Java 8：按多個字段分組，然后根據值排序

Question

我有一個名為Info的類

public class Info {
    private String account;
    private String opportunity;
    private Integer value;
    private String desc;
}

我想按兩個字段（帳戶和機會）對Info列表進行分組，然后根據值對其進行排序。 即分組和排序前的信息列表：

List<Info> infos = [(account = 1234, opportunity = abs, value = 4, desc= test),
(account = 1234, opportunity = abs, value = 5, desc = testing),
(account = 1234, opportunity = abss, value = 10, desc = vip),
(account = 123, opportunity = abss, value = 8, desc = vip),
(account = 12, opportunity = absddd, value = 4, desc = production),
(account = 12, opportunity = absooo, value = 2, desc = test)]

分組排序后我預期的結果，

Map<String, Map<String, List<Info>>> result = {
    1234 = {
        abss = [(account = 1234, opportunity = abss, value = 10, desc = vip)],
        abs = [(account = 1234, opportunity = abs, value = 5, desc = testing),  
               (account = 1234, opportunity = abs, value = 4, desc = test)]
    },
    123 = {
        abss = [(account = 123, opportunity = abss, value = 8, desc = vip)]
    },
    12 = {
        absddd = [(account = 12, opportunity = absddd, value = 4, desc = production)],
        absooo = [(account = 12, opportunity = absooo, value = 2, desc = test)]
    }
}

o/p 根據值排序(10->(5+4)->8->4->2)

到目前為止，我已經嘗試過 = infos.stream().collect(Collectors.groupingBy(Info::getAccount, Collectors.groupingBy(r -> r.getOpportunity(), Collectors.toList())))但它的排序是隨機的。

Answer 1

要根據Info的value進行排序，

1. 通過比較value使流排序，這樣分組將按順序執行。
2. 調用groupingBy 時，在mapFactory指定LinkedHashMap以保留插入順序。

下面的程序演示了如何實現。

---

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Info {
    private String account;
    private String opportunity;
    private Integer value;
    private String desc;

    private Info(String account, String opportunity, Integer value, String desc) {
        super();
        this.account = account;
        this.opportunity = opportunity;
        this.value = value;
        this.desc = desc;
    }

    public static void main(String[] args) {
        List<Info> infos = new ArrayList<>();
        infos.add(new Info("12", "absddd", 4, "production"));
        infos.add(new Info("1234", "abss", 10, "vip"));
        infos.add(new Info("1234", "abs", 4, "test"));
        infos.add(new Info("1234", "abs", 5, "testing"));
        infos.add(new Info("123", "abss", 8, "vip"));
        infos.add(new Info("12", "absooo", 2, "test"));
        Map<String, Map<String, List<Info>>> sortedResult = infos.stream().sorted(Info::compareByValueDesc)
                .collect(Collectors.groupingBy(Info::getAccount, LinkedHashMap::new,
                        Collectors.groupingBy(r -> r.getOpportunity(), LinkedHashMap::new, Collectors.toList())));
        sortedResult.forEach((key, value) -> System.out.println(key + value.toString()));
    }

    public static int compareByValueDesc(Info other1, Info other2) {
        return -other1.value.compareTo(other2.value);
    }

    public String getAccount() {
        return account;
    }

    public void setAccount(String account) {
        this.account = account;
    }

    public String getOpportunity() {
        return opportunity;
    }

    public void setOpportunity(String opportunity) {
        this.opportunity = opportunity;
    }

    public String toString() {
        return this.value.toString();
    }
}