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Java Comparator.comparing比較導(dǎo)致空指針異常

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)將報(bào)空指針異常

替代方案

Comparator.comparing(Department::getOrder, Comparator.nullsFirst(Comparator.naturalOrder()))

替代方案好處：

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));
}

會(huì)先取出keyExtractor.apply(c1)和keyExtractor.apply(c2)，放入比較器進(jìn)行比較

而Comparator.nullsFirst作為比較器，會(huì)創(chuàng)建一個(gè)Comparators.NullComparator比較器

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

Comparators.NullComparator比較器的compare接口實(shí)現(xiàn)中先進(jìn)行空值判斷處理，不為空的再進(jìn)行代入比較器比較

/**
 * 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());
    }
}

Comparator中comparing方法的學(xué)習(xí)

例子：

我們需要根據(jù)對(duì)象中的name字段進(jìn)行不規(guī)則排序

排序規(guī)則為(PPD > 政府 > 合作)

public class Obj {
    private String name;
    private BigDecimal price;
    ......
}

@Test
    public void sort() {
        List<Obj> list =  Arrays.asList(
                new Obj("政府", null),
                new Obj("政府", new BigDecimal("1216.23")),
                new Obj("商業(yè)", new BigDecimal("123.23")),
                new Obj("PPD", new BigDecimal("123.23")),
                new Obj("合作", new BigDecimal("127.23")),
                new Obj(null, new BigDecimal("125.23")));
        List<String> sortList =  Arrays.asList("PPD","政府","合作");
        List<Obj> result = list.stream().sorted(
                //先按照name排序（模擬需求的a屬性排序）
                Comparator.comparing(Obj::getName,(x,y)-> {
                    if(x == null && y != null){
                        return 1;
                    }else if(x !=null && y == null){
                        return -1;
                    }else if(x == null && y == null){
                        return -1;
                    } else {
                    for(String sort : sortList){
                        if(sort.equals(x) || sort.equals(y)){
                            if(x.equals(y)){
                                return 0;
                            }else if(sort.equals(x)){
                                return -1;
                            }else{
                                return 1;
                            }
                        }
                    }
                    return 0;
                    }
                })).collect(Collectors.toList());
        System.out.println(result);
    }