Returning List of given argument type - java

Question

I have the following method

static <T> List<T> foo(List<T> arg) {

I want to create a List of the same type, call it outcome, as arg that I will eventually return. For example if

ArrayList<Integer> arg = new ArrayList<Integer>();

I want foo to return an ArrayList<Integer> as well. If

LinkedList<Integer> arg = new LinkedList<Integer>();

then I want foo to return a LinkedList<Integer>

I know List<T> outcome = new List<T>(); will not work since List is abstract, so what am I suppose to write to do what I want?

Answer 1

You can use reflection

List<T> list = (List<T>)arg.getClass().newInstance(); // throws

You can further constraint your method signature

static<L extends List<?>> L foo(L arg) { ... }


LinkedList<Integer> x = foo( new ArrayList<Integer>() ); // doesn't compile

Answer 2

When you have a certain return type, you can return any subclass of that return type, so you can return an ArrayList<T>.

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EDIT (1): If you want to return a List of the same type as your argument, you could use instanceof:

static <T> List<T> foo(List<T> arg) {
    List<T> outcome = null;

    if (arg instanceof ArrayList)
        outcome = new ArrayList<T>();

    if (arg instanceof LinkedList)
        outcome = new LinkedList<T>();

    ...

    return outcome;
}

You can find a full list of the classes that implement List here.

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EDIT (2): Here's another idea:

static <T, U extends List<T>> U foo(U arg) {
    U outcome = arg;    
    ... 
    return outcome;
}

You might destroy the list you pass as arg, however, but you should be able to pass a copy.