Creating Map composed of 2 Lists using stream().collect in Java

问题

As for example, there are two lists:

List<Double> list1 = Arrays.asList(1.0, 2.0);
List<String> list2 = Arrays.asList("one_point_zero", "two_point_zero");

Using Stream, I want to create a map composed of these lists, where list1 is for keys and list2 is for values. To do it, I need to create an auxiliary list:

List<Integer> list0 = Arrays.asList(0, 1);

Here is the map:

Map<Double, String> map2 = list0.stream()
                .collect(Collectors.toMap(list1::get, list2::get));

list0 is used in order list1::get and list2::get to work. Is there a simpler way without creation of list0? I tried the following code, but it didn't work:

Map<Double, String> map2 = IntStream
                .iterate(0, e -> e + 1)
                .limit(list1.size())
                .collect(Collectors.toMap(list1::get, list2::get));

回答1:

Instead of using an auxiliary list to hold the indices, you can have them generated by an IntStream.

Map<Double, String> map = IntStream.range(0, list1.size())
            .boxed()
            .collect(Collectors.toMap(i -> list1.get(i), i -> list2.get(i)));

回答2:

Indeed the best approach is to use IntStream.range(startInclusive, endExclusive) in order to access to each element of both lists with get(index) and finally use Math.min(a, b) to avoid getting IndexOutOfBoundsException if the lists are not of the exact same size, so the final code would be:

Map<Double, String> map2 = IntStream.range(0, Math.min(list1.size(), list2.size()))
    .boxed()
    .collect(Collectors.toMap(list1::get, list2::get));

回答3:

This works for me but is O(n^2):

    Map<Double, String> collect =
            list1.stream()
                    .collect(
                            toMap(Double::doubleValue, 
                                    item -> list2.get(list1.indexOf(item))));

来源：https://stackoverflow.com/questions/39962796/creating-map-composed-of-2-lists-using-stream-collect-in-java

标签

java-8

java-stream

collectors