What's difference between these generic methods?

Question

What's difference between these?

public <T extends MyClass> void myMethod (ArrayList<T> list) {
}

public void myMethod (ArrayList<? extends MyClass> list) {
}

Possible duplicate of http://stackoverflow.com/questions/16707340/when-to-use-wildcards-in-java-generics — drrob, Aug 19 '15 at 09:44
See also https://docs.oracle.com/javase/tutorial/extra/generics/wildcards.html — drrob, Aug 19 '15 at 09:48

score 3 · Accepted Answer · answered Aug 19 '15 at 09:28

<T extends MyClass>
- Denotes a specific sub-type of MyClass, called T.
- You can actually refer to the type T within the method implementation. For example, you can assign elements from the list to new variables of type T.
- You can both add and remove elements from the list.
<? extends MyClass>
- Denotes a whole family of sub-types of MyClass. The exact substitute is unknown at compile-time and thus, cannot be referred, nor new variables of this very type can be introduced.
- You cannot add new elements to the list (because the actual type is unknown)
- You can only get elements from the list. They, however, can be only assigned to variables of type MyClass. Additional down-casting may be needed for some specific sub-types of MyClass.

score 2 · Answer 2 · answered Aug 19 '15 at 09:21

2

In the first case, you can refer to T as being the type of the MyClass or sub-class.

In the second case T can be what ever you have defined outside this method (or nothing if you don't have a type called T)

answered Aug 19 '15 at 09:21

Peter Lawrey

2 Answers2