DataInputStream异常

DataInputStream异常：

package mygame;
 
  import java.io.*;
  import javax.microedition.lcdui.*;
  import javax.microedition.lcdui.game.*;
 
  public class Screen extends GameCanvas implements Runnable {
 
  private Sprite oRole;
  private LayerManager oMap;
  private TiledLayer oLayer;
  public Screen()
  {
  super(false);
  InitMap("/mygame/tile.png","/mygame/map.dat");
  }
 
  private void CreateTiledLayer(String sPathName)
  {
  try
  {
  Image oTiles=Image.createImage(sPathName);
  oLayer=new TiledLayer(100,100,oTiles,20,20);
  }
  catch(Exception e)
  {
  e.printStackTrace();
  }
  }
 
  public void InitMap(String sPathName,String MapData)
  {
  CreateTiledLayer(sPathName);
  int iRows=oLayer.getHeight()/oLayer.getCellHeight();
  int iColumes=oLayer.getWidth()/oLayer.getCellWidth();
 
  int iCells=iRows*iColumes;
  InputStream is=null;
  is=getClass().getResourceAsStream(MapData);
  byte [] ba=new byte[50];
  try
  {
 
  //////////////////////////////////////////////////////////////////////
  int count=is.read(ba); 为什么到这里就不进行下去了
  /////////////////////////////////////////////////////////////////////
  }
  catch(Exception e)
  {
  e.printStackTrace();
  }
  for (int i=0;i<iRows;i++)
  {
  for(int j=0;j<iColumes;j++)
  {
  try
  {
  oLayer.setCell(i,j,ba[i*5+j]);
  }
  catch (Exception e)
  {
  oLayer.setCell(i,j,0);
  }
 
  }//for 1
  }//for 2
  oMap.append(oLayer);
  }
 
  public void run()
  {
  Graphics g=this.getGraphics();
  while (true)
  {
  oMap.paint(g,0,0);