android开发：退出程序（对话框、两次返回键退出）

private void exitDialog()
{
AlertDialog.Builder aa=new AlertDialog.Builder(this);
aa.setTitle("消息提示");
aa.setMessage("确认退出？");
aa.setIcon(R.drawable.icon);
aa.setPositiveButton("确定",new DialogInterface.OnClickListener() {

@Override
public void onClick(DialogInterface dialog, int which) {
// TODO Auto-generated method stub
MainActivity.this.finish();
}
});
aa.setNegativeButton("取消", null);
aa.create();
aa.show();
}

@Override
public boolean onKeyDown(int keyCode, KeyEvent event)
{
if(keyCode == KeyEvent.KEYCODE_BACK)
{
exitDialog();
}
return true;

}

private static boolean isExit=false;
private static boolean hasTask=false;
Timer tExit=new Timer();
TimerTask task=new TimerTask() {

@Override
public void run() {
// TODO Auto-generated method stub
isExit=false;
hasTask=true;
}
};

@Override
public boolean onKeyDown(int keyCode, KeyEvent event)
{
if(keyCode == KeyEvent.KEYCODE_BACK)
{
//exitDialog();
if(isExit==false)
{
isExit=true;
Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
if(!hasTask) {
tExit.schedule(task, 2000);
}
}
else
{
finish();
System.exit(0);
}

}
//return true;
return false;

}