Lambda表达式排序，出现空指针异常

问题描述：对象集合中，根据对象的某个属性进行升降排序时，未考虑对应属性的null值，导致出现空指针异常。
解决方法：Comparator.nullsFirst和Comparator.nullsLast

List<User> nList = list.stream().sorted(
    Comparator.comparing(User::getCode, Comparator.nullsFirst(String::compareTo)))
    .collect(Collectors.toList());
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下见源码：

 /**
     * Returns a null-friendly comparator that considers {@code null} to be
     * less than non-null. When both are {@code null}, they are considered
     * equal. If both are non-null, the specified {@code Comparator} is used
     * to determine the order. If the specified comparator is {@code null},
     * then the returned comparator considers all non-null values to be equal.
     *
     * <p>The returned comparator is serializable if the specified comparator
     * is serializable.
     *
     * @param  <T> the type of the elements to be compared
     * @param  comparator a {@code Comparator} for comparing non-null values
     * @return a comparator that considers {@code null} to be less than
     *         non-null, and compares non-null objects with the supplied
     *         {@code Comparator}.
     * @since 1.8
     */
public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {
    return new Comparators.NullComparator<>(true, comparator);
}

   /**
      * 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());
    }
}