具有通用返回类的Java方法签名

Question

What is the difference between the following 2 method signature:

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

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

Answer 1

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.

Answer 2

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.