我正在尝试使用Java-8 lambdas解决以下问题：
给定一个List<Transaction>，对于每一个Category.minorCategory我需要的总和Transaction.amount每Category.minorCategory和Map的Transaction.accountNumber与总和Transaction.amount每Transaction.accountNumber。根据下面的代码，我正在工作。我现在有一个分组依据Category.majorCategory，本质上是返回一个Map<String, Map<String, MinorCategorySummary>>key on Category.majorCategory。
在分组之前，我正在做所有工作，Category.majorCategory但是很难找到解决方案。lambda编程的范式转变证明了陡峭的学习曲线。
TransactionBreakdown是动作发生的地方，也是我想要返回的地方Map<String, Map<String, MinorCategorySummary>>。
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());
}
}
你的班级设计对我来说似乎很奇怪。为什么只将类别放入摘要类中，然后将类别作为映射键？拥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，您的两个问题已得到明确解决，并且没有多余的内容：
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()
)
));
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