import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;

import java.io.*;
import java.nio.*;
import java.util.*;
import java.nio.channels.FileChannel;
import java.util.function.Supplier;
import java.util.stream.Stream;

public class Main {
    public static void main(String... args) {
        try (RandomAccessFile f = new RandomAccessFile(new File("/tmp/wtf"), "r")) {
            long fileSize = f.length();
            MappedByteBuffer mem = f.getChannel().map(FileChannel.MapMode.READ_ONLY, 0, fileSize);
            Map<Integer, Long> freqStat =  Stream.generate(unsignedByteSupplier(mem))
                                                 .limit(fileSize)
                                                 .collect(groupingBy(identity(), counting()));
            Long topFreq = topFrequency(freqStat);
            showKeysByValue(freqStat, topFreq);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    static Supplier<Integer> unsignedByteSupplier(ByteBuffer buff) {
        return () -> Byte.toUnsignedInt(buff.get());
    }

    static Long topFrequency(Map<Integer, Long> stat) {
        return stat.values().stream().max(Long::compare).get();
    }

    static void showKeysByValue(Map<Integer, Long> map, Long value) {
        map.entrySet().stream()
           .filter(e -> Objects.equals(e.getValue(), value))
           .map(Map.Entry::getKey)
           .forEach(System.out::println);
    }
}