Simplify the following 2 for-loop using java streams

I am trying to achieve the following by converting two for loops using Java Streams.

    Set<Type2> set = new HashSet<>();
    for (Test1 test1 : testlist1){ // testlist1 is a list
      for (Test2 test2 : test1.getList()){ // test1.getList returns a list
        set.add(test2);
      }
    }

New to Java streams and trying to convert it.

>Solution :

You can use Stream.flatMap() to achieve the same result:

Set<Type2> set = testlist1.stream()             // Stream<Type1>
    .flatMap(test1 -> test1.getList().stream()) // Stream<Type2>
    .collect(Collectors.toSet());

Another option is to use Java 16 Stream.mapMulti(), which would be more performant than flatMap() in case if lists returned by test1.getList() would contain only a few elements or even can be empty.

Set<Type2> set = testlist1.stream()
    .<Type2>mapMulti((test1, c) -> test1.getList().forEach(c))
    .collect(Collectors.toSet());