将对象列表收集到使用Java 8的LinkedHashmap中

发布于 2025-01-24 04:44:05 字数 609 浏览 0 评论 0 原文

我有配置文件对象 list＆lt; profile＆gt;列表。

我需要将其转换为 linkedhashmap＆lt; string，string＆gt; 。

如果对象配置文件包括：

public class Profile {
    private String profileId;
    private String firstName;
    private String lastName;
}

我尝试了以下操作：

Map<String, String> map = list.stream()
    .collect(Collectors.toMap(Profile::getFirstName, 
                              Profile::getLastName));

但是它不起作用，我遇到了汇编错误：

Incompatible parameter types in method reference expression

原文

I have a list of Profile objects List<Profile> list.

Which I need to convert into a LinkedHashMap<String, String>.

Where object Profile is consisted of:

public class Profile {
    private String profileId;
    private String firstName;
    private String lastName;
}

I have tried the following:

Map<String, String> map = list.stream()
    .collect(Collectors.toMap(Profile::getFirstName, 
                              Profile::getLastName));

But it did not work, I'm getting a compilation error:

Incompatible parameter types in method reference expression

分享到QQ

分享到微博

如果你对这篇内容有疑问，欢迎到本站社区发帖提问参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。

发布评论

需要登录才能够评论，你可以免费注册一个本站的账号。

冬天旳寂寞 2025-01-31 04:44:05

方法参考表达式中的不兼容参数类型

确保您不使用 Row Type 作为流源的列表。 IE检查是否缺少通用类型参数：列表（必须为 list＆lt; profile＆gt; list ），否则列表的所有元素都为类型对象和配置文件>类的方法将无法访问。

默认为linkedhashmap。

默认情况下， tomap 为您提供了 map （目前是 hashmap ，但它可能会在将来发生变化）。

为了将流元素收集到 MAP 接口的特定实现中，您需要使用 collectors.tomap（） 期望四个参数：

keymapper - 生产密钥，
< 代码> valueMapper - 产生值的映射函数，
MergeFunction - 函数，旨在解决与同一密钥相关的值之间的碰撞，
mapfactory - 供应商提供新的空图将插入结果。

在下面的代码中， MergeFunction 没有做任何有用的事情，它只需要存在才能使用 tomap（）的版本，该版本允许指定 mapFactory 。

Map<String, String> map = list.stream()
        .collect(Collectors.toMap(
            Profile::getFirstName,
            Profile::getLastName,
            (left, right) -> left,
            LinkedHashMap::new
        ));

注释如果可能在某些情况下，一个以上值与同一键关联，则需要提供适当的实现 MergeFunction （ 查看特定的值或汇总值等，或使用），或使用 groupingby（）作为收集器，它将允许保留与特定密钥相关的所有值。

Incompatible parameter types in method reference expression

Make sure that you're not using a list of row type as a stream source. I.e. check if the generic type parameter is missing: List list (it has to be List<Profile> list), otherwise all elements of the list as being of type Object and methods from the Profile class would not be accessible.

Collecting into a LinkedHashMap

By default, toMap provides you with a general purpose implementation of the Map (for now it's HashMap but it might change in the future).

In order to collect stream elements into a particular implementation of the Map interface, you need to use a flavor of Collectors.toMap() that expects four arguments:

keyMapper - a mapping function to produce keys,
valueMapper - a mapping function to produce values,
mergeFunction - function that is meant to resolve collisions between value associated with the same key,
mapFactory - a supplier providing a new empty Map into which the results will be inserted.

In the code below, mergeFunction isn't doing anything useful, it just has to be present in order to utilize the version of toMap() that allows to specify the mapFactory.

Map<String, String> map = list.stream()
        .collect(Collectors.toMap(
            Profile::getFirstName,
            Profile::getLastName,
            (left, right) -> left,
            LinkedHashMap::new
        ));

Note if there could be cases when more than one value gets associated with the same key, you need either to provide a proper implementation of mergeFunction (to peek a particular value or aggregate values, etc.), or use groupingBy() as a collector, which will allow to preserve all values associated with a particular key.