找不到符号方法名称错误

Question

   /**
    * Returns a string describing the song object
    */
   public String toString()
   {
      return "Name of Song:" + this.title + "Sang by:" + getSingerName("") + this.getLanguageOfSong("") + "Alert needed:" + this.isAlertNeeded();
   }

为什么我在getSingerName和getLanguageOfSong上找到无法找到符号的方法？