Collectors.toMap() keyMapper——更简洁的表达方式?

2022-01-22 00:00:00 lambda collections java-8 java java-stream

我正在尝试为以下 Collectors.toMap() 调用中的keyMapper"函数参数提出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，但我想不出一个可以编译的.(我对 lambdas 很陌生，所以这不足为奇.)

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:

如已接受的答案中所述

Person::getLast

是我一直在寻找的，也是我尝试过的.然而，Eclipse 4.3 的 BETA_8 nightly build 是个问题——它标记为错误.从命令行编译时(我应该在发布之前完成)，它起作用了.因此，是时候向 eclipse.org 提交错误了.

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，您可以通过 使用 方法参考:::

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

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

而不是 Function.identity，您可以简单地使用等效的 lambda:

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

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

如果您使用 Netbeans，只要匿名类可以被 lambda 替换，您就会得到提示.

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

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