Java solution of Pramp - Word Count Engine problem

Solution.java

import java.io.*;
import java.util.*;

class Solution {
  static String tokenizeString(String str) {
    int len = str.length();
    String result = "";
    
    for (int i = 0; i < len; i++) {
      char c = str.charAt(i);
      
      // upper-case
      if (c >= 65 && c <= 90) {
        c += 32;
        result += c;
      }
      // lower-case
      else if (c >= 97 && c <= 122) {
        result += c;
      }
      // space
      else if (c == 32) {
        if (result.charAt(result.length()-1) != 32) {
          result += c;
        }
      } 
      // anything else
      else {
        continue;
      }
    }
    
    return result;
  }
  
  static String[][] wordCountEngine(String document) {
    document = tokenizeString(document);
    HashMap<String, Integer> hm = new HashMap<String, Integer>();
    String str = "";
    int length = document.length();
    int maxCount = 0;
    
    for (int i = 0; i < length; i++) {
        char c = document.charAt(i);
        int count = 0;
        if (c == 32) {
            count = addToHash(hm, str);
            str = "";
        } else if (i == length-1) {
            str += c;
            count = addToHash(hm, str);
        } else {
            str += c;
        }
        
        maxCount = Math.max(maxCount, count);
    }

    ArrayList<ArrayList<String>> bucketList = new ArrayList<ArrayList<String>>();
    for (int i = 0; i < maxCount + 1; i++) {
        bucketList.add(new ArrayList<String>());
    }
    
    for (String key : hm.keySet()) {
        int count = hm.get(key);
        bucketList.get(count).add(key);
    }
    
    for (int i = 0; i < bucketList.size(); i++) {
        Collections.sort(bucketList.get(i), String.CASE_INSENSITIVE_ORDER);
    }
    
    String[][] result = new String[hm.size()][2];
    int index = 0;
    for (int i = maxCount; i > 0; i--) {
        for (int j = 0; j < bucketList.get(i).size(); j++) {
            result[index][0] = bucketList.get(i).get(j);
            result[index++][1] = Integer.toString(i);
        }
    }
    
    
    return result;
  }

  private static int addToHash(HashMap<String, Integer> hm, String key) {
      int count = 0;
      if (hm.containsKey(key)) {
          count = (int) hm.get(key) + 1;
          hm.put(key, count);
      } else {
          count = 1;
          hm.put(key, count);
      }
      
      return count;
  }

  public static void main(String[] args) {
    String str = "Practice makes perfect. you'll only get Perfect by practice. just practice!";
    Solution.wordCountEngine(str);
    
  }

}