java - 使用Comparator.nullsLast时发生NullPointerException

我下面的代码带有2个类MyRange和MyCustomValue-

class MyRange {
    private Long id;
    private Double minValue;
    private Double maxValue;

    // getters and setters
    // equals, hashCode and toString

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null || getClass() != obj.getClass())
            return false;
        MyRange other = (MyRange) obj;
        return Objects.equals(this.id, other.id) &&
               Objects.equals(this.minValue, other.minValue) &&
               Objects.equals(this.maxValue, other.maxValue);
    }
}

class MyCustomValue {
    private String value;
    private MyRange myrange;

    //getters and setters
    // equals, hashCode and toString

}

如果value在MyCustomValue中为空，我最后要它。所以我像下面这样写比较器

public static final Comparator<MyCustomValue> externalMVComparator = (emv1, emv2) -> {
    if(emv1.getValue() != null && emv2.getValue() == null) {
        return -1;
    } else if (emv1.getValue() == null && emv2.getValue() != null) {
        return 1;
    } else {
        return myrangeMinValueComparator.compare(emv1, emv2);
    }
}

private static final Comparator<MyRange> minValueComparator =  Comparator.nullsLast(Comparator.comparingDouble(value -> value.getMinValue()));
private static final Comparator<MyCustomValue> myrangeMinValueComparator = Comparator.nullsLast(Comparator.comparing(MyCustomValue::getMyrange, minValueComparator));

上面的比较器工作正常。因此，我决定像下面那样更改externalMVComparator（即，使用thenComparing可以提高可读性）

private static final Comparator<MyCustomValue> valueComparator = Comparator.nullsLast(Comparator.comparing(MyCustomValue::getValue));
public static final Comparator<MyCustomValue> externalMVComparator2 = Comparator.nullsLast(valueComparator.thenComparing(myrangeMinValueComparator));

但是用externalMVComparator2对列表进行排序会导致NullPointerException。我是代码错误，那是什么错？

用于测试的代码-

MyCustomValue emv1 = new MyCustomValue("v1", new MyRange(1L, 0.71, 0.79));
MyCustomValue emv2 = new MyCustomValue(null, new MyRange(2L, 0.53, 0.65));
MyCustomValue emv3 = new MyCustomValue("v2", new MyRange(3L, 0.28, 0.42));
MyCustomValue emv4 = new MyCustomValue(null, new MyRange(4L, 0.06, 0.27));
List<MyCustomValue> shuffledList1 = Arrays.asList(emv1, emv2, emv3, emv4);
Collections.shuffle(shuffledList1);
shuffledList1.sort(MyCustomValue.externalMVComparator2);
Assert.assertEquals(shuffledList1, Arrays.asList(emv3, emv1, emv4, emv2));

错误堆栈跟踪-

    Exception in thread "main" java.lang.NullPointerException
    at java.util.Comparator.lambda$comparing$77a9974f$1(Comparator.java:469)
    at java.util.Comparator.lambda$thenComparing$36697e65$1(Comparator.java:216)
    at java.util.Comparators$NullComparator.compare(Comparators.java:83)
    at java.util.Comparators$NullComparator.compare(Comparators.java:83)
    at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
    at java.util.TimSort.sort(TimSort.java:220)
    at java.util.Arrays.sort(Arrays.java:1438)
    at java.util.Arrays$ArrayList.sort(Arrays.java:3895)
    at TestNullComparator.main(TestNullComparator.java:15)

最佳答案

问题在以下行中（为清楚起见，我删除了Comparator.）：

Comparator<MyCustomValue> valueComparator = nullsLast(comparing(MyCustomValue::getValue));

您创建的Comparator将处理null类型的MyCustomValue值。它不会处理null返回的getValue。您必须使用Comparator.comparing的2参数版本，并为这些值提供一个null安全的比较器：

valueComparator = comparing(MyCustomValue::getValue, nullsLast(naturalOrder()));

上面的方法可以处理通常要按value排序的常见情况。当我查看您的代码时，我认为您的意思是仅将value用于null检查，否则不希望对其进行排序。在这种情况下，可以将nullsLast( (x,y) -> 0)用作comparing的null安全第二个参数，它将所有字符串视为相等。您还可以使用valueComparator = comparing(mcv -> mcv.getValue() == null)，因为true按照自然顺序排在false之后，但这可能不太清楚。

如果您还想处理null的MyCustomValue，则必须再次将其包装在nullsLast中。