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There is Data Set<Long> input and Map<Long, List<String>> store.

And I wanna get List only 10 size.

Map<Long, List<String>> datas = {...{;
Set<Long> input = {....};
List<String> output = new ArrayList<>();
for(Long key : input) {
    if(output.size() >= 10) break;
    List<String> tmp = datas.get(key);
    for(String val : tmp) {
        if(output.size() >= 10) break;
        output.add(val);
I can't use java 9 doWhile in stream
A corresponding stream pipeline can be
List<String> output = input.stream()
    .flatMap(i -> datas.get(i).stream()).limit(10)
    .collect(Collectors.toList());
Test:
Map<Long, List<String>> datas = new HashMap<>();
datas.put(1l, Arrays.asList("a", "b", "c", "d", "e", "f", "g"));
datas.put(2l, Arrays.asList("z", "y", "x", "w", "v", "u", "t"));
Set<Long> input = new LinkedHashSet<>(Arrays.asList(1l, 2l, 3l, 4l, 5l));
System.out.println(input.stream()
        .flatMap(i -> datas.get(i).stream()).limit(10)
        .collect(Collectors.toList()));
And that prints [a, b, c, d, e, f, g, z, y, x]
                @EdgarHan It doesn't. limit(10) will stop it. Test the code (or add .peek(System.out::println) before collect to see what keys were looked up. And this code would throw a nullpointerexception if all keys from input were looked up.
– ernest_k
                Feb 26, 2021 at 1:44
                @ernest_k it may or may not see "w", that’s an implementation detail. With Java 8 before update 222, as well as Java 9, the peek before limit will even see "v", "u", and "t". See this Q&A. With a parallel stream, even the exceptions due to calling stream() on null may occur. It’s not a good idea to rely on the laziness…
– Holger
                Feb 26, 2021 at 9:07
                Yes that’s only an issue of the example program; for the OP’s task, the approach is sufficient.
– Holger
                Feb 26, 2021 at 9:49
        
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