reader.readLine() of BufferedReader causes this: Exception in thread "main" java.io.IOException: Stream closed

Question

Here is a code snippet from my main Java function:

try (MultiFileReader multiReader = new MultiFileReader(inputs)) {
        PriorityQueue<WordEntry> words = new PriorityQueue<>();
        for (BufferedReader reader : multiReader.getReaders()) {
            String word = reader.readLine();
            if (word != null) {
                words.add(new WordEntry(word, reader));
            }
        }
    }

Here is how I get my BufferedReader readers from another Java file:

public List<BufferedReader> getReaders() {
        return Collections.unmodifiableList(readers);
    }

But for some reason, when I compile my code here is what I get:

The error happens exactly at the line where I wrote String word = reader.readLine(); and what's weird is that reader.readLine() is not null, in fact multiReader.getReaders() returns a list of 100 objects (they are files read from a directory). I would like some help solving that issue.

I posted where the issue is, now let me provide a broader view of my code. To run it, it suffices to compile it under the src/ directory doing javac *.java and java MergeShards shards/ sorted.txt provided that shards/ is present under src/ and contains .txt files in my scenario.

• This is MergeShards.java where I have my main function:

import java.io.BufferedReader;
import java.io.Writer;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.List;
import java.util.Objects;
import java.util.PriorityQueue;
import java.util.stream.Collectors;

public final class MergeShards {
    public static void main(String[] args) throws Exception {
        if (args.length != 2) {
            System.out.println("Usage: MergeShards [input folder] [output file]");
            return;
        }

        List<Path> inputs = Files.walk(Path.of(args[0]), 1).skip(1).collect(Collectors.toList());
        Path outputPath = Path.of(args[1]);

        try (MultiFileReader multiReader = new MultiFileReader(inputs)) {
            PriorityQueue<WordEntry> words = new PriorityQueue<>();
            for (BufferedReader reader : multiReader.getReaders()) {
                String word = reader.readLine();
                if (word != null) {
                    words.add(new WordEntry(word, reader));
                }
            }

            try (Writer writer = Files.newBufferedWriter(outputPath)) {
                while (!words.isEmpty()) {
                    WordEntry entry = words.poll();
                    writer.write(entry.word);
                    writer.write(System.lineSeparator());
                    String word = entry.reader.readLine();
                    if (word != null) {
                        words.add(new WordEntry(word, entry.reader));
                    }
                }
            }
        }
    }

    private static final class WordEntry implements Comparable<WordEntry> {
        private final String word;
        private final BufferedReader reader;

        private WordEntry(String word, BufferedReader reader) {
            this.word = Objects.requireNonNull(word);
            this.reader = Objects.requireNonNull(reader);
        }

        @Override
        public int compareTo(WordEntry other) {
            return word.compareTo(other.word);
        }
    }
}

• This is my MultiFileReader.java file:

import java.io.BufferedReader;
import java.io.Closeable;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public final class MultiFileReader implements Closeable {

    private final List<BufferedReader> readers;

    public MultiFileReader(List<Path> paths) {
        readers = new ArrayList<>(paths.size());
        try {
            for (Path path : paths) {
                readers.add(Files.newBufferedReader(path));
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            close();
        }
    }

    public List<BufferedReader> getReaders() {
        return Collections.unmodifiableList(readers);
    }

    @Override
    public void close() {
        for (BufferedReader reader : readers) {
            try {
                reader.close();
            } catch (Exception ignored) {
            }
        }
    }
}

Answer 1

The finally block in your constructor closes all of your readers. Remove that.

public MultiFileReader(List<Path> paths) {
    readers = new ArrayList<>(paths.size());
    try {
        for (Path path : paths) {
            readers.add(Files.newBufferedReader(path));
        }
    } catch (IOException e) {
        e.printStackTrace();
    } /* Not this. finally {
        close();
    } */
}