Most programmers learn by looking at computer programs. This book teaches you design patterns in exactly this way: by looking at computer programs and analyzing them in terms of the patterns that they use. Consequently, you learn how the patterns actually occur in the real world, and how to apply the patterns to solve real problems. This book also looks at the broader context of OO (Object-Oriented) design and how the patterns solve commonplace OO design problems. It covers many of the principles of OO design - principles not covered by most books on Java - and shows you how to apply these principles to make your code easier to maintain and debug.  The existing books on design patterns take a "catalog" approach, where they show the individual design patterns in isolation. This approach is fundamentally flawed because you can’t see how the design patterns actually function in the real world. The patterns in real programs interact in complex ways, working in concert to solve many problems simultaneously. By seeing the patterns actually applied in a realistic way, you can really understand how to use them in your own code.

Holub on Patterns: Learning Design Patterns by Looking at Code PDF

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