code: Map<String, Integer> map = new HashMap<String, Integer>();
  map.put("hello", 1);
  map.put("world", 2);
  map.put("!", 3);
  
  // iterating over key-value pairs:
  for (Map.Entry<String, Integer> e : map.entrySet()) {
      String key = e.getKey();
      Integer value = e.getValue();
      System.out.println("key = " + key + ", value = " + value);
  }
  
  // iterating over keys:
  for (String key : map.keySet()) {
      System.out.println("key = " + key);
  }
  
  // iterating over values:
  for (Integer value : map.values()) {
      System.out.println("value = " + value);
  }
  