Java8在一次迭代中求和两个对象属性

Question

class LedgerEntry{
 private BigDecimal creditAmount;
 private BigDecimal debitAmount;

 //getters and setters
}

BigDecimal creditTotal = ledgeredEntries.stream().map(p ->p.getCreditAmount()).
reduce(BigDecimal.ZERO, BigDecimal::add);
BigDecimal debitTotal = ledgeredEntries.stream().map(p ->p.getDebitAmount()).
reduce(BigDecimal.ZERO, BigDecimal::add);

//...
//Use creditTotal, debitTotal later

Java 8之前版本

BigDecimal creditTotal = BigDecimal.ZERO;
BigDecimal debitTotal = BigDecimal.ZERO;
for(LedgerEntry entry : ledgerEntries){
  creditTotal = creditTotal.add(entry.getCreditAmount());
  debitTotal = debitTotal.add(entry.getDebitAmount());
}

Answer 1

您可以减少到一个总计条目：

LedgerEntry totalsEntry = entries.stream().reduce(new LedgerEntry(), (te, e) -> {
    te.setCreditAmount(te.getCreditAmount().add(e.getCreditAmount()));
    te.setDebitAmount(te.getDebitAmount().add(e.getDebitAmount()));

    return te;
});

更新

在注释中，正确地指出reduce()不应修改初始标识符值，而collection()应用于可变约简。下面是使用collection()的版本（使用相同的biconsumer作为累加器和组合器）。如果未设置creditamount和/或debitamount值，它还解决了潜在的NPE问题。

BiConsumer<LedgerEntry, LedgerEntry> ac = (e1, e2) -> {
    BigDecimal creditAmount = e1.getCreditAmount() != null ? e1.getCreditAmount() : BigDecimal.ZERO;
    BigDecimal debitAmount = e1.getDebitAmount() != null ? e1.getDebitAmount() : BigDecimal.ZERO;

    e1.setCreditAmount(creditAmount.add(e2.getCreditAmount()));
    e1.setDebitAmount(debitAmount.add(e2.getDebitAmount()));
};

LedgerEntry totalsEntry = entries.stream().collect(LedgerEntry::new, ac, ac);