Comparator.nullsLast不能避免NullPointerException [英] Comparator.nullsLast does not avoid NullPointerException

问题描述

我想按可为空的字段之一对对象列表进行排序.

I want to sort a list of objects by one of nullable fields.

为了避免 NullPointerexception ，我使用了 Comparator.nullsLast .但是仍然会发生例外情况:

In order to avoid NullPointerexception I use Comparator.nullsLast. But the exception still occurs:

public class Test {

    public static void main(String[] args) {
        List<Bean> l = new ArrayList<>();
        for (int i=0;i<5;i++) {
            Bean b = new Bean("name_"+i,i);
            l.add(b);
        }
        l.get(2).setVal(null);
        System.out.println(l);
        Collections.sort(l, Comparator.nullsLast(Comparator.comparing(Bean::getVal)));
        System.out.println(l);
    }

    static class Bean{
        String name;
        Integer val;
        // omit getters & setters & constructor
    }

}

如何对这种列表进行排序?

How can I sort this kind of list?

推荐答案

您应该两次使用 Comparator.nullsLast :

list.sort(nullsLast(comparing(Bean::getVal, nullsLast(naturalOrder()))));

• 第一个 nullsLast 将处理 Bean 对象为null的情况.
• 第二个 nullsLast 将处理 Bean :: getVal 的返回值为null的情况.
• First nullsLast will handle the cases when the Bean objects are null.
• Second nullsLast will handle the cases when the return value of Bean::getVal is null.

如果您确定列表中没有任何 null 值，则可以省略第一个 nullsLast (如@Holger所指出):

In case you're sure there aren't any null values in your list then you can omit the first nullsLast (as noted by @Holger) :

list.sort(comparing(Bean::getVal, nullsLast(naturalOrder())));

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