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Creating a custom Map Item with Maps API for Java ME

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This article explains how to implement a CustomItem that displays a map by using the HERE Maps API for Java ME

Contents

Introduction

JavaMELocationAPI CustomItem.png

The HERE Maps API for Java ME offers a MapDisplay class that is used to display a portion of a geographical map. By using the methods of this class, and specifically its paint() method, it is possible to build a CustomItem that embeds a MapDisplay, and so to show a geographical map in a Java ME Form screen.

Implementing the MapCustomItem

Since MapCustomItem extends the CustomItem class, its base skeleton can be written as:

public class MapCustomItem extends CustomItem
{
MapDisplay map = null;
}

The MapCustomItem defines a MapDisplay variable that will hold a reference to the MapDisplay object used to display the geographical map.

The constructor

Now, it is necessary to define an explicit constructor, since the CustomItem class does not define a default one. The class constructor should initialize the MapDisplay object that will be used to show the map within the item itself: this can be done by using the MapFactory class, whose purpose is to instantiate MapDisplay and MapObject implementations.

When creating a MapFactory object, three parameters are necessary: the size of map tiles, the map width, and the map height. The tile size must be one of the values defined by the MapResolutionEnum class.

So, the arguments that can be passed to the MapCustomItem constructor can be:

  • the Item label
  • the map width
  • the map height

And the constructor can be written as:

public MapCustomItem(String label, int width, int height)
{
super(label);
 
MapFactory mapFactory = MapFactory.createMapFactory(
128,
width,
height );
 
this.map = mapFactory.createMapDisplay();
}

Implementing CustomItem methods

When implementing a CustomItem, some size-related methods need to be implemented, in order to tell what are the minimum and preferred size of the item itself. For the MapCustomItem, a trivial implementation could assume that both the preferred and minimum size are equal to the map width and height.

protected int getMinContentHeight() {
return map.getHeight();
}
 
protected int getMinContentWidth() {
return map.getWidth();
}
 
protected int getPrefContentHeight(int arg0) {
return map.getHeight();
}
 
protected int getPrefContentWidth(int arg0) {
return map.getWidth();
}

Painting the map

The CustomItem paint() method is responsible for painting the item content: since the MapCustomItem has to show the MapDisplay object, it would be enough to call its own paint() method, as shown below:

protected void paint(Graphics graphics, int width, int height)
{
this.map.paint(graphics);
}

Exposing the MapDisplay object

Since it would be useful to get a reference to the MapDisplay object from outside the CustomMapItem class, in order to add some interactivity to the map, a new method is added:

public MapDisplay getMapDisplay() {
return map;
}

Using the MapCustomItem

A base MIDlet that may want to use and display the MapCustomItem inside a Form can be written as follows.

public class MapFormMIDlet extends MIDlet implements ItemStateListener {
 
MapCustomItem mapItem = null;
 
protected void startApp() throws MIDletStateChangeException {
 
ApplicationContext ctx = ApplicationContext.getInstance();
 
ctx.setAppID("MyAppId");
ctx.setToken("MyToken");
 
Form f = new Form("Map Form");
 
mapItem = new MapCustomItem("A map", 220, 220);
 
mapItem.getMapDisplay().setZoomLevel(12, 0, 0);
 
mapItem.getMapDisplay().setCenter(new GeoCoordinate(41.90311, 12.49576, 0.0f));
 
f.append(mapItem);
 
Display.getDisplay(this).setCurrent(f);
}
protected void destroyApp(boolean unconditional)
throws MIDletStateChangeException {
 
}
 
protected void pauseApp() {
}
}

In the above code, the getMapDisplay() method is used to grab a reference to the MapDisplay object and call some of its own methods (for instance, to set a center and a default zoom level for the map).

Interacting with other Form items

A Gauge item can be added to the Form in order to let the user change the zoom level of the map. The Gauge maximum and initial values can be dynamically set by retrieving the maximum and current zoom values of the MapDisplay object (retrieved with the MapCustomItem.getMapDisplay() method).

Since the MIDlet has to be notified when the Gauge value changes, it also has to implement the ItemStateListener interface.

public class MapFormMIDlet extends MIDlet implements ItemStateListener
{
MapCustomItem mapItem = null;
 
Gauge zoomGauge = null;
 
protected void startApp() throws MIDletStateChangeException
{
[...]
 
int maxZoom = (int)mapItem.getMapDisplay().getMaxZoomLevel();
int initialZoom = (int)mapItem.getMapDisplay().getZoomLevel();
 
zoomGauge = new Gauge("Map zoom", true, maxZoom, initialZoom);
 
f.append(zoomGauge);
 
f.setItemStateListener(this);
}
}

The ItemStateListener interface defines a method, itemStateChanges(), that must be implemented by the MapFormMIDlet class in order to detect changes to Form items. When the Gauge value gets changed by the user, this method has to modify the map zoom value, and this can be performed by using the MapDisplay.setZoomLevel() method, as shown in the code snippet below:

public void itemStateChanged(Item item)
{
if(item == zoomGauge)
{
double newZoom = zoomGauge.getValue();
 
mapItem.getMapDisplay().setZoomLevel(newZoom, 0, 0);
}
}


Resources

A sample MIDlet showing the MapCustomItem in action is available here: File:JavaMELocationAPI MapCustomItem.zip

The source code of the MapCustomItem class is available for download here: File:MapCustomItem.java.zip

The source code of the MapFormMIDlet midlet is available for download here: File:MapFormMIDlet.java.zip

See Also

Article Metadata
Code ExampleTested with
Devices(s): X3-02, Asha 311, Asha 501
Compatibility
Device(s): All
Dependencies: HERE Maps API for Java ME v1.3
Article
Keywords: HERE Maps, Java ME, Form, CustomItem, Gauge
Created: jappit (28 Jun 2011)
Reviewed: skalogir (06 Oct 2011)
Last edited: hamishwillee (25 Jun 2013)
This page was last modified on 25 June 2013, at 06:48.
111 page views in the last 30 days.

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