java8 集合笔记

class User(){
private String userCode;
private String userName;
}

List<User> userList;

//list 集合提取字段数组
String [] productCodes = userList.stream().map(User::getUserCode).toArray(String[]::new);

//list 集合提取某个字段 转换为新的list
List<String> userCodes = userList.stream().map(user->user.getUserCode()).collect(Collectors.toList());

//list 按照字段分组 依据字段等值的 放在同一个集合并指向KEY
Map<String, List<User>> userListMap = userList.stream().collect(Collectors.groupingBy(User::getUserCode));