在方法引用的返回值上调用方法 [英] Call a method on the return value of a method reference

问题描述

我有一个文件流要根据文件名的结尾进行过滤：

I have a stream of files that I want to filter based on the ending of the file name:

public Stream<File> getFiles(String ending) throws IOException {
    return Files.walk(this.path)
            .filter(Files::isRegularFile)
            .map(Path::toFile)
            .filter(file -> file.getName().endsWith(ending));
}

虽然最后一行的lambda还不错，但我想我可以用方法也在那里引用，如下所示：

While the lambda in the last line is not bad, I thought I could use method references there as well, like so:

 .filter(File::getName.endsWith(ending));

或者用括号括起来。但是，这失败了此表达式的目标类型必须是功能接口

Or alternatively wrapped in parentheses. However, this fails with The target type of this expression must be a functional interface

你能解释为什么这个没有工作？

Can you explain why this doesn't work?

推荐答案

你能解释为什么这不起作用吗？

Can you explain why this doesn't work?

方法引用是l​​ambda表达式的语法糖。例如，方法引用 File :: getName 与（文件  f）  - >  f.getName（）相同。

Method references are syntactical sugar for a lambda expression. For example, the method reference File::getName is the same as (File f) -> f.getName().

Lambda表达式是用于定义功能接口实现的方法文字，例如 Function ， Predicate ，供应商等

Lambda expressions are "method literals" for defining the implementation of a functional interface, such as Function, Predicate, Supplier, etc.

为了使编译器知道您正在实现什么接口，lambda或方法引用必须具有目标类型：

For the compiler to know what interface you are implementing, the lambda or method reference must have a target type:

// either assigned to a variable with =
Function<File, String> f = File::getName;
// or assigned to a method parameter by passing as an argument
// (the parameter to 'map' is a Function)
...stream().map(File::getName)...

或（异常）投射到某物：

or (unusually) cast to something:

((Function<File, String>) File::getName)

赋值上下文，方法调用上下文和强制转换上下文都可以为lambda或方法引用提供目标类型。 （在上述所有3种情况中，目标类型为功能<文件， 字符串> 。）

Assignment context, method invocation context, and cast context can all provide target types for lambdas or method references. (In all 3 of the above cases, the target type is Function<File, String>.)

编译器告诉你的是你的方法引用没有目标类型，所以它不知道如何处理它。

What the compiler is telling you is that your method reference does not have a target type, so it doesn't know what to do with it.

这篇关于在方法引用的返回值上调用方法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！