How to process stream of streams?

Question

Consider I want to println the lines of a list of files, using the Java 8 streams API only. How would I write that?

This is not my real case, just a very simplified form of it.

File[] files;
Arrays.stream(files). // what now?

I thought about mapping it from File to Stream<String> but then I got stuck.

Answer 1

public class AllFilesLines {
  public static void main(String[] args) {
      File[] files = …

      Arrays.stream(files).flatMap(AllFilesLines::lines)
            .forEach(System.out::println);
  }
  static Stream<String> lines(File f) {
      try { return Files.lines(f.toPath()); }
      catch (IOException e) { throw new UncheckedIOException(e); }
  }
}

Answer 2

You can do something like this.

package de.lhorn.so;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.util.Arrays;

public class SOPlayground {

    public static void main(String[] args) throws Exception {
        File[] files = {new File("/tmp/1.txt"), new File("/tmp/2.txt")};

        Arrays.stream(files).forEach(f -> { // Stream of File
            try (InputStream fis = new FileInputStream(f);
                    Reader isr = new InputStreamReader(fis);
                    BufferedReader reader = new BufferedReader(isr)) {
                reader.lines().forEach(line -> { // Stream of String
                    System.out.println(line);
                });
            } catch (IOException ex) {
                System.err.println(ex);
            }
        });
    }
}

Edit: Using Files:

package de.lhorn.so;

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.util.Arrays;

public class SOPlayground {

    public static void main(String[] args) throws Exception {
        File[] files = {new File("/tmp/1.txt"), new File("/tmp/2.txt")};

        Arrays.stream(files).forEach(f -> {
            try {
                Files.lines(f.toPath()).forEach(System.out::println);
            } catch (IOException ex) {
                System.err.println(ex);
            }
        });
    }
}

Answer 3

You can also use Files.lines.

By default it considers that the characters in the file are encoded in UTF-8, but you can specify a Charset if you want.

File[] files = {new File("file1.txt"), new File("file2.txt")};
for(File f : files){
    Files.lines(f.toPath()).forEach(System.out::println);
}