如何使用java 8将二维对象Set / ArrayList转换为一个Flat Set / List

Question

I'm new in java 8 , I have Set of Set for example:

Set<Set<String>> aa = new HashSet<>();
Set<String> w1 = new HashSet<>();

w1.add("1111");
w1.add("2222");
w1.add("3333");

Set<String> w2 = new HashSet<>();
w2.add("4444");
w2.add("5555");
w2.add("6666");

Set<String> w3 = new HashSet<>();
w3.add("77777");
w3.add("88888");
w3.add("99999");

aa.add(w1);
aa.add(w2);
aa.add(w3);

EXPECTED RESULT: FLAT SET...something like:

But it doesn't work!

// HERE I WANT To Convert into FLAT Set 
// with the best PERFORMANCE !!
Set<String> flatSet = aa.stream().flatMap(a -> setOfSet.stream().flatMap(ins->ins.stream().collect(Collectors.toSet())).collect(Collectors.toSet()));

Any ideas?

Answer 1

You only need to call flatMap once :

Set<String> flatSet = aa.stream() // returns a Stream<Set<String>>
                        .flatMap(a -> a.stream()) // flattens the Stream to a 
                                                  // Stream<String>
                        .collect(Collectors.toSet()); // collect to a Set<String>

Answer 2

作为@Eran正确答案的替代方案，您可以使用3参数collect ：

Set<String> flatSet = aa.stream().collect(HashSet::new, Set::addAll, Set::addAll);