関連ワード:  java 8 null or empty check    java 8 check null or empty object    java 8 check string null or empty    java 8 check collection null or empty    java 8 check array null or empty    java 8 list null and empty check    java 8 optional null and empty check    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