AspectJ is a simple aspect-oriented extension to the Java™ programming language that enables the clean modularization of crosscutting concerns such as: design patterns, error checking and handling, synchronization constraints and performance optimizations.

AOP introduces a new unit of modularity, called an aspect, that is not present in other types of programming. Aspects are suited for modularizing crosscutting concerns, which are difficult to address in OO and procedural languages.
 

Open implementation is a software design technique that helps write modules that are both reusable and efficient for a wide range of clients.  In this approach, the client can tailor a module's implementation strategy to better suit their needs, effectively making the module more reusable and the client code more simple.

The metaobject protocol approach is based on the idea that one can and should allow users to adjust the design and implementation of a language in order to suit their particular needs.  In a language based upon metaobject protocols, the language implementation itself is structured as an object-oriented program, permitting the power of object-oriented programming techniques to be exploited in order to make the language more flexible.

Browsing a large hierarchy, like an organization chart or a directory structure, is difficult because the hierarchies typically grow exponentially.  However, hyperbolic geometry affords an exponentially growing space, and as such is better suited to displaying hierarchical information.

The design and specification of OO programs is challenging because extensibility comes from allowing the user to override parts of the implementation.  This raises issues in both how the programs are designed and how their specification is written.  The related papers are: Issues In the Design and Specification of Class Libraries, Typing the Specialization Interface, and Methods as Assertions.