Programming Question-4: Fast file reading using java

import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class test {

    private static int[] a;

    static private int readInt(ByteBuffer buffer) throws IOException {
        int res = 0;
        boolean started = false;
        for (int pos = buffer.position(); pos < buffer.limit(); pos++) {
            int v = buffer.get() - '0';
            if (v >= 0 && v < 10) {
                if (!started) started = true;
                res = res * 10 + v;
            } else if (started)
                break;
        }
        return res;
    }

    public void process(String filename) throws IOException{
        long time = -System.nanoTime();
        int n = 5105042;
        a = new int[n];
        RandomAccessFile ra = new RandomAccessFile(filename, "r");
        long size = ra.length();
        FileChannel channel = ra.getChannel();
        ByteBuffer buffer = ByteBuffer.allocate((int) size);
        channel.read(buffer);
        buffer.position(0);
        Map<Integer,List<Integer>> g = new HashMap<Integer, List<Integer>>();
        for(int i = 0; i < n; i++) a[i] = readInt(buffer);
        for(int i = 0; i < n/2; i++){
            int src = a[i/2];
            int dst = a[i/2+1];
            List<Integer> edges = g.get(src);
            if (edges == null){
                edges = new ArrayList<Integer>();
                g.put(src, edges);
            }
            edges.add(dst);
        }
        long rtime = System.nanoTime()+time;
        time  += System.nanoTime();
        System.out.println("Reading time (ms)   : " + rtime/1E6);
        System.out.println("Total time (ms)     : " + time/1E6);
    }

    static public void main(String[] args) throws IOException {
        new test().process(args[0]);
    }
}