为什么File :: isDirectory作为FileFilter工作正常？ [英] Why File::isDirectory works fine as a FileFilter?

问题描述

为什么 File :: isDirectory 在以下示例中作为FileFilter正常工作？

Why File::isDirectory works fine as a FileFilter in the below example?

File[] files = new File(".").listFiles(File::isDirectory);

listFiles方法需要FileFilter作为参数

The listFiles method requires a FileFilter as a parameter

public File[] listFiles(FileFilter filter) {
    ...
}

FileFilter是一个功能接口，它有一个方法 accept ，带有File参数

The FileFilter is a functional interface which has one method accept with a File parameter

boolean accept(File pathname);

File类中的isDirectory方法没有参数

And isDirectory method from the File class has no parameters

public boolean isDirectory() {
   ...
}

推荐答案

为了使事情更清楚，方法引用 File :: isDirectory 等同于以下lambda ：

To make things clearer the method reference File::isDirectory is equivalent to the following lambda:

file -> file.isDirectory()

如你所见我们传递的是文件参数然后在它上面调用 isDirectory 返回 boolean 因此满足 FileFilter 界面中的SAM。

As you can see we're passing a File parameter then calling isDirectory upon it which returns boolean hence satisfies the SAM in the FileFilter interface.

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