从Java对象计数值？

您可以利用Java 8 Stream API并实现如下所示的内容：

public static void main(String[] args) {    PaymentDetailsItem payment = new PaymentDetailsItem("test", "100.00", 10, "1");    PaymentDetailsItem payment2 = new PaymentDetailsItem("test number 2", "250.00", 10, "2");    List<PaymentDetailsItem> payments = new ArrayList<>();    payments.add(payment);    payments.add(payment2);    List<String> amounts = payments.stream().map(PaymentDetailsItem::getAmount).collect(Collectors.toList());    System.out.println("Here we have the extracted List of amounts: " + amounts);    String totalAmount = amounts.stream() .reduce((amount1, amount2) -> String.valueOf(Float.valueOf(amount1) + Float.valueOf(amount2))).get();    System.out.println("Total amount obtained by using .reduce() upon the List of amounts: " + totalAmount);    System.out.println("Or you can do everything at once: " + payments.stream().map(PaymentDetailsItem::getAmount) .reduce((amount1, amount2) -> String.valueOf(Float.valueOf(amount1) + Float.valueOf(amount2))).get());}

记住要为amount属性实现getter。