使用Comparator.nullsLast时的NullPointerException

Question

我有下面两个类MyRange和MyCustomValue的代码-

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MyCustomValue中的value为null，我想把它放在最后。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。所以我决定像下面这样修改externalMVComparator (也就是说，使用thenComparing来提高可读性)

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

但是在NullPointerException中使用externalMVComparator2对列表进行排序。我在代码中做错了什么？

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));

堆栈跟踪错误-

    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)

Answer 1

问题出在下面这一行(为了清楚起见，我删除了Comparator. )：

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

您创建的Comparator将处理MyCustomValue类型的null值。它不会处理getValue返回的null%s。您必须使用双参数版本的Comparator.comparing，并为值提供一个null-safe比较器：

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

上面的代码将处理您实际想要按value排序的常见情况。当我查看您的代码时，我认为您的意思是只使用value进行null检查，否则就不想按它排序。如果是这样的话，您可以使用nullsLast( (x,y) -> 0)作为comparing的空安全第二个参数，该参数将把所有字符串视为相等。您也可以使用valueComparator = comparing(mcv -> mcv.getValue() == null)，因为true以自然的顺序使用false，但这可能不太清楚。

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