関連ワード:  java 8 check if null    java 8 check if string is null or empty    java 8 check if list is null or empty using optional    java 8 check list is null or empty    java 8 check if list is null or empty before stream    java 8 check if list is null or empty    java 8 check if object is null    java 8 check if map is null or empty    java 8 check if collection is null or empty    java 8 check if object is null or empty