用于在 Java 8 Stream 中排序的空安全日期比较器
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Null safe date comparator for sorting in Java 8 Stream
提问by Andrew
I'm using this to get the newest item. How can I get this to be null safe and sort with null dates last (oldest). createDt is a joda LocalDate object.
我正在使用它来获取最新的项目。我怎样才能让它成为空安全并用空日期最后（最旧）排序。createDt 是一个 joda LocalDate 对象。
Optional<Item> latestItem = items.stream()
.sorted((e1, e2) -> e2.getCreateDt().compareTo(e1.getCreateDt()))
.findFirst();
采纳答案by Paul Boddington
If it's the Items that may be null, use @rgettman's solution.
如果Items 可能为空，请使用@rgettman 的解决方案。
If it's the LocalDates that may be null, use this:
如果LocalDates 可能为空，请使用：
items.stream()
.sorted(Comparator.comparing(Item::getCreateDt, Comparator.nullsLast(Comparator.reverseOrder())));
In either case, note that sorted().findFirst()is likely to be inefficient as most standard implementations sort the entire stream first. You should use Stream.mininstead.
在任何一种情况下，请注意这sorted().findFirst()可能是低效的，因为大多数标准实现首先对整个流进行排序。您应该改用Stream.min。
回答by rgettman
You can turn your own null-unsafe Comparatorinto an null-safe one by wrapping it Comparator.nullsLast. (There is a Comparator.nullsFirstalso.)
您可以Comparator通过包装它来将您自己的 null-unsafe变成 null-safe Comparator.nullsLast。（还有一个Comparator.nullsFirst。）
Returns a null-friendly comparator that considers
nullto be greater than non-null. When both arenull, they are considered equal. If both are non-null, the specifiedComparatoris used to determine the order.
返回一个认为
null大于非空的对空友好的比较器。当两者都是 时null，它们被认为是相等的。如果两者都不为空，Comparator则使用指定的来确定顺序。
.sorted(Comparator.nullsLast(
(e1, e2) -> e2.getCreateDt().compareTo(e1.getCreateDt())))
.findFirst();
回答by Mohsen Kashi
write your custom one:
写你的自定义：
stream().sorted((o1, o2) ->
(o1.getF1() == null || o2.getF1() == null) ? 1 :
o1.getF1().compareTo(o2.getF1())
)
This is nulls first sorting, change the return value of if statement from 1 to -1 for nulls last
这是空值优先排序，最后将if语句的返回值从1改为-1
