Comparator.nullsFirst与null-safe-comparator [英] Comparator.nullsFirst with null-safe-comparator

问题描述

我想要一个空安全的比较器，但是它不起作用:

I want to have a null-safe comparator, but it does not work:

Comparator<Item> sort_high = (i1, i2)-> Double.compare(i2.getUser().getValue(), i1.getUser().getValue());

items.sort(Comparator.nullsFirst(sort_high));

但是，如果item.getUser().getValue()(或item.getUser())为null，我会得到一个NPE.

However, I get a NPE, if item.getUser().getValue() (or item.getUser()) is null.

at Item.lambda$1(Item.java:270)
    at java.util.Comparators$NullComparator.compare(Comparators.java:83)
    at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
    at java.util.TimSort.sort(TimSort.java:220)
    at java.util.Arrays.sort(Arrays.java:1438)
    at java.util.List.sort(List.java:478)

怎么了?

推荐答案

nullsFirst将处理null Item s("i1")，但是一旦找到两个有效对象，您的Comparator为被调用，您需要处理内部null引用.

nullsFirst will take care of null Items ("i1"), but once two valid objects are found, your Comparator is invoked and you need to handle internal null references.

在您的情况下，您可以使用类似以下内容的

In your case, you could use something like:

items.sort(
  Comparator.nullsFirst(
    Comparator.comparing(Item::getUser,
      Comparator.nullsFirst(Comparator.comparingDouble(User::getValue))
    )
  )
);

(假设getValue()返回double)

但是我几乎不推荐这样复杂的代码.

But I hardly recommend such convoluted code.

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