code: static <K, V> void putAllIfAbsent(Map<K, V> target, Map<K, V> source) {
  
          for (Map.Entry<K, V> entry : source.entrySet()) {
              if (!target.containsKey(entry.getKey())) {
                  target.put(entry.getKey(), entry.getValue());
              }
          }
      }
docs: Puts entries from the {@code source} map into the {@code target} map, but without overriding
  any existing entry in {@code target} map, i.e. put only if the key does not exist in the
  {@code target} map.
  
  @param target The target map where to put new entries.
  @param source The source map from which read the entries.