関連ワード:  how to use optional in java    how to use optional in java 8    how to use optional in java 8 streams    how to use optional in java 8 for null check    how to use optional class in java    how to use optional parameter in java    how to use optional ofnullable in java    how to use optional parameter in javascript    how to use optional class in java 8    how to use optional string in java