JavaStreamreduce累加方法详解

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int sum = numbers.stream()
                 .reduce(0, Integer::sum);
System.out.println(sum); // 输出 15

List<Integer> emptyList = Collections.emptyList();
Optional<Integer> result = emptyList.stream()
                                     .reduce(Integer::sum);
if (result.isPresent()) {
    System.out.println("总和为：" + result.get());
} else {
    System.out.println("列表为空");
}

class Product {
    String name;
    double price;
    public Product(String name, double price) {
        this.name = name;
        this.price = price;
    }
    public double getPrice() { return price; }
}

List<Product> products = Arrays.asList(
    new Product("A", 100),
    new Product("B", 200),
    new Product("C", 300)
);

double totalPrice = products.stream()
                            .mapToDouble(Product::getPrice)
                            .reduce(0.0, Double::sum);
System.out.println(totalPrice); // 输出 600.0