是否存在通过多次调用函数来填充列表的Java方法？

Question

Is there something like this in the standard library? It fills a List<T> by calling source.get() :

public static <T> List<T> listFromFunc(int size, Supplier<T> source) {
    return IntStream.range(0, size)
            .mapToObj(i -> source.get())
            .collect(Collectors.toList());
}

Answer 1

I am afraid not, but you can improve readability of your code with this construct

Stream.generate(source).limit(size).collect(Collectors.toList())

Although as Brian Goetz mentioned your original version will parallelize much better.

Answer 2

Here's another variation:

static <T> List<T> listFromFunc(int size, Supplier<T> source) {
    return Collections.nCopies(size, "").stream()
        .map(o -> source.get())
        .collect(Collectors.toCollection(() -> new ArrayList<>(size)));
}

This enables stream to be SIZED throughout. Collections.nCopies().stream() actually uses IntStream.range() internally so it isn't faster or anything, though.

Using Collectors.toCollection() enables you to control the concrete type of the list, which toList() does not. In the sequential case, this also allows you to pre-size the list, avoiding copying that might occur as elements are added to the list.

(Fair warning: in JDK 8, toList() creates an ArrayList , but we might change this in JDK 9 to return a list type that's more amenable to appending.)