TK2023 Object-Oriented Software Engineering

1. TK2023 Object-Oriented Software Engineering CHAPTER 13 Introduction to Design Patterns and GRASP

2. INTRODUCTION TO DESIGN PATTERNS • Experienced OO developers normally build up a collection of both general principles and idiomatic solutions that guide them in the creation of software. • In OO design, a (design) pattern is a named description of a problem and solution that can be applied to new contexts. It advises the reader on how to apply it in novel situations and discusses its trade-offs, implementations, variations and so on.

3. Example of a pattern: Pattern name: Information Expert Problem: What is a basic principle by which to assign responsibilities to objects? Solution: Assign a responsibility to the class that has the information needed to fulfill it.

4. The name of a design pattern is important for the following reasons: • It supports chunking and incorporating that concept into our understanding and memory. • It facilitates communication. Jill: “Hey Jack, for the persistence subsystem, let’s expose the services with a Façade. We’ll use an Abstract Factory for Mappers, and Proxies for lazy materialization.” Jack: “Huh?”

5. It is important to understand that patterns do not describe new ideas. They describe existing tried-and-true knowledge, idioms, and principles. • Examples of patterns: GOF patterns, GRASP

6. INTRODUCTION TO GRASP • General Responsibility Assignment Software Patterns (GRASP) is a set of patterns that defines nine basic OO design principles. • Larman, in his book, uses GRASP as a tool to help master the basics of OO design and understand responsibility assignment in object design.