有没有办法在一个Java8流中读取两个或更多的文件？ [英] Is there any way for reading two or more files in one Java8-stream?

问题描述

我喜欢新的Java8 StreamAPI，并且希望不仅仅用于一个文件。
通常，我使用下面的代码：

  Stream< String> lines = Files.lines（Paths.get（/ somepathtofile））; 
  

但是如果可能的话，如何在一个流中读取两个文件呢？

解决方案

没有任何额外的辅助函数或外部库，最简单的是：

 流<字符串> lines1 = Files.lines（Paths.get（/ somepathtofile））; 
 Stream< String> lines2 = Files.lines（Paths.get（/ somepathtoanotherfile））; 
 
 Stream.concat（lines1，lines）
 .filter（...）
 .forEach（...）; 
  

如果 Files.lines 声明抛出一个检查的异常，你可以做

$ $ $ $ $ $ c $ Stream.of（/ file1，/ file2 ）
.map（Paths :: get）
.flatMap（Files :: lines）....

但是，唉，我们不能这样做。有几个解决方法。一个是使你自己的版本 Files.lines 调用标准版本，捕获 IOException ，并重新抛出 UncheckedIOException 。另一种方法是从抛出检查异常的方法中进行功能的更一般的方法。它看起来像这样：

pre $ $ $ $ $ $ $ $ $ $ $ public interface ThrowingFunction< T，R>扩展Function< T，R> {

@Override
public default R apply（T t）{
try {
return throwingApply（t）;
catch（Exception e）{
throw new RuntimeException（e）;
}
}

public static< T，R>功能< T，R> （ThrowingFunction< T，R> f）{
return f;
}

R throwingApply（T t）throws Exception;

$ / code>

然后是

<$ Stream.of（/ somefile，/ someotherfile，/ yetanotherfile）
.map（Paths :: get）
.flatMap（ThrowingFunction。 wrap（Files :: lines））
.....

在那里经历了为每个功能界面写上类似上面的东西的麻烦。

I like new Java8 StreamAPI and want use it not only for one file. As usually, I use this code:

Stream<String> lines = Files.lines(Paths.get("/somepathtofile"));

But how read two file in one stream if it possibly?

解决方案

Without any extra helper functions or outside libraries, the easiest is:

Stream<String> lines1 = Files.lines(Paths.get("/somepathtofile"));
Stream<String> lines2 = Files.lines(Paths.get("/somepathtoanotherfile"));

Stream.concat(lines1, lines)
    .filter(...)
    .forEach(...);

If Files.lines hadn't been declared to throw a checked exception, you'd be able to do

Stream.of("/file1", "/file2")
     .map(Paths::get)
     .flatMap(Files::lines)....

But, alas, we can't do that. There are several workarounds. One is to make your own version of Files.lines that calls the standard one, catches IOException and rethrows as an UncheckedIOException. Another approach is a more general way to make functions out of methods that throw checked exceptions. It would look something like this:

@FunctionalInterface
public interface ThrowingFunction<T,R> extends Function<T,R> {

    @Override
    public default R apply(T t) {
        try {
            return throwingApply(t);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    public static<T,R> Function<T,R> wrap(ThrowingFunction<T,R> f) {
        return f;
    }

    R throwingApply(T t) throws Exception;
}

and then

Stream.of("/somefile", "/someotherfile", "/yetanotherfile")
        .map(Paths::get)
        .flatMap(ThrowingFunction.wrap(Files::lines))
        .....

There are several libraries out there that went through the trouble of writing something like the above out for every functional interface.

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