Core Objective C Class Extensions

Objective-C has a feature alien to Java developers, categories. A category is a kind of mix-in, which adds messages to an existing type without needing to modify it. J2ObjC's JRE emulation library uses categories to make the core Foundation classes more supportive of Java applications.

NSObject

The root of the Foundation framework is NSObject, which is similar to java.lang.Object. Several methods were added to this class via the NSObject+JavaObject category:

• clone(): the Foundation framework has a NSCopying protocol that is similar to Object.clone(), but several corner cases found it incompatible. [Clone Clone support] was added to the translator, which required this method.

• compareTo(): although compareTo() is not an Object method, the Comparable contract requires that a ClassCastException be thrown if the specified object's type prevents it from being compared. This method therefore throws a ClassCastException, and is overridden by any class implementing Comparable.

• getClass(): returns an IOSClass instance that maps to java.lang.Class. This distinction is necessary because Java considers Class instances to be true objects, while Objective-C doesn't.

• notify(), notifyAll(), wait(): threading primitives that work with Objective-C's @synchronized support.

NSString

NSString is the Foundation framework's answer to java.lang.String, but the two have very different capabilities and APIs, making mapping difficult. String functions are generally performance-sensitive, too. So rather than have translation support for the unmappable methods, they are implemented in the NSString+JavaString category.