JDK8 List.stream()用法

1.取出某个属性返回list

List<Long> ids= records.stream().map(User::getId).collect(Collectors.toList());

List<String> names= records.stream().map(User::getName).collect(Collectors.toList());

2.取出属性,用逗号分割拼接返回String

String nameStr= records.stream().map(User::getName).collect(Collectors.joining(","));

3.按id对实体分组：

Map<Long, User> userMap = records.stream().collect(Collectors.toMap(User::getId, Function.identity()));

4.按id对属性分组：

Map<Long, String> nameMap= records.stream().collect(Collectors.toMap(User::getId, User::getName));

5.从全部的集合中，过滤出符合条件的集合

//allList 全部集合

//includeIds 需要取出的 ids集合

Set<Long> includeIds= allList.stream().map(User::getUserId).collect(Collectors.toSet());

//hitList 最终的集合

List<User> hitList = allList.stream().filter(ent -> includeIds.contains(ent.getId())).collect(Collectors.toList());

//过滤 不符合条件的集合

List<User> missList = allList.stream().filter(ent -> !includeIds.contains(ent.getId())).collect(Collectors.toList());