java 8,list以映射部门和地址列表

Question

我有一个类，像classe User{String department；String address；}我有一个用户列表，我能用java 8 stream/map/collect得到下面的输出吗

List<User> userList=getUserList();

Map<String,List<String>> userAddressMap=new HashMap<String,List<String>>();

for(User user : userList){

   List<String> addressList=userAddressMap.get(user.getDepartment());

   if(addressList==null){
        userAddressMap.put(user.getDepartment(),addressList);
   }

   addressList.add(user.getAddress);
}

Answer 1

在下游使用groupingby收集器和mapping收集器。

import static java.util.stream.Collectors.*;

Map<String, List<String>> userAddressMap = userList.stream()
    .collect(groupingBy(
        User::getDepartment,
        mapping(User::getAddress, toList())
    ));