The diverse inheritance mechanisms provided by Smalltalk, Beta, and CLOS are interpreted as different uses of a single underlying construct. Smalltalk a.nd Beta differ primarily in the direction of class hierarchy growth. These inheritance mechanisms are subsumed in a new inheritance model ba.sed on composition of mixins, or abstract subclasses. This form of inheritance can also encode a CLOS multiple-inherita.nce hierarchy, although changes to the encoded hierarchy that would violate encapsulation are difficult. Practical applica.tion of mixin-based inheritance is illustrated in a sketch of an extension to Modula-3.

Class loaders are a powerful mechanism for dynamically loading software components on the Java platform.They are unusual in supporting all of the following features:laziness, type-q? linkage, user-defined extensibility, and multiple communicating namespaces.We present the notion of class loaders and demonstrate some of their interesting uses. In addition, we discuss how to maintain type safety in the presence of user-defined dynamic class loading.

We present GJ, a design that extends the Java programming language with generic types and methods. These are both explained and implemented by translation into the unextended language. The translation closely mimics the way generics are emulated by programmers: it erases all type parameters, maps type variables to their bounds, and inserts casts where needed. Some subtleties of the translation are caused by the handling of overriding.GJ increases expressiveness and safety: code utilizing generic libraries is no longer buried under a plethora of casts, and the corresponding casts inserted by the translation are guaranteed to not fail.GJ is designed to be fully backwards compatible with the current Java language, which simplifies the transition from non-generic to generic programming. In particular, one can retrofit existing library classes with generic interfaces without changing their code.An implementation of GJ has been written in GJ, and is freely available on the web.