Group Anagrams

import java.util.Arrays;
import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
import java.util.stream.Collectors;

class Main {
    public static void main(String[] args) {
        /**
         * Given an array of strings strs, group the anagrams together.
         * 
         * Input: strs = ["eat","tea","tan","ate","nat","bat"]
         * Output: [["bat"],["nat","tan"],["ate","eat","tea"]]
         */
         
         String[] strs = {"eat","tea","tan","ate","nat","bat"};
         
        // Map to collect the sorted string as key and the ocurrences in the list as value
        Map<String, List<String>> occurrencesGroups = new TreeMap<>();
                
        // Iterating the original strings to check one by one
         for (String str : strs) {
             // Sorting string to prepare it for searching
             String sortedStr = sortString(str);
             
             // Value reference to add occurrences
             List<String> occurrences = null;
             // If the string has occurrences, then get the List of occurrences
             // Otherwise, put the string as key and add a new list to add new 
             // occurrences
             if (occurrencesGroups.containsKey(sortedStr)) {
                 occurrences = occurrencesGroups.get(sortedStr);
             } else {
                 occurrences = new ArrayList<String>();
                 occurrencesGroups.put(sortedStr, occurrences);
             }
             // Adding occurrence to the reference list
             occurrences.add(str);
         }
         
         // Collecting values to match expected output format
         List<List<String>> occurrencesList = new ArrayList<>(occurrencesGroups.values());
         System.out.println(occurrencesList);
    }
    
    private static String sortString(String str) {
        return Arrays.stream(str.split(""))
                    .sorted()
                    .collect(Collectors.joining());
    }
}