Lambda表达式排序，出现空指针异常

你的名字 2023-07-11 05:50 22阅读 0赞

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());  
1  
2  
3  
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());  
    \}  
\}