Design Patterns:Elemente of Rousable Object-Oriented Software for their development effort andfor patience despite what must appear to have been our insatiableappetite for revision.A special acknowledgment is due I8M Resea h,which contir to underwrite much of this activity. We also thankthe reviewers,including Robert Brunner,Sandeep Dani,Bob Koss,ScottMeyers,Stefan Schulz,and the Patterns Discussion Group at theUniversity of Illinois Urbana-Champaign.Their advice led to at leastone major redesign and several minor one8】 Finally,we thank all who have taken time to comment on DesignPatterns.Your feed nvaluable to us as we striveto better our understanding and presentation of this material Zurich,Switzerland E.G. Sydney,Australia R.H. Urbana,Illinois R.J. Hawthorne,New York J.V. August 1997 6 Design Patterns: Elements of Reusable Object-Oriented Software 6 for their development effort andfor patience despite what must appear to have been our insatiableappetite for revision. A special acknowledgment is due IBM Research,which continues to underwrite much of this activity. We also thankthe reviewers, including Robert Brunner, Sandeep Dani, Bob Koss, ScottMeyers, Stefan Schulz, and the Patterns Discussion Group at theUniversity of Illinois Urbana-Champaign. Their advice led to at leastone major redesign and several minor ones. Finally, we thank all who have taken time to comment on DesignPatterns. Your feedback has been invaluable to us as we striveto better our understanding and presentation of this material. Zurich, Switzerland E.G. Sydney, Australia R.H. Urbana, Illinois R.J. Hawthorne, New York J.V. August 1997