null-safe mapping Comparator using default implementations

问题

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)

回答1:

Found a possible solution:

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

来源：https://stackoverflow.com/questions/28499846/null-safe-mapping-comparator-using-default-implementations

标签

java

java-8

comparator