使用Java Stream API对BigDecimals进行分组和相加

Question

我有一个income对象列表。它们都有一个类型。我想对它们进行分组，并将amount字段加起来。

对于本例：

Income income1 = new Income();
income1.setType(IncomeType.COMMON);
income1.setAmount(BigDecimal.valueOf(1000));

Income income2 = new Income();
income2.setType(IncomeType.COMMON);
income2.setAmount(BigDecimal.valueOf(2000));

Income income3 = new Income();
income3.setType(IncomeType.MARCIN);
income3.setAmount(BigDecimal.valueOf(100));

List<Income> incomes = Arrays.asList(income1, income2, income3);

Answer 1

最简单的解决方案是使用包含合并函数的collectors::tomap。

Map<IncomeType, BigDecimal> grouping = incomes.stream()
    .collect(Collectors.toMap(Income::getType, Income::getAmount, BigDecimal::add));