java - Java 8流-合并共享相同ID的对象的集合

我有发票的集合:

class Invoice {
  int month;
  BigDecimal amount
}

我想合并这些发票，所以我每个月收到一张发票​​，金额是该月发票金额的总和。

例如:

invoice 1 : {month:1,amount:1000}
invoice 2 : {month:1,amount:300}
invoice 3 : {month:2,amount:2000}

输出:

invoice 1 : {month:1,amount:1300}
invoice 2 : {month:2,amount:2000}

我该如何使用Java 8流？

编辑:由于我的发票类是可变的，修改它们不是问题，所以我选择了尤金的解决方案

Collection<Invoice>  invoices = list.collect(Collectors.toMap(Invoice::getMonth, Function.identity(), (left, right) -> {
                left.setAmount(left.getAmount().add(right.getAmount()));
                return left;
            })).values();

最佳答案

如果可以，则返回Collection，如下所示:

Collection<Invoice>  invoices = list.collect(Collectors.toMap(Invoice::getMonth, Function.identity(), (left, right) -> {
                left.setAmount(left.getAmount().add(right.getAmount()));
                return left;
            })).values();

如果您确实需要List:

 list.stream().collect(Collectors.collectingAndThen(Collectors.toMap(Invoice::getMonth, Function.identity(), (left, right) -> {
                left.setAmount(left.getAmount().add(right.getAmount()));
                return left;
            }), m -> new ArrayList<>(m.values())));

两者显然都认为Invoice是可变的...