Java 8 lambda 嵌套映射

Question

我正在尝试使用 Java-8 lambda 来解决以下问题:

给定一个 List<Transaction> , 对于每个 Category.minorCategory我需要 Transaction.amount 的总和每Category.minorCategory和一个 Map的 Transaction.accountNumber总和为 Transaction.amount每Transaction.accountNumber .根据下面的代码，我有这个工作。我现在需要按 Category.majorCategory 分组，本质上返回一个 Map<String, Map<String, MinorCategorySummary>>键入 Category.majorCategory .

在按 Category.majorCategory 分组之前，我已经完成了所有工作但很难看到解决方案；事实证明，使用 lambda 进行编程的范式转变是一条陡峭的学习曲线。

TransactionBreakdown是 Action 发生的地方，也是我想返回 Map<String, Map<String, MinorCategorySummary>> 的地方.

public class Transaction {

    private final String accountNumber;
    private final BigDecimal amount;
    private final Category category;

}

public class Category {

    private final String majorCategory;
    private final String minorCategory;

}

public class MinorCategorySummary {

    private final BigDecimal sumOfAmountPerMinorCategory;
    private final Map<String, BigDecimal> accountNumberSumOfAmountMap;
    private final Category category;

}

public class TransactionBreakdown {

    Function<Entry<String, List<Transaction>>, MinorCategorySummary> transactionSummariser = new TransactionSummariser();

    public Map<Object, MinorCategorySummary> getTransactionSummaries(List<Transaction> transactions) {
        return transactions
                .stream()
                .collect(groupingBy(t -> t.getCategory().getMinorCategory()))
                .entrySet()
                .stream()
                .collect(
                        toMap(Entry::getKey,
                                transactionSummariser));
    }

}

public class TransactionSummariser implements Function<Entry<String, List<Transaction>>, MinorCategorySummary> {

    @Override
    public MinorCategorySummary apply(
            Entry<String, List<Transaction>> entry) {
         return new MinorCategorySummary(
                    entry.getValue()
                            .stream()
                            .map(Transaction::getAmount)
                            .collect(reducing(BigDecimal.ZERO, BigDecimal::add)),
                    entry.getValue()
                            .stream()
                            .collect(
                                    groupingBy(Transaction::getAccountNumber,
                                            mapping(Transaction::getAmount,
                                                    reducing(BigDecimal.ZERO, BigDecimal::add)))), 
                                                    entry.getValue().get(0).getCategory());
    }

}

Answer 1

我觉得你的类(class)设计很奇怪。为什么只将类别放入摘要类中，然后将类别作为映射键？有一个没有类别的摘要类会更有意义:

public class TransactionSummary {

    private final BigDecimal amount;
    private final Map<String, BigDecimal> acctToTotal;

    TransactionSummary(Map<String, BigDecimal> acctToTotal) {
        this.acctToTotal = Collections.unmodifiableMap(acctToTotal);
        this.amount = acctToTotal.values().stream()
                .reduce(BigDecimal.ZERO, BigDecimal::add);
    }

    public static Collector<Transaction, ?, TransactionSummary> collector() {
        // this can be a static constant
        return collectingAndThen(
            toMap(Transaction::getAccountNumber,Transaction::getAmount,BigDecimal::add),
            TransactionSummary::new
        );
    }

    // getters
}

现在你的两个问题已经清楚且没有冗余地解决了:

Map<String, TransactionSummary> minorSummary = transactions.stream()
        .collect(groupingBy(
                t -> t.getCategory().getMinorCategory(),
                TransactionSummary.collector()
        ));

Map<String, Map<String, TransactionSummary>> majorMinorSummary = transactions.stream()
        .collect(groupingBy(
                t -> t.getCategory().getMajorCategory(),
                groupingBy(
                        t -> t.getCategory().getMinorCategory(),
                        TransactionSummary.collector()
                )
        ));