Collectors.toMap（）keyMapper - 更简洁的表达方式？ [英] Collectors.toMap() keyMapper -- more succinct expression?

问题描述

我正在尝试为以下 Collectors.toMap（）调用中的keyMapper函数参数提供一个更简洁的表达式：

I'm trying to come up with a more succinct expression for the "keyMapper" function parameter in the following Collectors.toMap() call:

List<Person> roster = ...;

Map<String, Person> map = 
    roster
        .stream()
        .collect(
            Collectors.toMap(
                new Function<Person, String>() { 
                    public String apply(Person p) { return p.getLast(); } 
                },
                Function.<Person>identity()));

似乎我应该能够使用lambda表达式内联它，但我无法想出一个编译。 （我对lambdas很新，所以这并不奇怪。）

It seems that I should be able to inline it using a lambda expression, but I cannot come up with one that compiles. (I'm quite new to lambdas, so that's not much of a surprise.)

谢谢。

- >更新：

如接受的答案中所述

Person::getLast

是我要找的东西，也是我尝过的东西。然而，Eclipse 4.3的BETA_8每晚构建是问题 - 它标记为错误。从命令行编译时（我应该在发布之前完成），它可以工作。所以，是时候用eclipse.org提交bug了。

is what I was looking for, and is something I had tried. However, the BETA_8 nightly build of Eclipse 4.3 was the problem -- it flagged that as wrong. When compiled from the command-line (which I should have done before posting), it worked. So, time to file a bug with eclipse.org.

谢谢。

推荐答案

你可以使用lambda：

You can use a lambda:

Collectors.toMap(p -> p.getLast(), Function.identity())

或者更简洁地说，你可以使用方法参考使用 :: ：

or, more concisely, you can use a method reference using :::

Collectors.toMap(Person::getLast, Function.identity())

而不是 Function.identity ，你可以简单地使用等价的lambda：

and instead of Function.identity, you can simply use the equivalent lambda:

Collectors.toMap(Person::getLast, p -> p)

如果使用Netbeans，只要匿名类可以被lambda替换，就应该得到提示。

If you use Netbeans you should get hints whenever an anonymous class can be replaced by a lambda.

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