Java 8，列表 <Employee> 映射部门和地址列表

Question

I have a class like classe User{ String department; String address; } i have list of user,can i get below output using java 8 stream/map/collect

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

Use groupingBy collector with mapping collector downstream.

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

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