java - 使用 Java 流 API 将 Map> 转换为 Map>>

Question

我从数据库收到一些结果集，并将每一行存储为 DbRow 类型的对象，这是类:

public class DbRow {

    private int clientNumber;
    private String recordNumber;
    private String takeoverMonth;
    private int takeovers;
    private int mainClaims;
    private int distinctContractDates;
    private LocalDate contractDate;

    public DbRow(int clientNumber, String recordNumber, String takeoverMonth, int takeovers,
                int mainClaims, int distinctContractDates, LocalDate contractDate) {
        super();
        this.clientNumber = clientNumber;
        this.recordNumber = recordNumber;
        this.takeoverMonth = takeoverMonth;
        this.takeovers = takeovers;
        this.mainClaims = mainClaims;
        this.distinctContractDates = distinctContractDates;
        this.contractDate = contractDate;
    }

    // getters & setters

    // hashCode, equals & toString
}

我目前将收到的对象存储在 Map<Integer, List<DbRow>> 中为了让它们由某个属性分隔(在本例中 clientNumber ，所有具有特定客户端编号的 DbRow 都存储在 List<DbRow>> 中，而它们的 key 就是这个特定客户端编号)。

我需要创建一个Map<Integer, Map<String, Map<LocalDate, Integer>>>我目前使用很多行代码来完成此操作。

期望的结果可以描述为
每个客户数量的每个收购月的不同契约(Contract)日期的数量
(如 Map< 客户编号 , Map< 接管月份 , Map< 契约(Contract)日期 , 计数 >>> )
其中接管月份是 String格式 "yyyy/MM" .

我当前的解决方案很丑陋，但有效。它包含子图的创建和填充以及检查我想要删除的现有键。请看一下:

Map<Integer, List<DbRow>> records = new TreeMap<>();

// fill records with database results ...

Map<Integer, Map<String, Map<LocalDate, Integer>>> results = new TreeMap<>();

records.forEach((clientNumber, dbRows) -> {
    Map<String, List<DbRow>> rowsPerMonth = dbRows.stream()
                    .collect(Collectors.groupingBy(DbRow::getTakeoverMonth));
    Map<String, Map<LocalDate, Integer>> monthResults = new TreeMap<>();

    rowsPerMonth.forEach((takeoverMonth, rows) -> {
        if (monthResults.containsKey(takeoverMonth)) {
            Map<LocalDate, Integer> contractDateCount = monthResults.get(takeoverMonth);
            rows.forEach(dbRow -> {
                LocalDate contractDate = dbRow.getContractDate();
                if (contractDateCount.containsKey(contractDate)) {
                    int count = contractDateCount.get(contractDate);
                    count++;
                    contractDateCount.put(contractDate, count);
                } else {
                    contractDateCount.put(contractDate, 1);
                }
            });
            monthResults.put(takeoverMonth, contractDateCount);
        } else {
            Map<LocalDate, Integer> contractDateCount = new TreeMap<>();
            rows.forEach(dbRow -> {
                LocalDate contractDate = dbRow.getContractDate();
                if (contractDateCount.containsKey(contractDate)) {
                    int count = contractDateCount.get(contractDate);
                    count++;
                    contractDateCount.put(contractDate, count);
                } else {
                    contractDateCount.put(contractDate, 1);
                }
            });
            monthResults.put(takeoverMonth, contractDateCount);
        }
    });

    results.put(clientNumber, monthResults);
});

有人知道(如果有)一种使用流 API 以更短的方式完成此操作的方法吗？我真的很想有一个漂亮的解决方案。

Answer 1

您可能正在寻找类似的东西:

Map<Integer, Map<String, Map<LocalDate, Long>>>  countDistinctDatesPerMonthPerClient(List<DbRow> input) {
    return input.stream()
            .collect(Collectors.groupingBy(DbRow::getClientNumber,
                    Collectors.groupingBy(DbRow::getTakeoverMonth,
                            Collectors.groupingBy(DbRow::getContractDate, Collectors.counting()))));
}