Question

我有一个比较器，用于比较对象的嵌套字段。这里是price的{{1}}。

问题：如果该字段为null，则显示NullPointerException。

问题：如何让比较器忽略比较字段为null的对象，而无需（！）事先过滤列表？ “忽略”是指将它们放在列表的末尾。

product

结果：

public class SorterTest {
    private static final Comparator<Product> PRODUCT_COMPARATOR = 
           Comparator.comparing(p -> p.details.price);

    static class Product {
        String name;
        Details details;

        static class Details {
            BigDecimal price;
        }
    }

    @Test
    public void test() {
        List<Product> products = Arrays.asList(
                createProduct("A", new BigDecimal(30.00)),
                createProduct("B", new BigDecimal(55.00)),
                createProduct("C", new BigDecimal(20.00)),
                createProduct("D", null),
                createProduct("E", null),
                createProduct("F", null)
        );

        Collections.sort(products, PRODUCT_COMPARATOR);
        assertEquals("C", products.get(0).name);
        assertEquals("A", products.get(1).name);
        assertEquals("B", products.get(2).name);
        assertEquals("D", products.get(3).name);
        assertEquals("E", products.get(4).name);
        assertEquals("F", products.get(5).name);
    }

    private Product createProduct(String name, BigDecimal price) {
        Product p = new Product();
        p.details = new Product.Details();
        p.name = name;
        p.details.price = price;
        return p;
    }
}

Answer 1

您可以使用Comparator的nullsLast方法：

   private static final Comparator PRODUCT_COMPARATOR = 
           Comparator
           .nullsLast(Comparator.comparing(p -> p.details.price, Comparator.nullsLast(Comparator.naturalOrder())));

Product的第一个null持续时间为null，价格的第二个null为空。

Answer 2

由于价格为null，因此您将收到NullPointerException。因此，使用if语句检查价格值，如果为null，则使用其他值（如0）替换价格。代码可能像：

public class SorterTest {
    private static final Comparator<Product> PRODUCT_COMPARATOR =
            Comparator.comparing(p->
                    {
                        if (p.details != null && p.details.price != null) {
                            return p.details.price;
                        } else {
                            return new BigDecimal(0);
                        }
                    }
            );

    static class Product {
        String name;
        Details details;

        static class Details {
            BigDecimal price;
        }
    }

    @Test
    public void test() {
        List<Product> products = Arrays.asList(
                createProduct("A", new BigDecimal(30.00)),
                createProduct("B", new BigDecimal(55.00)),
                createProduct("C", new BigDecimal(20.00)),
                createProduct("D", null),
                createProduct("E", null),
                createProduct("F", null)
        );

        Collections.sort(products, PRODUCT_COMPARATOR);
        assertEquals("D", products.get(0).name);
        assertEquals("E", products.get(1).name);
        assertEquals("F", products.get(2).name);
        assertEquals("C", products.get(3).name);
        assertEquals("A", products.get(4).name);
        assertEquals("B", products.get(5).name);
    }

    private Product createProduct(String name, BigDecimal price) {
        Product p = new Product();
        p.details = new Product.Details();
        p.name = name;
        p.details.price = price;
        return p;
    }
}

Answer 3

您可以使用Optional <>来查看getter的结果是否为null或有效值。对于空值，可以通过始终将其comparer（）结果设为-1或1来将它们放在尾部或头部。

private static final Comparator<Product> PRODUCT_COMPARATOR = 
           Comparator.comparing(p -> {
               Optional<BigDecimal> res = Optional.ofNullable(p.getPrice);
               if(res.isPresent())
                   return p.getPrice();
               else
                   return -1;
           });
           ...
static class Product {
        String name;
        BigDecimal price;

        BigDecimal getPrice(){
            return this.price;
        }
        ...
}

如何忽略比较器中的空值？

3 个答案: