使用默认实现的空安全映射比较器 [英] null-safe mapping Comparator using default implementations

问题描述

是否有可能在 Java 8 中创建空安全映射比较器而无需编写自己的 Comparator 实现?

运行以下代码时，会导致 NPE，因为 Comparator.comparing() 的 keyExtractor 参数可能返回 null 值:

公共类 ToSort{私人字符串 sortBy;公共 ToSort(字符串 sortBy){this.sortBy = sortBy;}公共字符串 getSortBy(){返回 sortBy;}public static void main(String[] args){//映射比较器比较器comp = Comparator.comparing(ToSort::getSortBy);SortedSetset = new TreeSet<>(comp);ToSort o1 = new ToSort("1");ToSort o2 = new ToSort(null);set.add(o1);System.out.println(set.contains(o2));//NPE 因为 o2.getSortBy() == null}}

<块引用>

线程main"中的异常java.lang.NullPointerException在 java.util.Comparator.lambda$comparing$77a9974f$1(Comparator.java:469)在 java.util.Comparator$$Lambda$2/1480010240.compare(来源不明)在 java.util.Comparators$NullComparator.compare(Comparators.java:83)在 java.util.TreeMap.getEntryUsingComparator(TreeMap.java:376)在 java.util.TreeMap.getEntry(TreeMap.java:345)在 java.util.TreeMap.containsKey(TreeMap.java:232)在 java.util.TreeSet.contains(TreeSet.java:234)在 test.ToSort.main(ToSort.java:48)

使用

Comparatorcomp = Comparator.nullsFirst(Comparator.comparing(ToSort::getSortBy));

也不起作用，因为只有 null 的 ToSort 对象被正确处理.

我知道如何编写自己的 Comparator 实现，我只是在寻找更优雅"的解决方案，例如

Comparator.comparingNullsFirst(ToSort::getSortBy)

解决方案

找到了可能的解决方案:

Comparator.comparing(ToSort::getSortBy,Comparator.nullsFirst(Comparator.naturalOrder()))

Is there a build-in possibility to create a null-safe mapping comparator in Java 8 without writing a own implementation of Comparator?

When running the following code, it causes a NPE because the keyExtractor argument of Comparator.comparing() may return a null value:

public class ToSort
{

    private String sortBy;

    public ToSort(String sortBy)
    {
        this.sortBy = sortBy;
    }

    public String getSortBy()
    {
        return sortBy;
    }

    public static void main(String[] args)
    {
        // mapping comparator
        Comparator<ToSort> comp = Comparator.comparing(ToSort::getSortBy);                          
        SortedSet<ToSort> set = new TreeSet<>(comp);
        ToSort o1 = new ToSort("1");
        ToSort o2 = new ToSort(null);

        set.add(o1);

        System.out.println(set.contains(o2)); //NPE because o2.getSortBy() == null

    }
}

Exception in thread "main" java.lang.NullPointerException at java.util.Comparator.lambda$comparing$77a9974f$1(Comparator.java:469) at java.util.Comparator$$Lambda$2/1480010240.compare(Unknown Source) at java.util.Comparators$NullComparator.compare(Comparators.java:83) at java.util.TreeMap.getEntryUsingComparator(TreeMap.java:376) at java.util.TreeMap.getEntry(TreeMap.java:345) at java.util.TreeMap.containsKey(TreeMap.java:232) at java.util.TreeSet.contains(TreeSet.java:234) at test.ToSort.main(ToSort.java:48)

Using

Comparator<ToSort> comp = Comparator.nullsFirst(Comparator.comparing(ToSort::getSortBy));

does not work either as only ToSort objects that are null are treaded properly.

I know how to write my own Comparator implementation, I`m just searching a more "elegant" solution like

Comparator.comparingNullsFirst(ToSort::getSortBy)

解决方案

Found a possible solution:

Comparator.comparing(ToSort::getSortBy, 
      Comparator.nullsFirst(Comparator.naturalOrder())
)

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