Kotlin sorting nulls last

Question

What would be a Kotlin way of sorting list of objects by nullable field with nulls last?

Kotlin object to sort:

@JsonInclude(NON_NULL)
data class SomeObject(
    val nullableField: String?
)

Analogue to below Java code:

@Test
public void name() {
    List<SomeObject> sorted = Stream.of(new SomeObject("bbb"), new SomeObject(null), new SomeObject("aaa"))
            .sorted(Comparator.comparing(SomeObject::getNullableField, Comparator.nullsLast(Comparator.naturalOrder())))
            .collect(toList());

    assertEquals("aaa", sorted.get(0).getNullableField());
    assertNull(sorted.get(2).getNullableField());
}

@Getter
@AllArgsConstructor
private static class SomeObject {
    private String nullableField;
}

Answer 1

You can use these functions from the kotlin.comparisons package:

• fun <T: Comparable<T>> nullsLast(): Comparator<T?>, which constructs a comparator of something comparable that just puts nulls after all not-null values;

• fun <T, K> compareBy(comparator: Comparator<in K>, selector: (T) -> K): Comparator<T>, which accepts a comparator and a function that provides values for the comparator, combining them into a new comparator;

This will let you make a comparator that compares SomeObject by nullableField putting nulls last. Then you can simply pass the comparator to
fun <T> Iterable<T>.sortedWith(comparator: Comparator<in T>): List<T>, which sorts an iterable into a list using a comparator:

val l = listOf(SomeObject(null), SomeObject("a"))

l.sortedWith(compareBy(nullsLast<String>()) { it.nullableField }))
// [SomeObject(nullableField=a), SomeObject(nullableField=null)]