如何在java中将自定义对象列表重新组合到Map中？

9q78igpj 于 2021-08-20 发布在 Java

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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)));

我想将上面的内容转移到map，键应该是customerid，值应该是帐户列表

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？

Java grouping java-stream java-8

来源：https://stackoverflow.com/questions/68303749/how-to-regroup-a-list-of-custom-objects-into-a-map-in-java

3条答案

按热度按时间

1tuwyuhd1#

你可以用 Collectors.groupingBy .

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

演示

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tcbh2hod2#

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);
}

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bkkx9g8r3#

我宁愿使用 Collectors.toMap 方法具有三个参数，以确保清晰：

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())));

赞(0）回复(0）举报 2021-08-20

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如何在java中将自定义对象列表重新组合到Map中？

3条答案

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