Modify file using Files.lines

Question

I\'d like to read in a file and replace some text with new text. It would be simple using asm and int 21h but I want to use the new java 8 streams.

    File         


        

Answer 1

The Files.write(..., Iterable, ...) method seems tempting here, but converting the Stream to an Iterable makes this cumbersome. It also "pulls" from the Iterable, which is a bit odd. It would make more sense if the file-writing method could be used as the stream's terminal operation, within something like forEach.

Unfortunately, most things that write throw IOException, which isn't permitted by the Consumer functional interface that forEach expects. But PrintWriter is different. At least, its writing methods don't throw checked exceptions, although opening one can still throw IOException. Here's how it could be used.

Stream<String> stream = ... ;
try (PrintWriter pw = new PrintWriter("output.txt", "UTF-8")) {
    stream.map(s -> s.replaceAll("foo", "bar"))
          .forEachOrdered(pw::println);
}

Note the use of forEachOrdered, which prints the output lines in the same order in which they were read, which is presumably what you want!

If you're reading lines from an input file, modifying them, and then writing them to an output file, it would be reasonable to put both files within the same try-with-resources statement:

try (Stream<String> input = Files.lines(Paths.get("input.txt"));
     PrintWriter output = new PrintWriter("output.txt", "UTF-8"))
{
    input.map(s -> s.replaceAll("foo", "bar"))
         .forEachOrdered(output::println);
}