Java Comparator.comparing比较导致空指针异常

Comparator.comparing(Department::getOrder)

原因：

public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
        Function<? super T, ? extends U> keyExtractor)
{
     
    Objects.requireNonNull(keyExtractor);
    return (Comparator<T> & Serializable)
        (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
}

如果keyExtractor.apply(c1)，那么keyExtractor.apply(c1).compareTo(XX)将报空指针异常

替代方案
Comparator.comparing(Department::getOrder, Comparator.nullsFirst(Comparator.naturalOrder()))

替代方案好处：

public static <T, U> Comparator<T> comparing(
        Function<? super T, ? extends U> keyExtractor,
        Comparator<? super U> keyComparator)
{
     
    Objects.requireNonNull(keyExtractor);
    Objects.requireNonNull(keyComparator);
    return (Comparator<T> & Serializable)
        (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
                                          keyExtractor.apply(c2));
}

会先取出keyExtractor.apply(c1)和keyExtractor.apply(c2)，放入比较器进行比较
而Comparator.nullsFirst作为比较器，会创建一个Comparators.NullComparator比较器

public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {
     
    return new Comparators.NullComparator<>(true, comparator);
}

Comparators.NullComparator比较器的compare接口实现中先进行空值判断处理，不为空的再进行代入比较器比较

/**
 * Null-friendly comparators
 */
final static class NullComparator<T> implements Comparator<T>, Serializable {
     
    private static final long serialVersionUID = -7569533591570686392L;
    private final boolean nullFirst;
    // if null, non-null Ts are considered equal
    private final Comparator<T> real;

    @SuppressWarnings("unchecked")
    NullComparator(boolean nullFirst, Comparator<? super T> real) {
     
        this.nullFirst = nullFirst;
        this.real = (Comparator<T>) real;
    }

    @Override
    public int compare(T a, T b) {
     
        if (a == null) {
     
            return (b == null) ? 0 : (nullFirst ? -1 : 1);
        } else if (b == null) {
     
            return nullFirst ? 1: -1;
        } else {
     
            return (real == null) ? 0 : real.compare(a, b);
        }
    }

    @Override
    public Comparator<T> thenComparing(Comparator<? super T> other) {
     
        Objects.requireNonNull(other);
        return new NullComparator<>(nullFirst, real == null ? other : real.thenComparing(other));
    }

    @Override
    public Comparator<T> reversed() {
     
        return new NullComparator<>(!nullFirst, real == null ? null : real.reversed());
    }
}