This is a common source of confusion - generics are not covariant in Java.
For a really good explanation of this please see Java theory and practice: Generics gotchas:
While you might find it helpful to think of collections as being an abstraction of arrays, they have some special properties that collections do not. Arrays in the Java language are covariant -- which means that if Integer extends Number (which it does), then not only is an Integer also a Number, but an Integer[] is also a Number[], and you are free to pass or assign an Integer[] where a Number[] is called for. (More formally, if Number is a supertype of Integer, then Number[] is a supertype of Integer[].) You might think the same is true of generic types as well -- that List is a supertype of List, and that you can pass a List where a List is expected. Unfortunately, it doesn't work that way.