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