How to specify prefixes for package names.

Name Mapping

Java uses packages to informally define namespaces; while Objective C++ has C++ namespaces, Objective C doesn't. To preserve name uniqueness when using classes from multiple packages, J2ObjC prepends a camel-cased version of the package to the type name. For example, java.util.Map is renamed to JavaUtilMap.

Unfortunately, camel-cased package names can reduce readability of the generated code, especially with long package names. For example, Google Guava's Beta annotation is in the com.google.common.annotations package, and ComGoogleCommonAnnotationsBeta is harder to read than Beta.

Defining a Package Prefix

The Objective-C convention for defining informal namespaces is to use a shared prefix, usually two uppercase letters. The iOS Foundation Framework uses "NS" (from NeXTStep), for instance. To simplify Google Guava's Beta name, a prefix such as "GG" would improve readability by referring to Beta as GGBeta.

J2ObjC supports developers specifying their own prefixes to map to package names. This is done on the command line using "--prefix package=prefix. To shorten all the class names in Beta's package, the "--prefix com.google.common.annotations=GG"" would be used. A separate prefix declaration is needed for each package.

Defining a Single Prefix for Multiple Packages

Smaller libraries often have Java class names that don't conflict, and so can share a single prefix with a wildcarded package specification. For example, all of the Joda-Time packages can share the same JT prefix, using "--prefix 'org.joda.time.*=JT'". The only wildcard character supported is '*', which matches the same way the command-line shell does with filenames.

Defining Multiple Package Prefixes

To simplify specifying several prefix definitions, a properties file can be used with the "--prefixes file" argument:

$ cat prefixes.properties
com.google.common.annotations: GG
com.google.common.base: GG

# While GG can be used for all packages, let's mix it up.
com.google.common.collect: GC
com.google.common.io: GIO        # A prefix can be more than two characters,
com.google.common.net: GuavaNet  # a lot more!
...
$ j2objc --prefixes prefixes.properties <args>

Prefixed Classes at Runtime

Since the finished app has classes with prefixes, they cannot be located using the original Java class name by default. However, if the app has a file named prefixes.properties in its resource bundle with the prefixes used for translation, Class.forName(javaName) will find the mapped class.

To add the above prefixes.properties to an iOS app in Xcode, open the build target's Build Phases tab, expand its Copy Bundle Resources section, and add the prefixes.properties file to that list. [[Java Resources]] has further information on how Java resource concepts map to iOS resources.