尝试使用Comparator按名称排序，忽略大小写以及首先为null [英] Attempting to use Comparator to sort by name, ignore case, as well as nulls first

问题描述

使用Java 8 Comparator类对项目列表进行排序时遇到问题。

I am having issues using the Java 8 Comparator class to sort a list of items.

我当前的工作比较器如下：

My current working comparator is below:

comparator = Comparator.comparing(Person::getName, Comparator.nullsFirst(Comparator.naturalOrder()));

此方法有效：它首先按名称对列表进行排序，并先使用空值。但是，我现在试图忽略名称的大小写。

This works: it orders the list by name with the null values first. However, I am now attempting to ignore case of the names.

我知道我可以编写一个新的getter来将名称全部小写，但是我不想

I know that I can write a new getter that returns the name all lowercase, but I do not want to go with this approach as I have to do this for multiple attributes.

在线查找，看来我应该使用 String.CASE_INSENSITIVE_ORDER ，但是我看到的唯一示例不包括空排序规范。

Looking online, it looks like I should be using String.CASE_INSENSITIVE_ORDER, but the only examples I see do not include the null ordering specification.

我可以做这样的事情：

comparator = Comparator.comparing(Person::getName, String.CASE_INSENSITIVE_ORDER);

但是，每当我尝试包含 Comparator.nullsFirst 我最终遇到类型错误，并对如何继续感到困惑。

However, whenever I try to include the Comparator.nullsFirst I end up getting type errors, and am confused on how to continue.

我尝试做类似于

thenComparing(Comparator.nullsFirst(Comparator.naturalOrder))

但这还是行不通的。

有人可以向我提供一些建议，让我知道如何将它们链接在一起以按名称排序（不区分大小写） ），然后对空值进行排序。我似乎对类型感到困惑。

Could someone lend me some advice on how I can chain these together to sort by name (not case sensitive) and then order the nulls. I seem to be confusing myself with the types.

推荐答案

null s您可以在列表中。您可以具有 null Person 引用，并且可以具有 Person s为 null 名称。

There are two types of nulls you can have in your list. You can have null Person references, and you can have Persons with null names.

在第一种情况下，您必须应用 nullsFirst 到要使用的基本比较器：

In the first case, you have to apply nullsFirst to the base comparator you want to use:

comparator = Comparator.nullsFirst(
        Comparator.comparing(Person::getName, String.CASE_INSENSITIVE_ORDER));

如果您可以使用 null 名称，则需要确保您的密钥永远不会返回 null ，或者将 nullsFirst 应用于 String.CASE_INSENSITIVE_ORDER 。第二种选择当然容易得多：

If you have the possibility of null names, you need to make sure that your key never returns a null, or that you apply nullsFirst to String.CASE_INSENSITIVE_ORDER. The second option is of course much easier:

comparator = Comparator.comparing(
        Person::getName, Comparator.nullsFirst(String.CASE_INSENSITIVE_ORDER));

如果您同时具有两种选择（ null 引用和 null 名称），则必须将两个版本组合在一起，并两次应用 nullsFirst ：

If you have both options (null references and null names), you will have to combine both versions and apply nullsFirst twice:

comparator = Comparator.nullsFirst(
        Comparator.comparing(
                Person::getName,
                Comparator.nullsFirst(String.CASE_INSENSITIVE_ORDER)
        )
);

如果要像这样链接多个比较，则外部 nullsFirst 可确保对 null Person 进行正确排序，可将其应用于整个链： / p>

If you are chaining multiple comparisons like this, the outer nullsFirst, which ensures that null Persons get sorted properly, can be applied to the entire chain:

comparator = Comparator.nullsFirst(
        Comparator.comparing(
                Person::getName,
                Comparator.nullsFirst(String.CASE_INSENSITIVE_ORDER)
        ).thenComparing(...)
);

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