Question

在isBigOrder方法中，如果订单中产品的总价格大于1000，则必须返回true。我怎么用java 8编写它？我写了总和部分，但我无法完成它。

public Function<Order, Boolean> isBigOrder() {

        Function<Order, Optional<Long>> sum = a -> a.getProducts()
                .stream()
                .map(P -> P.getPrice())
                .reduce((p1,p2)->p1+p2);

        Predicate <Optional<Long>> isBig =  x -> x.get() > 1000 ;

        return ????;
    }

如果需要其他课程：

enum OrderState { CONFIRMED, PAID, WAREHOUSE_PROCESSED, READY_TO_SEND, DELIVERED }

enum ProductType { NORMAL, BREAKABLE, PERISHABLE }

public class Product {
    private String code;
    private String title;
    private long price;
    private ProductState state;
    private ProductType type;

    //all fields have getter and setter

    public Product price(long price) {
        this.price = price;
        return this;
    }
}

public class Order {

    private String code;
    private long price;
    private String buyyer;
    private OrderState state;
    private List<Product> products = new ArrayList<>();

    //all fields have getter and setter

    public Order price(long price) {
        this.price = price;
        return this;
    }

    public Order product(Product product) {
        if (products == null) {
            products = new ArrayList<>();
        }
        products.add(product);
        return this;
    }    
}

Answer 1

您不需要Predicate。只需计算总和并检查它是否＆gt; 1000.

public Function<Order, Boolean> isBigOrder() {
    return o -> o.getProducts()
                 .stream()
                 .mapToLong(Product::getPrice)
                 .sum() > 1000;
}

或者，正如Holger所评论的那样，Predicate接口是一个更合适的函数接口，当你想用一个返回boolean的参数实现一个函数时：

public Predicate<Order> isBigOrder() {
    return o -> o.getProducts()
                 .stream()
                 .mapToLong(Order::getPrice)
                 .sum() > 1000;
}

Answer 2

假设您无法在写作状态下组合这两个函数，例如，因为它们来自不同的地方，您可以按如下方式组合它们：

public static Function<Order,Boolean> combine(
    Function<Order, Optional<Long>> f
,   Predicate <Optional<Long>> pred
) {
    return a -> pred.test(f.apply(a));
}

在Java 8中结合函数和谓词

2 个答案: