MergeSort implementation

MergesortFiles.java

import java.io.*;

public class MergesortFiles {

    public static void main(String[] bla) throws Exception {

        final int MAX_READ_STEP = 15;

        final File f1 = new File("/tmp/test.txt");
        final File f2 = new File("/tmp/test2.txt");

        try (final Reader fr1 = new BufferedReader(new FileReader(f1));
             final Reader fr2 = new BufferedReader(new FileReader(f2))) {
            while (true) {

                fr1.mark(MAX_READ_STEP);
                fr2.mark(MAX_READ_STEP);

                final Integer i1 = readNextInt(fr1);
                final Integer i2 = readNextInt(fr2);

                if(i1 == null || i2 == null) {
                    Reader notEmptyReader = i1 == null ? fr2 : fr1;
                    notEmptyReader.reset();
                    printTheRest(notEmptyReader);
                    return;
                }

                if(i1 >= i2) {
                    System.out.println(i2);
                    fr1.reset();
                } else {
                    System.out.println(i1);
                    fr2.reset();
                }
            }


        }

    }

    private static void printTheRest(Reader r) throws IOException {
        while (true) {
            final Integer i = readNextInt(r);
            if (i == null) {
                break;
            }
            System.out.println(i);
        }
    }

    private static Integer readNextInt(Reader fr) throws IOException {
        char c;
        final StringBuilder sb = new StringBuilder();
        do {
            int i = fr.read();

            if(i == -1) {
                return null;
            }

            c = (char)i;

            if (c == ' ' || c == '\n') {
                break;
            }
            sb.append(c);
        } while (fr.ready());

        return Integer.parseInt(sb.toString());
    }

}