Java流乘法并返回乘法之和

Question

目前我正在使用 for 循环来执行简单的乘法并对获得的值求和:

BigDecimal serviceOrderTotal = new BigDecimal(0.00);
for (int i = 0; i < orderPresenter.getServiceList().size(); i++) {
    System.out.println("orderPresenter.getServiceList().get(i).getServiceValue()");
    System.out.println(orderPresenter.getServiceList().get(i).getServiceValue());
    System.out.println("orderPresenter.getServiceList().get(i).getServiceQuantity()");
    System.out.println(orderPresenter.getServiceList().get(i).getServiceQuantity());
    serviceOrderTotalold = serviceOrderTotal.add(orderPresenter.getServiceList().get(i).getServiceValue().multiply(orderPresenter.getServiceList().get(i).getServiceQuantity()));
}

我需要使用流来做同样的事情。对于每个 orderPresenter，我需要乘以数量值并返回这些值的总和。

如果我在 stream 中使用 forEach 来执行此操作，我无法返回该值，因为变量必须是 final 或原子变量在 forEach 内部使用。我最终没有成功:

BigDecimal serviceOrderTotal = new BigDecimal(0.00);
serviceOrderTotal = orderPresenter.getServiceList().stream()
               .forEach(servicePresenter -> {
                    System.out.println("orderPresenter.getServiceValue()");
                    System.out.println(orderPresenter.getServiceValue());
                    System.out.println("orderPresenter.getServiceQuantity()");
                    System.out.println(orderPresenter.getServiceQuantity());
                    serviceOrderTotalold = serviceOrderTotalold.add(orderPresenter.getServiceValue().multiply(orderPresenter.getServiceQuantity()));
                });

Answer 1

您可以修改您的实现以使用 foreach，例如:

BigDecimal serviceOrderTotal = BigDecimal.ZERO;
for (Service service : orderPresenter.getServiceList()) {
    serviceOrderTotal = serviceOrderTotal.add(service.getServiceValue().multiply(service.getServiceQuantity()));
}

这也可以使用 java-stream 的 map 和 reduce 操作进一步转换为:

BigDecimal serviceOrderTotal = orderPresenter.getServiceList().stream()
        .map(service -> service.getServiceValue().multiply(service.getServiceQuantity()))
        .reduce(BigDecimal.ZERO, BigDecimal::add);