如何正确使用hashSet？ （从C ++移到Java）

Question

I'm approaching (after c++) to java and, in particular, with android app development (Android studio) but I've some trouble with hashSet usage. Here's is the issue: I cannot find a way to store map's keys obtained from "keySet()" method in any container; I tried to store keys in a string vector or a list but an error occurs. How can I fix that?

This is a piece of code:

 private void fillMap(Map<String, String> map, String[] strings){

    String str;

    map.put("Question1", "answer1");
    map.put("Question2", "answer2");
    map.put("Question3", "answer3");

    Set<String> keys = map.keySet();


}

As you can see I created a map whose keys are questions and its values are answers. Then I created a Set where to put all the keys; The problem is that I would try to store the content of the set in an ordered container but everything I tried gives me error.

Answer 1

Assume you have a Map<String, Integer> then its keySet() is a Set<String> an interface derived from the even more general Collection<String> . This collection you can add to everything.

Set<String> keys = map.keySet();
List<String> list = new ArrayList<>(keys); // 1
list.sort();

Collections.addAll(list, keys); // or 2
Collections.sort(list); 

SortedSet<String> set = new TreeSet<>(keys);

And a warning to a C++ programmer: Vector is a very old inefficient class; use ArrayList instead, hence variables, parameters, result types List.