CSC/ECE 517 Fall 2010/ch7 7b JB

Information Expert Pattern

Introduction

Information Expert Pattern is a principle for assigning responsibility in object-oriented design. The fundamental idea is to identify the class with the most information (expertise) and assign responsibility to it.

GRASP

In object-oriented design, the overall problems of assigning responsibility are dealt with by GRASP 1, or General Responsibility Assignment Software Patterns (or Principles). GRASP has nine principles:

1. Information Expert
2. Creator
3. Low Coupling
4. High Cohesion
5. Controller
6. Polymorphysm
7. Pure Fabrication
8. Indirection
9. Protected Variations

There are often situations in programming object-oriented designs when data needed by an object is spread out across several other objects. Rather than access each object individually and sequentially to get the details one at a time, the Information Expert pattern can be used to keep the program organized and straightforward. This will help to enable later extension of the program to enable new functionality without adding a lot of complexity.

Process

Using this approach, the programmer would:

1. Identify a responsibility
2. Determine what information is needed to fulfill it,
3. Locate where that information is stored
4. Identify which class contains the largest subset of the required data
5. Assign responsibility to that class to gather the remaining data

In some circumstances, this method could be used in a tree structure to have a small number of experts who are responsible for specific types of data. Each contacts a subset of the group of objects containing the needed data and may do some simple processing or combination of the data before passing it back to the original caller.

An example of when this might be retrieval of a database record. The record may be comprised of data from many different databases. One object would be the assigned as the caller. It would be responsible for fetching the necessary data and building the final record. It would contact each database controller, which may each be asked to retrieve records from one or more tables. They would return the requested records (possibly after some minor editing of them). The caller would then extract the fields it needed and build the new record from them. This new record would be returned.

References

1. Wikipedia's GRASP article 1
2. Information Expert Pattern overview with diagrams 2
3. Creator and Information Expert Principles with sample code 3