Generic multiple parameter of same type

Question

I am trying to understand generics. In the below code getDuplicate() return type PlaceHolder<X,X> has same parameter X which compiles fine. But when I use the same parameter type in MyClass<T,T> it says "type variable T already defined". Can someone explain how it's possible with getDuplicate method?

class PlaceHolder<K,V> {
   public K k;
   public K v;

   public PlaceHolder(K k, K v){
       this.k = k;
       this.v = v;
   }

   public K get(){ return k;    }

   public static <X> PlaceHolder<X,X> getDuplicateHolder(X x){
        return new PlaceHolder<X,X>(x,x);
   }

}

class MyTest<T,T> {}

Answer 1

The difference is that X is declared once and used twice where T is being declared twice.

In methods, you can declare type parameters with <> after modifiers but before the return type. Likewise, in classes and interfaces, you can declare type parameters after the class name but before any implements and/or extends clauses and before the class/interface body.

You may use these type parameters in the scope in which they're declared. Declaring a return type of PlaceHolder<X, X> is using X twice, but declaring a class MyText<T, T> is attempting to declare T twice.

A variable-equivalent analogy would be declaring a variable and using it twice:

int x;
...
x + x

vs. attempting to declare two variables with the same name.

int t;
int t;

You just need to make sure you know when you're declaring a type parameter and when you're using an existing type parameter.