How to create a mobile oriented, multiplatform, shared library in Java?

Question

I have a Java application that runs on BlackBerry (JDE 4.5). I want to port this application to Android, and be able to maintain the 2 applications simultaneously. I may also want to port this application to other Java platforms (J2ME ?).

I understand that a good part of the code will have to be specific to each platform (UI and other stuff). But I also feel that a lot of the code could (should) be shared (domain related classes).

What is the best way to achieve this, and what are the pitfalls to avoid?

I have been able so far to create a JAR with all my shared classes, that I have been able to integrate into my BlackBerry application (using preverify and rapc). But:

• The JAR is a J2SE library. How can I make sure that it will run (or even compile) on BlackBerry, Android or J2ME?
• I am also using a JSON library targeting J2ME (https://github.com/upictec/org.json.me/). This library seems to make use of some kind of preprocessing directives (CLDC, see https://github.com/upictec/org.json.me/blob/master/src/main/java/org/json/me/JSONObject.java#L392). How can I use (or convert) this library using the right preprocessing definitions?

Answer 1

This is likely to be difficult:

• As you have already identified, the UI code will have to be different for each platform.
• There are major differences between Java SE / Android and Java ME-based platforms. For example, ME doesn't have the Collections framework, or the java.io or java.nio stacks.

It is hard to predict from the information you've provided, but there's a fair chance that you'll spend more time fighting the platform dependencies than you are saving by sharing the code-base.

Answer 2

These days, the biggest stumbling block to sharing code this way is that the BlackBerry VM and Android VM both support different versions of the Java language. BlackBerry uses a subset of Java 1.3, Android uses a subset of Java 1.5. (As an aside, neither platform implements a Java VM, both use their own VMs. Java is used as the programming language. Java bytecodes must be transformed to the appropriate native VM format before they can run on the platform.)

The biggest difference you will find as a library implementor is that the BlackBerry lacks the things that were introduced in 1.5, very important things like generics and enums. Even worse, the Collections classes are missing from the BlackBerry. It is unfortunate, but that is the way it has been for a long time now.

This means that to be truly portable you have to write to the lowest-common denominator, which means using (very) old-style classes like Hashtable and Vector, not having generics, rolling your own enums (as in the 1st edition of Effective Java) and so on.

Or you build two libraries, a modern version for Android and a stripped-down version (with just the bare stuff you need) for the BlackBerry.

Hard to say what`s right for you.