Lecture Notes for Advanced Programming II

25 January 2001 - The Bridge Object Pattern

1. consider the bag data structure - like a set but with multiple elements

   1. operations - add, get, member, count, clear

2. possible implementations

   1. array - simple and fast, but fixed

   2. linked list - not so simple or fast, but dynamic

   3. hash table - hard, but fast (usually) and dynamic

3. from this get the usual inheritance hierarchy

   1. class Bag

   2. class ArrayBag : Bag

   3. class ListBag : Bag

   4. class HeapBag : Bag

   5. is this a good hierarchy

      1. what's the relation

      2. what about related data types

   6. the problem is behavior and implementation are being mixed

4. the bridge pattern - separating behavioral and implementation hierarchies

   1. have two hierarchies - one for behavior, the other for implementation

   2. the two top-level classes form the bridge between the two hierarchies

   3. why bother

      1. clarity - distinct concepts (behavior, implementation) are distinct

      2. reduced work - only one implementation needed

      3. flexibility - implementation choice, even dynamically

This page last modified on 25 January 2001.