Will �cross platform app� run on one platform?

Question

Hi there,
please forgive my stupidity but if I have a J2ME app which contains one function calling functions from Blackberry library. Can this app be installed and run ok on non Blackberry phones, such as Nokia (suppose that Blackberry related code will not be called under this non-Blackberry phone), within Nokia emulator environment, or real Nokia phones?
It would be much appreciated if someone could shed some light here so that my time and effort to try it out could be saved.
Many thanks in advance,
qmei from London

Answer 1

Here, let me actually be helpfull...
It depends on the phone.
To be able to use the exact same app in multiple devices, when some are calling device specific code is possible, but it really does depend on a few factors...
1) You must not ever reference or instantiate any classes that import the device specific libraries. If any such classes are referenced directly in your application, the offending class will be loaded into memory, at which point the device's VM will notice that it's trying to reference a library that it doesn't recognize, and likely crash out.
2) You must only load such offending classes by name, and even then, there must only reference offending classes through an interface. So if you have a canvas class that imports nokia libraries, as well as other canvas classes that import other manufacturer libraries, you must make them all implement the same interface, and do something like this...
try{
Class cls = Class.forName("com.nokia.mid.ui.FullCanvas");
cls = Class.forName("NokiaGameScreen");
Object o = (cls.newInstance());
gameScreen = (GameScreen)o;
gameScreen.init(this);
}catch(Exception e){
//cant' use nokia!
//try another manufacturer...
}... so I'll explain in greater detail...
1) Class cls = Class.forName("com.nokia.mid.ui.FullCanvas");This line with throw an exception if no class matches the supplied argument. Generally you'd do something like this to check for the existance of one of the classes you intend to import. If this line does not throw an Exception, than instantiating the desired display should be fine. In this case, if we don't throws an Exception, we can be sure that we are running on a Nokia device.
2) cls = Class.forName("NokiaGameScreen");This line actually grabbs the class you intend to instantiate, which contains the offending import. Only when we reach this point does the VM load the class into memory, and validate it's imports. We have already determined that we are running on a Nokia device from the previous line.
3) Object o = (cls.newInstance()); Get an instance of the desired class.
4) gameScreen = (GameScreen)o;gameScreen is of the type 'GameScreen', which is an interface I defined and is implemented by all of my different manufacturer/device specific Canvas's.
5) gameScreen.init(this);I am simply calling the 'public void init(Midlet)' function I created that is exposed through my 'GameScreen' interface.
As you can see in the above example, I never directly reference the 'NokiaGameScreen' class, and instead indirectly reference it through an interface it's implementing called 'GameScreen'.
This type of 'cross platform' coding works well on many, but not all devices. It's a headache, but some devices go through the entire .jar file and validate each and every .class file during the installation process. These annoying phones will cancel the install if they find any imports that are not recognized. So even if the class is simply accidentally left in the .jar, with no possible way to ever load or instantiate it, the device may decide it won't let you run or install.
Because of the problem devices, I always create a separate build for each manufacturer, and exclude the other manufacturer's classes from the build entirely.
Message was edited by:
hooble