Why are arrays covariant but generics are invariant?

Question

From Effective Java by Joshua Bloch,

1. Arrays differ from generic type in two important ways. First arrays are covariant. Generics are invariant.

2. Cov

Answer 1

May be this help:-

Generics are not covariant

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.

It turns out there's a good reason it doesn't work that way: It would break the type safety generics were supposed to provide. Imagine you could assign a List to a List. Then the following code would allow you to put something that wasn't an Integer into a List:

List li = new ArrayList();
List ln = li; // illegal
ln.add(new Float(3.1415));

Because ln is a List, adding a Float to it seems perfectly legal. But if ln were aliased with li, then it would break the type-safety promise implicit in the definition of li -- that it is a list of integers, which is why generic types cannot be covariant.