如何在Java中将自定义对象列表重新组合到映射中

Question

我有以下对象列表

class Account {
int id;
String type;
int balance;
Customer customer;
// getters setters
}
class Customer {
int customerID;
}

List<Account> accounts = new ArrayList<>();
accounts.add(new Account(1, "abc", 17998210, new Customer(190)));
accounts.add(new Account(2, "hsj", 6786179, new Customer(190)));
accounts.add(new Account(4, "ioip", 246179, new Customer(191)));
accounts.add(new Account(4, "ewrew", 90179, new Customer(191)));

我想把上面的转移到Map，密钥应该是customerID，值应该是Account 的列表

Map<Integer, List<Account>>
Key            Value
190 -> Account(1, "abc", 17998210, 190)
Account(2, "hsj", 6786179, 190)
191 -> Account(4, "ioip", 246179, 191)
Account(4, "ewrew", 90179, 191)

如何做到这一点？

Answer 1

您可以使用Collectors.groupingBy。

Map<Integer, List<Account>> map =
accounts.stream().collect(Collectors.groupingBy(Account::getCustomerID));

演示

Answer 2

List<Account> accounts = new ArrayList<>();
accounts.add(new Account(1, "abc", 17998210, 190));
accounts.add(new Account(2, "hsj", 6786179, 190));
accounts.add(new Account(4, "ioip", 246179, 191));
accounts.add(new Account(4, "ewrew", 90179, 191));
Map<Integer, List<Account>> accountsMap = new HashMap<>();
for (Account account : accounts) {
accountsMap.computeIfAbsent(account.customerID, k -> new ArrayList<>()).add(account);
}

Answer 3

为了清晰起见，我更喜欢使用带有三个参数的Collectors.toMap方法：

Map<Integer, List<Account>> map = accounts.stream()
.collect(Collectors.toMap(
// key - customerID
e -> e.getCustomer().getCustomerID(),
// value - List<Account>
e -> List.of(e),
// merge two lists
(l1, l2) -> Stream.of(l1, l2)
.flatMap(List::stream)
.collect(Collectors.toList())));