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It has been influential to the field of software engineering and is regarded as an important source for object-oriented design theory and practice. More than 500,000 copies have been sold in English and in 13 other languages. 90, “Towards an Architecture Handbook”, run by Bruce Anderson, where Erich Gamma and Richard Helm met and discovered their common interest. They were later joined by Ralph Johnson and John Vlissides. The original publication date of the book was October 21, 1994 with a 1995 copyright, hence it is often cited with a 1995-year, despite being published in 1994.
The book was first made available to the public at the OOPSLA meeting held in Portland, Oregon, in October 1994. Programming Languages Achievement Award to the authors, in recognition of the impact of their work “on programming practice and programming language design”. As of March 2012, the book was in its 40th printing. They warn that the implementation of a subclass can become so bound up with the implementation of its parent class that any change in the parent’s implementation will force the subclass to change.
Furthermore, they claim that a way to avoid this is to inherit only from abstract classes—but then, they point out that there is minimal code reuse. Using inheritance is recommended mainly when adding to the functionality of existing components, reusing most of the old code and adding relatively small amounts of new code. To the authors, ‘delegation’ is an extreme form of object composition that can always be used to replace inheritance. Delegation involves two objects: a ‘sender’ passes itself to a ‘delegate’ to let the delegate refer to the sender.
Thus the link between two parts of a system are established only at runtime, not at compile-time. These allow any type to be defined without specifying all the other types it uses—the unspecified types are supplied as ‘parameters’ at the point of use. Dynamic, highly parameterized software is harder to understand and build than more static software. Sometimes acquaintance is called ‘association’ or the ‘using’ relationship. Acquaintance objects may request operations of each other, but they aren’t responsible for each other.
The chapter goes through seven problems that must be addressed in order to properly design Lexi, including any constraints that must be followed. Each problem is analyzed in depth, and solutions are proposed. Finally, each solution is associated directly with one or more design patterns. It is shown how the solution is a direct implementation of that design pattern. The document is “an arrangement of basic graphical elements” such as characters, lines, other shapes, etc.
The structure of the document contains a collection of these elements, and each element can in turn be a substructure of other elements. The implementation should treat complex and simple structures the same way. It should not have to know the difference between the two. Specific derivatives of abstract elements should have specialized analytical elements. Each node in the structure knows of its own children and its parent. When an operation is performed on the parent, that operation is recursively passed down the hierarchy.