The Location API for J2ME
The Location API for J2ME specification defines an optional package, javax.microedition.location, that enables developers to write wireless location-based applications and services for resource-limited devices like mobile phones, and can be implemented with any common location method. The compact and generic J2ME location APIs provide mobile applications with information about the device's present physical location and orientation (compass direction), and support the creation and use of databases of known landmarks, stored in the device.
JSR 179 requires the Connected Device Configuration (CDC) or version 1.1 of the Connected Limited Device Configuration (CLDC). CLDC 1.0 isn't adequate because it doesn't support floating-point numbers, which the API uses to represent coordinates and other measurements. The Location API doesn't depend on any particular profile -- it can be used with MIDP or the Personal Profile.
The hardware platform determines which location methods are supported. If it doesn't support at least one location provider, LBS won't be possible. Applications can request providers with particular characteristics, such as a minimum degree of accuracy. Some location methods may be free; others may entail service fees. The application should warn the user before any charges are incurred.
It is up to the application to determine the criteria for selecting the location method. Criteria fields include: accuracy, response time, need for altitude, and speed. Once the application obtains a LocationProvider instance that meets the criteria, it can use that object to obtain the location, in either of two ways:
- Invoke a method synchronously to get a single location.
- Register a listener and get periodic updates at application-defined intervals.
The Location class abstracts the location results. Its object contains coordinates, speed if available, textual address if available, and a time stamp that indicates when the location measurements were made.
Coordinates are represented by either of two classes:
- A
Coordinates object represents a point's latitude and longitude in degrees, and altitude in meters.
- A
QualifiedCoordinates object contains latitude, longitude, and altitude, and also an indication of their accuracy, represented as the radius of an area.
The following segment of code demonstrates how to obtain the present location of the device synchronously:
...
// Set criteria for selecting a location provider:
// accurate to 500 meters
horizontally
Criteria cr= new Criteria();
cr.setHorizontalAccuracy(500);
// Get
an instance of the provider
LocationProvider lp= LocationProvider.getInstance(cr);
//
Request the location, setting a one-minute timeout
Location l = lp.getLocation(60);
Coordinates
c = l.getQualifiedCoordinates();
if(c != null ) {
// Use coordinate information
double lat = c.getLatitude();
double lon = c.getLongitude();
}
...
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Landmarks
A landmark is a location associated with a name and a description. Landmarks can be stored in a device-based database, where they can be shared among all J2ME applications. Landmarks can store frequently used locations: home, office, favorite restaurants, and so on. Each is represented by a Landmark instance, and the database by a LandmarkStore. You can create multiple named LandmarkStores to group locations into categories such as cinemas, museums, or customer sites.
If the device includes a compass, the application may be able to determine not only its location but its orientation, which is useful in navigational applications. The Orientation class represents the device's azimuth as an angle from due north, which the application can easily convert to a compass direction.
