Group a list of strings by length

Question

Is it possible to write this using the stream element? I can get the most of it with stream but it is the add adding string to list which is in the map that I am caught on.

/**
 * Given a list of strings, returns a map where the keys are the unique strings lengths found in the list and the values are the strings matching each length
 * [hello,world,cat,ambulance] would return {5 -> [hello, world], 3 -> [cat] -> 3, 9 -> [ambulance]}
 * @param strings
 * @return
 */
public Map<Integer,List<String>> mapLengthToMatchingStrings(List<String> strings){
 Map<Integer,List<String>> mapILS = new HashMap<Integer, List<String>>();
 for (String str : strings) {
 int length = str.length();
 if (mapILS.containsKey(length)) {
 // Add str to list at key
 //Get list at key
 List<String> rs = mapILS.get(length);
 //Add str to list
 rs.add(str);
 //Update key in map
 mapILS.put(length, rs);
 }else {
 // Add new key list to map
 //Create list
 List<String> rs = new ArrayList<String>();
 //Add string
 rs.add(str);
 //Add key and list to map
 mapILS.put(length, rs);
 }
 } 
 return mapILS;
} 

• Mast: For anyone voting to close this question: yes, it could be worded better. But the code seems to work and there's a description in the title & comments. The specific request is usually off-topic, but it would've been the likely outcome in a review anyway. John, for your next question, please read our FAQ on asking questions.

Answer 1

It's not only possible but also a great example of how functional programming can make code much simpler and readable:

 public Map<Integer,List<String>> mapLengthToMatchingStrings(List<String> strings){
 return strings.stream().distinct().collect(Collectors.groupingBy(String::length));
 }

(The distinct part is optional, regarding whether you want duplicates in the lists)

• John O Sullivan: Wow, that is great. So to the point.