FilenameFilter的java 8 lambda表达式 [英] java 8 lambda expression for FilenameFilter
问题描述
我正在浏览java 8中的lambda表达式
I am going through the lambda expression in java 8
当我更改了线程代码时它工作正常
when i changed the code of thread it's working fine
new Thread(new Runnable() {
@Override
public void run() {
System.out.println("run");
}
}).start();
转换为lambda表达式为
is converted to lambda expression as
new Thread(
() -> System.out.println("Hello from thread")
).start();
但我无法转换FilenameFilter表达式
But i am not able to convert the FilenameFilter Expression
File file = new File("/home/text/xyz.txt");
file.list(new FilenameFilter() {
@Override
public boolean accept(File dir, String name) {
name.endsWith(".txt");
return false;
}
});
并且未成功转换为
file.list(new FilenameFilter () {
(File a1, String a2) -> {
return false;
}
});
它在eclipse中给出错误
it's giving error as in eclipse as
此行的多个标记
- 语法错误,插入;完成语句
- 语法错误,插入}以完成阻止
- 语法错误,插入AssignmentOperator表达式以完成作业
Multiple markers at this line
- Syntax error, insert ";" to complete Statement
- Syntax error, insert "}" to complete Block
- Syntax error, insert "AssignmentOperator Expression" to complete Assignment
推荐答案
首先,你的格式是可怕,整理出来!
First things first, your formatting is horrible, sort it out!
现在,lambda语法;转换匿名类:
Now, lambda syntax; to convert the anonymous class:
final FilenameFilter filter = new FilenameFilter() {
@Override
public boolean accept(File dir, String name) {
return false;
}
};
我们首先用单一方法的等效lambda替换匿名类 accept(文件目录,字符串名称)
:
We start by replacing the anonymous class with an equivalent lambda for the single method accept(File dir, String name)
:
final FilenameFilter filter = (File dir, String name) -> {
return false;
};
但我们可以做得更好,我们不需要定义类型 - 编译器可以工作那些out:
But we can do better, we don't need to define the types - the compiler can work those out:
final FilenameFilter filter = (dir, name) -> {
return false;
};
我们可以做得更好,因为该方法返回布尔值
;如果我们有一个评估为 boolean
的语句,我们可以跳过 return
和大括号:
And we can do better still, as the method return a boolean
; if we have a single statement that evaluates to a boolean
we can skip the return
and the braces:
final FilenameFilter filter = (dir, name) -> false;
这可以是任何声明,例如:
This can be any statement, for example:
final FilenameFilter filter = (dir, name) -> !dir.isDirectory() && name.toLowerCase().endsWith(".txt");
然而,文件
API是非常旧,所以不要使用它。使用 nio API
。自2011年Java 7以来一直存在,所以确实有没有借口:
However, the File
API is very old, so don't use it. Use the nio API
. This has been around since Java 7 in 2011 so there is really no excuse:
final Path p = Paths.get("/", "home", "text", "xyz.txt");
final DirectoryStream.Filter<Path> f = path -> false;
try (final DirectoryStream<Path> stream = Files.newDirectoryStream(p, f)) {
stream.forEach(System.out::println);
}
事实上你的例子中有一个内置于文件
接受一个Glob :
And in fact your example has a specific method built into Files
that takes a Glob:
final Path p = Paths.get("/", "home", "text", "xyz.txt");
try (final DirectoryStream<Path> stream = Files.newDirectoryStream(p, "*.txt")) {
stream.forEach(System.out::println);
}
或者,使用更现代的 Files.list
:
Or, using the more modern Files.list
:
final Path p = Paths.get("/", "home", "text", "xyz.txt");
final PathMatcher filter = p.getFileSystem().getPathMatcher("glob:*.txt");
try (final Stream<Path> stream = Files.list(p)) {
stream.filter(filter::matches)
.forEach(System.out::println);
}
这里 filter :: matches
是方法引用,因为方法 PathMatcher.matches
可用于实现功能接口 Predicate< Path>
因为它需要路径
并返回布尔值
。
Here filter::matches
is a method reference because the method PathMatcher.matches
can be used to implement the functional interface Predicate<Path>
as it takes a Path
and returns a boolean
.
暂且不说:
f.list(new FilenameFilter() {
@Override
public boolean accept(File dir, String name) {
name.endsWith(".txt");
return false;
}
});
这没有任何意义......
This makes no sense...
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