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Abstraction mechanisms.  Although there is some agreement about the core 

abstraction mechanisms, there are still a number of differences between the common 

languages. We discuss alternative mechanisms for supporting classification and 

composition, single and multiple inheritance, inner versus super, genericity versus 

virtual classes, encapsulation, dynamic versus static typing, contravariance versus 

covariance, and types versus classes. 


Class-based versus prototype-based languages.  Prototype-based languages are one 

of the most interesting developments within object-orientation. Prototype-based 

languages make it possible to program without classes, but they do in general not 

support programming using a class-like style. We argue that the class-like style should 

be supported and that it would be desirable to unify the two approaches. 


Concurrency.  Simula includes a notion of quasi-parallel processes in the form of 

active objects that are used for representing concurrent processes. They have also been 

the basis for the design of concurrency in BETA. No other object-oriented language 

seem to have adapted the Simula notion of active object. There are a number of 

proposals for concurrency in object-oriented languages, but no single model has been 

widely accepted. We argue that the Simula approach is (still) considered a good 

approach to concurrency. 

The rest of this paper consists of a section corresponding to each item in the above list of 

issues, followed by a conclusion. Due to the Simula/BETA background of the author,  there 

will often be arguments in favour of the approaches taken in BETA, which is a statically-



 class-based object-oriented language. Some knowledge about BETA is assumed, 

although we have tried to limit this as much as possible. An introduction to BETA may be 

found in Reference [Mad94]. 

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