Why is it wrong to supply type parameter in the constructor of a generic class (Java)?

Question

I\'m just learning about generics in Java from a textbook, where it talks about a class GenericStack implemented with an ArrayList

Answer 1

You are confusing a method's declaration with its call site (that is, where you use it). If you want to use a generic that's declared on the class, only ever need to provide the or <> at the call site — not the declaration. And in that regard, a constructor is declared like any other method:

public GenericStack {
    public E get() { ...            // correct
    public  put(E element) { ... // bad (see below)
    public GenericStack() { ...     // correct, like the get()

Redeclaring the generic (as in the put case above) actually shadows it; the E in put(E) is actually a different E than in GenericStack, despite having the same name. This is confusing, and you should avoid doing it.