In this article it is described and developed a fully MIDlet application that gets coordinates from the mobile device GPS module and use them to display a Google static map for that location. The application can be tested on the emulator or on a real device that has a GPS module incorporated.
You can download the full source code, including a sample test MIDlet.
If you have a mobile device that doesn’t have a GPS incorporated, this solution will not work because the J2ME Location API doesn’t have methods for connecting through Bluetooth to that external module. In this case, you need another solution (I will post it in another article) that
- uses the Bluetooth API (JSR 82) to discover and to connect to the external GPS module;
- parse the received GPS NMEA strings.
How to use J2ME Location API (JSR-179) and get the GPS coordinates
Midlet that displays GPS coordinates
- create a javax.microedition.location.Criteria instance, used to select the location provider; although there are multiple options (described by the JSR-179 documentation), their default values are the least restrictive; in this example we choose explicitly to allow a cost for the service (it’s free) and we don’t have a power consumption requirement; this step is not so important because the device has one GPS module and these are the criteria for selecting between multiple modules;
import javax.microedition.location.*; ... Criteria criteria = new Criteria(); //same as default value criteria.setCostAllowed(true); //same as default value criteria.setPreferredPowerConsumption(Criteria.NO_REQUIREMENT);
- obtain the location provider reference, a LocationProvider instance, using previous defined criteria and use it to query the GPS module for the current location; the getLocation() method has a timeout parameter indication how long we are willing to wait (seconds) for the data;
- get the coordinates and extract the latitude and longitude; Attention ! the coordinates are returned as double and only the CLDC 1.1 supports floating point data processing (set the device configuration to CLDC 1.1 for this project because CLDC 1.0 does not support double type):
LocationProvider provider = null; double latitude; double longitude; try { //get the location provider provider = LocationProvider.getInstance(criteria); //set a timeout of 60 seconds Location location = provider.getLocation(60); //get the coordinates Coordinates coordinates = location.getQualifiedCoordinates(); if (coordinates != null) { //get the latitude and longitude of the coordinates latitude = coordinates.getLatitude(); longitude = coordinates.getLongitude(); } else { //no coordinates } } catch (LocationException ex) { System.out.println("Problems with location provider ! " + ex.getMessage()); ex.printStackTrace(); } catch (InterruptedException ex) { System.out.println(ex.getMessage()); ex.printStackTrace(); }
In the final application, the previous code sample is part of the
getGPSData() method. This method defines a inner class that extends
Thread because we want to query the GPS module on another thread than
the main one. In this way, the application will respond to commands
while it is waiting for the GPS to respond.How to get GPS data on regular intervals
If you want to develop a J2ME application that will receive GPS coordinates on regular intervals (not the case for this example) you must define a handler used by the LocationProvider to notify the application. To do this, you must implement the LocationListener interface that has 2 abstract methods:- locationUpdated(LocationProvider provider, Location location) – method called by the provider at regular intervals to provide the current location;
- providerStateChanged(LocationProvider provider, int newState) – method called by the provider to announce if its new state (LocationProvider.OUT_OF_SERVICE, LocationProvider.AVAILABLE, LocationProvider.TEMPORARILY_UNAVAILABLE);
public void locationUpdated(LocationProvider arg0, Location arg1) { if (arg1 != null && arg1.isValid()) { //get the coordinates Coordinates coordinates = arg1.getQualifiedCoordinates(); if (coordinates != null) { //get the latitude and longitude of the coordinates. latitude = coordinates.getLatitude(); longitude = coordinates.getLongitude(); } else { //no valid coordinates } } } public void providerStateChanged(LocationProvider arg0, int arg1) { if (arg1 == LocationProvider.OUT_OF_SERVICE || arg1 == LocationProvider.TEMPORARILY_UNAVAILABLE) { System.out.println("GPS inactive"); } }
provider.setLocationListener(this, 60, -1, -1);
If you want to stop the updates (this consumes battery power), this is done calling one again the setLocationListener() method:
provider.setLocationListener(null, -1, -1, -1);
How to use Google Static Map API
Google Static Maps is a free service offered by Google to developers that want to embed maps into their applications based on URL parameters sent through a simple HTTP query string without requiring JavaScript or any dynamic page loading. The service sends an image as response.So, in order to get an 300×300 pixels map image for the 44.435251 latitude and 26.1024 longitude, with a moderate zoom, you make a HTTP request using:
http://maps.google.com/maps/api/staticmap?center=44.435251,26.1024&zoom=14&size=300×300&sensor=false
Link: How to use GPS location services and Google Static Map API in J2ME MIDlets _ IT&C Solutions
