How to declare an array of method references?

Question

I know how to declare an array of other things, like strings, in this way:

String[] strings = { "one", "two", "tree" };
// or
String[] strings = new String[] { "one", "two", "tree" };

But when it comes to method references, I can't figure out how to avoid having to create a list and add every item individually.

Example: Call the method smartListMerge on several diferent matching lists from two sources:

List<Function<TodoUser, TodoList>> listGetters = new ArrayList<>(3);
listGetters.add(TodoUser::getPendingList);
listGetters.add(TodoUser::getCompletedList);
listGetters.add(TodoUser::getWishList);

TodoUser userA = ..., userB = ...;
for (Function<TodoAppUser, TodoList> listSupplier : listGetters) {
    TodoList sourceList = listSupplier.apply(userA);
    TodoList destinationList = listSupplier.apply(userB);

    smartListMerge(sourceList, destinationList);
}

What is the right way to declare an array of method references?

Do you want a List or an array? – Eran Mar 12 '18 at 12:13 — Eran, Mar 12 '18 at 12:13

score 3 · Accepted Answer · answered Mar 12 '18 at 12:15

There are shorter ways to create a List:

List<Function<TodoUser, TodoList>> listGetters = Arrays.asList(TodoUser::getPendingList,
                                                               TodoUser::getCompletedList,
                                                               TodoUser::getWishList);

or (in Java 9):

List<Function<TodoUser, TodoList>> listGetters = List.of(TodoUser::getPendingList,
                                                         TodoUser::getCompletedList,
                                                         TodoUser::getWishList);

`What is the right way to declare an array of method references` - OP asks, `List#of` or `Arrays#asList` the answer :) +1, but still funny — Eugene, Mar 12 '18 at 13:17

score 1 · Answer 2 · answered Mar 12 '18 at 12:32

In your question, you asked of an array of methods. You can define it like this:

Method toString = String.class.getMethod("toString"); // This needs to catch NoSuchMethodException.
Method[] methods = new Method[] { toString };

Yet, in your example you work with functionals, which you can also put into an array:

Function<String, String> toStringFunc = String::toString;
Function[] funcs = new Function[] { toStringFunc };

Keep in mind to import java.lang.reflection.Method for the first or java.util.function.Function for the second example, respectively.

As far as I know, there is no shorthand for defining a list.

`Function[] funcs` should trigger a warning – Eugene Mar 12 '18 at 12:36 — Eugene, Mar 12 '18 at 12:36

score 0 · Answer 3 · answered Mar 12 '18 at 12:30

0

You can't create a generic array, but you can declare one (there is warning though):

@SuppressWarnings("unchecked")
Function<String, Integer>[] arr = new Function[2];
arr[0] = String::length;

answered Mar 12 '18 at 12:30

Eugene

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How to declare an array of method references?

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