関連ワード:  use of optional class in java 8    use of optional class in java 8 example    why we use optional class in java 8    purpose of optional class in java 8    advantages of optional class in java 8    optional class in java 8    optional class in java 8 with example    optional class in java 8 interview questions    optional class in java 8 methods    optional class in java 8 example javatpoint