英文:
How is method reference syntax working in Java 8
问题
以下是翻译好的内容：
我对 Java 8 中方法引用的工作原理有些困惑。我为筛选文件夹中的隐藏文件编写了以下代码段。它们产生了正确的结果。我不明白代码段的选项 2 中 listFiles 方法的方法签名是如何工作的。
以下是我在 Java 8 文档中找到的内容：
File[] listFiles()File[] listFiles(FileFilter filter)File[] listFiles(FilenameFilter filter)
File[] hidden = f.listFiles((p) -> p.isHidden()); // 选项 1 - 函数签名匹配（根据我的理解）
for (int i = 0; i < hidden.length; i++) {
System.out.println(hidden[i]);
}
System.out.println("================================");
File[] hidden1 = f.listFiles(File::isHidden); // 选项 2 - 这个方法调用是如何工作的
for (int i = 0; i < hidden1.length; i++) {
System.out.println(hidden1[i]);
}
英文:
I am having some confusion of how method references work in Java 8. I wrote the following code segment for filtering hidden files in a folder. They are producing correct result. I am not understanding -> how method signature of listFiles method is working for option 2 of this code segment.
This is what I found in Java 8 documentation
File[] listFiles()File[] listFiles(FileFilter filter)File[] listFiles(FilenameFilter filter)
<!-- -->
File[] hidden = f.listFiles((p)-> p.isHidden()); //Option 1 - function signature matching (based on my understanding)
for (int i = 0; i < hidden.length; i++) {
System.out.println(hidden[i]);
}
System.out.println("================================");
File[] hidden1 = f.listFiles(File::isHidden); //Option 2 - how this method call is working
for (int i = 0; i < hidden1.length; i++) {
System.out.println(hidden1[i]);
}
答案1
得分: 3
什么是方法引用
您可以将方法引用视为调用现有方法的lambda表达式。
方法引用的种类
方法引用有四种种类：
- 对静态方法的引用：
ContainingClass::staticMethodName - 对特定对象的实例方法的引用：
containingObject::instanceMethodName - 对特定类型的任意对象的实例方法的引用：
ContainingType::methodName - 对构造函数的引用：
ClassName::new
如何理解它？
下面的表达式列出了所有隐藏文件：
f.listFiles(p -> p.isHidden());
这个表达式由实例方法listFiles(FileFilter)和lambda表达式p -> p.isHidden()组成，后者不是匿名方法，而是类File的现有实例方法。
注意，FileFilter是一个函数式接口，因此可以用作lambda表达式或方法引用的赋值目标。因此，您可以写出表达式f.listFiles(File::isHidden);
附加说明
1）您不需要括号括住p。为了更好地阅读，建议将(p)简化为p。因此，您的lambda表达式将变为p-> p.isHidden()。
2）您的for循环可以被增强的for循环替代：
for (File value : hidden) {
System.out.println(value);
}
文档：
英文:
What is a method refernce
You can see a method reference as a lambda expression, that call an existing method.
Kinds of method references
There are four kinds of method references:
- Reference to a static method:
ContainingClass::staticMethodName - Reference to an instance method of a particular object:
containingObject::instanceMethodName - Reference to an instance method of an arbitrary object of a particular type:
ContainingType::methodName - Reference to a constructor:
ClassName::new
How to understand it?
The following expression that lists all hidden files:
f.listFiles(p -> p.isHidden());
This expression is composed by the instance method listFiles(FileFilter) and your lambda expression p -> p.isHidden() that is not a anonymous method but an existing instance method of the class File.
Note that FileFilter is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference. Hence, you can write your expression f.listFiles(File::isHidden);
Side notes
- You don't need the parentheses surrounding the
p. For a better readibility, I would suggest to replace(p)with simply ap. Hence, your lambda expression will becomep-> p.isHidden(). - Your for loop can be replaced by an enhanced for loop:
for (File value : hidden) {
System.out.println(value);
}
Documentation:
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