code: @NullSafe
    @Override
    public boolean contains(KEY key) {
      return (key != null && this.map.containsKey(key));
    }
docs: Determines whether this {@link Cache} contains an entry mapped with the given {@link KEY key}.
  
  @param key {@link KEY key} to evaluate.
  @return a boolean value indicating whether this {@link Cache} contains an entry
  mapped with the given {@link KEY key}.