Design Patterns:Elemente of Rousable Object-Oriented Software Danny Sabbah,and Mark Wegman at IBMResearch for their untlagging support ot this work. Last but certainly not least,we thank everyone on the Internet andpoints beyond who commented on versions of the patterns,offeredencouraging words,and told us that what we were doing was worthwhile.These people include but are not limited toJon Avotins,Steve Berczuk,Julian Berdych,Matthias Bohlen,John Brant,Allan Clarke,Paul Chisholm,Jens Coldewey,Dave Collins,Jim Coplien,Don Dwiggins,Gabriele Elia,Doug Felt,Brian Foote,Denis Fortin,Ward Harold,Herman Hueni,Nayeemi m,Bikramjit Kalra,Paul Keefer,Th as Kof Lea,Da LaLiberte,James Long,Ann Louise Luu,Pundi Madhavan,Brian Marick,Robert Martin,Dave McComb,Carl McConnell,Christine Mingins,Hanspeter Nossenbock,Eric Newton,Marianne Ozkan,Roxsan Payette,Larry Podmolik,George Radin,Sita Ramakrishnan,Russ Ramirez,Alexander Ran,Dirk Riehle,Bryan Rosenburg,Aamod Sane,Duri Schmidt,Robert Seidl,Xin Shu,and Bill Walker We don't consider this collection of design patterns complete andstatic;it's more a recording of our current thoughts on design.Wewelcome comments on it, whether criticisms of our examples,referencesand known uses we've missed,or design patterns we should haveincluded.You can write us care of Addison-Wesley, o nd eEro to deian-pa iuc.ed Yo ci in th Sampie Code sections by sendin g themessage 'send design pattern source"to design-patterns-sourceecs.uiuc.edu.And now there's a Web page at http://st-www.cs.uiuc.edu/users/patterns/DPBook/DPBook.html for late-breaking information and updates. Mountain View,California E.G. Montreal.Quebec R.且 Urbana,I1linois R.J. Hawthorne,New Yor J.V. August 1994 8 Design Patterns: Elements of Reusable Object-Oriented Software 8 Danny Sabbah, and Mark Wegman at IBMResearch for their unflagging support of this work. Last but certainly not least, we thank everyone on the Internet andpoints beyond who commented on versions of the patterns, offeredencouraging words, and told us that what we were doing was worthwhile.These people include but are not limited toJon Avotins,Steve Berczuk,Julian Berdych,Matthias Bohlen,John Brant,Allan Clarke,Paul Chisholm,Jens Coldewey,Dave Collins,Jim Coplien,Don Dwiggins,Gabriele Elia,Doug Felt,Brian Foote,Denis Fortin,Ward Harold,Hermann Hueni,Nayeem Islam,Bikramjit Kalra,Paul Keefer,Thomas Kofler,Doug Lea,Dan LaLiberte,James Long,Ann Louise Luu,Pundi Madhavan,Brian Marick,Robert Martin,Dave McComb,Carl McConnell,Christine Mingins,Hanspeter Mössenböck,Eric Newton,Marianne Ozkan,Roxsan Payette,Larry Podmolik,George Radin,Sita Ramakrishnan,Russ Ramirez,Alexander Ran,Dirk Riehle,Bryan Rosenburg,Aamod Sane,Duri Schmidt,Robert Seidl,Xin Shu,and Bill Walker. We don't consider this collection of design patterns complete andstatic; it's more a recording of our current thoughts on design. Wewelcome comments on it, whether criticisms of our examples, referencesand known uses we've missed, or design patterns we should haveincluded. You can write us care of Addison-Wesley, or send electronicmail to design-patterns@cs.uiuc.edu. You can also obtainsoftcopy for the code in the Sample Code sections by sending themessage "send design pattern source" to design-patterns-source@cs.uiuc.edu. And now there's a Web page at http://st-www.cs.uiuc.edu/users/patterns/DPBook/DPBook.html for late-breaking information and updates. Mountain View, California E.G. Montreal, Quebec R.H. Urbana, Illinois R.J. Hawthorne, New York J.V. August 1994