userList.clear(); userList.add(new User(3, "Tom3")); userList.add(new User(2, "Tom2")); userList.add(new User(1, "Tom1")); Optional<List<User>> optional = Optional.ofNullable(userList); optional.ifPresent(new Consumer<List<User>>() { @Override public void accept(List<User> users) { users.stream().forEach(user -> { System.out.println(user.getName()); }); } }); // Tom3 // Tom2 // Tom1 Optional<List<User>> optional2 = Optional.ofNullable(userList); optional.ifPresent(users -> { users.stream().forEach(user -> { System.out.println(user.getName()); }); }); // Tom3 // Tom2 // Tom1 userList.clear(); Optional<List<User>> optional3 = Optional.ofNullable(userList); optional.ifPresent(users -> { users.stream().forEach(user -> { System.out.println(user.getName()); }); }); //输出无内容
更多详细请阅读:https://blog.csdn.net/aitangyong/article/details/54564100