What does Java's type parameter wildcard really mean? What's the real difference between Foo and Foo<?>?

Question

For a generic interface:

public interface Foo {
    void f(T t); 
} 

The difference between the two fields:

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Answer 1

Object is the supertype of all types in java. However, Foo is not the supertype of all Foo. The supertype of all Foo is Foo. see http://docs.oracle.com/javase/tutorial/extra/generics/wildcards.html for more details.