Android : Unable to enumerate usb device whenever it is connected while my application is running

I am building an app for UsbInterfacing and i need to enumerate usb devices continously while my application is running..i need to be notified when it is connected and disconnected.. My usb device's vendor id is 1204 and product-id is 64905..The enumeration works once. Will not work after i unplug the device and plug it back in..Below is my code:

package com.example.enumerate;

    import java.util.HashMap;
    import java.util.Iterator;
    import android.hardware.usb.UsbDevice;
    import android.hardware.usb.UsbManager;
    import android.os.Bundle;
    import android.widget.Toast;
    import android.app.Activity;
    import android.content.Context;

    public class Enumerate extends Activity {
    //My onResume method
        @Override
        protected void onResume() 
        {
            super.onResume();
            UsbManager manager = (UsbManager) getSystemService(Context.USB_SERVICE);
            HashMap<String, UsbDevice> deviceList = manager.getDeviceList();
            deviceIterator = deviceList.values().iterator();
//my UsbIterator    
        while(deviceIterator.hasNext()){
                device = deviceIterator.next();

        }
            if(device.getProductId()==64905) //my device ProductId
            {
                Toast t=Toast.makeText(this,"Connected",2000);
                t.show();

            }
            else
            {
                Toast t=Toast.makeText(this,"Not Connected",2000);
            }
            }
        UsbDevice device;
        Iterator<UsbDevice> deviceIterator;
        @Override
        protected void onCreate(Bundle savedInstanceState) 
        {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.main);

        }

    }