (and hence my question) is different.

Upvotes: 2

Views: 198

Answers (1)

Ted Hopp
Ted Hopp

Reputation: 234807

It's so you can do things like this:

List<Integer> intList = new ArrayList<Integer>();
// . . . populate intList

// now create an unmodifiable version:
List<Number> numList = Collections.unmodifiableList(intList);

Note that because generics are not covariant, so you cannot assign from a List<Integer> to a List<Number>:

numList = intList;

even though each element would be assignment-compatible. If you changed the signature of unmodifiableList as you suggest, it would run into the same problem (since the return type in my example would be forced to be List<Integer> once the parameter was bound).

Upvotes: 3

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