List轉(zhuǎn)換為map

如，一個(gè)用戶列表List<User>轉(zhuǎn)換為Map<id, User>

List<User> userList = UserMapper.getUserList();
...
Map<Integer, User> userMap = userList.stream()
            .collect(Collectors.toMap(
                        User::getId, user -> user)
            );

此時(shí)如果userList有重復(fù)user會(huì)報(bào)錯(cuò)，那么可以這樣處理：

List<User> userList = UserMapper.getUserList();
...
Map<Integer, User> userMap = userList.stream()
            .collect(Collectors.toMap(
                        User::getId, user -> user, (v1, v2) -> v2)
            );

也就是當(dāng)map key重復(fù)時(shí)用v2覆蓋v1.

單獨(dú)取對(duì)象List的某項(xiàng)屬性

比較簡(jiǎn)單，直接上代碼
(從User列表取全部Id)

List<User> userList = UserMapper.getUserList();
...
//map里的方法可以自己定義方法來調(diào)用
List<Integer> ids = userList.stream().map(User::getId).collect(Collectors.toList());

根據(jù)指定屬性排序

stream().sort()結(jié)合Comparator可以用于排序

List<User> userList = UserMapper.getUserList();
...
//根據(jù)score升序排序
List<User> sortedUserList = userList.stream().sort(Comparator.comparing(User::getScore)).collect(Collectors.toList());
//根據(jù)score降序排序
List<User> sortedUserList = userList.stream().sort(Comparator.comparing(User::getScore).reversed()).collect(Collectors.toList());