Java generics input type vs returned type

Question

I'm learning about generics and am slightly confused. I'm confused about the difference between the input type and returned type for a class using generics.

   Stack<Integer> even = new Stack<>();
    // pushing values in stack
    even.push(0);
    even.push(2);

    System.out.println(even.pop().getClass().getSimpleName());
    System.out.println("pop => " + even.pop());

We are pushing in an int value of 0 and 2. But the first print statement will print "Integer". If the stack is declared with Integer as its generic type, why are we able to push in a primitive "int"? If we can push in a primitive "int" why is the pop() class returning a wrapper class "Integer"?

I'm clearly misunderstanding something about generics.

Answer 1

You cannot have a generics declared as primitive types, because they are not classes. They are other thing (primitive types, of course). Instead, you use the corresponding wrapper class.

When you do the push(int), there is an implicit cast to the wrapper class Integer via the Integer#valueOf(int) method.

stack.push(0);

Is transformed, by the compiler and without asking into this

stack.push(Integer.valueOf(0))

For the same reason, you can do something like this

Integer n = 3;

And get no errors.