根据密钥大小对HashMap密钥进行排序

Question

I have a HashMap in which keys are Strings . I need to take the keys and sort it based on the key size ie String length and then store it in some ArrayList .

Note: If two keys size is same, then we can place it in any order.

For eg.

hm.put("abc","hi");
hm.put("A","hi");
hm.put("ab","hi");
hm.put("a","hi");

My ArrayList should have elements in order abc,ab,a,A .

Can someone let me know what's the best possible way to do it.

Answer 1

You can sort the keys by length using a custom Comparator like this:

List<String> s = new ArrayList<>(hm.keySet());
Collections.sort(s, new Comparator<String>(){
            @Override
            public int compare(String s1, String s2){
                 return Integer.compare(s2.length(), s1.length());
            }
});

---

Another possibility would be to use a TreeSet providing also here a custom Comparator to its constructor.

TreeSet<String> ts = new TreeSet<>(new Comparator<String>(){
                     @Override
                     public int compare(String s1, String s2){
                           int cmp = Integer.compare(s2.length(), s1.length());
                           return cmp != 0 ? cmp : s1.compareTo(s2);
                     }
});
ts.addAll(hm.keySet());

Just note here that if the length of the pair of Strings you compare is the same, you have to compare also their lexicographical order, otherwise the TreeSet would just contains the Strings that have different length (ie either "a" or "A" wouldn't be added to the set)