Kotlin排序最后为空 [英] Kotlin sorting nulls last

问题描述

科特林将对象列表按可空字段排序为空的最后一种方式是什么?

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

排序的Kotlin对象:

Kotlin object to sort:

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

与下面的Java代码类似:

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;
}

推荐答案

您可以从

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

• fun <T: Comparable<T>> nullsLast(): Comparator<T?> ，它构造了一个可比较的比较器，该比较器只将null s放在所有非空值之后；

• 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> ，它接受比较器和为比较器提供值的函数，将它们组合成新的比较器；

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;

这将使您创建一个比较器，该比较器通过将null放在最后来对SomeObject进行比较.然后，您可以简单地将比较器传递给
，它使用比较器将迭代器排序到列表中:

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)]

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