Java Method signature with generic return class

What is the difference between the following 2 method signature:

public <T extends MyClass> Set<T>开发者_运维问答 getMyList() {...}

public Set<? extends MyClass> getMyList() {...}

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In the first one, T is accessible in method body, while in the second one it is not. Whether that's useful depends on the method's job.

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For first one you can use it as:

Set<MyClass> set = thing.getMyList();
Set<MyDerivedClass> set = thing.getMyList();
Set<?> set = thing.getMyList();
Set<? extends MyClass> set = thing.getMyList();
Set<? extends MyDerivedClass> set = thing.getMyList();

For the second, you are more limited:

Set<?> set = thing.getMyList();
Set<? extends MyClass> set = thing.getMyList();

How would you expect to implement the first? The second is bad because it forces client code to use wildcards.