如何转换列表<set<string> &gt; 列出<list<string> &gt; 在 java 中？ </list<string></set<string>

Question

How can we convert List<Set> to List<List> in java easily? Context: I came across this issue where I have generated List<Set> while in function return expects List<List>.

Answer 1

Do you care about order in the Set s? If you do, you'll have to make sure you don't have them as Sets in the first place. If you don't, you can use the Java 8+ streams API:

list.stream().map(ArrayList::new).collect(Collectors.toList());

Or a for loop.

Answer 2

To convert from a Set to a List, you can simply do new ArrayList<>(mySet) . So you can do something like:

Set<String> set1 = new HashSet<>(Arrays.asList("a", "b"));
Set<String> set2 = new HashSet<>(Arrays.asList("c", "d"));
List<List<String>> result = new ArrayList<List<String>>();
List<Set<String>> listOfSets = new ArrayList<>();
listOfSets.add(set1);
listOfSets.add(set2);
for (Set<String> set : listOfSets) {
    result.add(new ArrayList<>(set));
}
System.out.println(result);

Output: [[a, b], [c, d]]