Java generic return generic list of type of parameter

Question

I'd like to create the following:

//infer the type from parameter but restrict it to one of
// Proxy's subtype. return a list of this sub type
public static List<T> process(<T extends Proxy> proxy)
{
  return new ArrayList<T>(); //just for example
}

Usage:

List<ConcreteTypeOfProxy> list = process(new ConcreteTypeOfProxy());

The above example has compilation issues. I think this should logically be available in java, just not sure what the syntax is

Answer 1

//Any array that extends T
public static  <T extends Proxy> List<T>  process(T proxy)
{
    return new ArrayList<T>(); //just for example
}

//Returns array that has T type of parameter
public static  <T> List<T>  process(T proxy)
{
    return new ArrayList<T>(); //just for example
}

//Returns a map of generic type which you define in the method
public static <T, E extends Proxy> Map<T, E>  process(T key, E value)
{
    Map<T, E> map =  new HashMap<T, E>();
    map.put(key, value);
    return map;
}

Answer 2

You don't need any method, and consequently you don't need any parameters, to do this:

List<ConcreteTypeOfProxy> list = new ArrayList<>();

Remember: there is no difference between ArrayList<ConcreteTypeOfProxy> and ArrayList<AnyOtherType> : it's just an ArrayList .

The type parameter is merely an instruction to the compiler to check the type of what is added - at compile time only - and to automatically cast values obtained from the list.