Characteristics of Good Object Oriented Design

Object-Oriented Design (OOD) is a way of designing software that focuses on objects—things in the real world that your software will interact with. Instead of writing code that deals with each step of the process separately, you group related parts together into objects. Each object has data (information about something) and methods (actions or behaviors it can perform).

In this article, we will look at what makes an Object-Oriented Design good and how you can make sure your design decisions are the best ones for your software project.

What is Object-Oriented Design?

Object-oriented design (OOD) is a method of designing software where we organize the program into objects that work together to solve a problem. Each object represents a real-world entity, like a car, a customer, or a bank account. These objects contain two important things:

• Data: Information about the object. For example, a car might have data like its color, model, and speed.
• Methods: Actions the object can perform. For example, a car might have methods like “drive” or “stop”.

The key idea behind OOD is that these objects communicate with each other to complete tasks instead of having one large chunk of code that does everything. This makes your software easier to manage, change, and expand over time.

Characteristics of Good Object-Oriented Design

Good OOD is all about creating clean and organized code that is easy to understand and maintain. Here are some important characteristics and guidelines of Good object-oriented design

1. Coupling Guidelines

Coupling is a term used to describe how connected different objects are. In a well-designed system, we want to reduce the amount of communication between objects. Why? Because when objects talk to each other too much, it becomes harder to change or fix things. So, we aim for loose coupling, meaning each object works mostly on its own and only communicates with others when necessary.

When designing your software, keep interactions between objects to a minimum. This will help ensure that your system is flexible and easier to maintain.

2. Cohesion Guidelines

Cohesion refers to how well the methods (actions) and data (information) in an object work together. A highly cohesive object is one where all the parts are focused on a single task. This makes the object easier to understand and use.

There are three levels of cohesion:

• Method Cohesion: Each method should do one specific job. For example, a “calculate” method should only be responsible for calculating, not for saving data.
• Class Cohesion: A class (group of related objects) should focus on one main responsibility. For instance, a “BankAccount” class should only deal with bank account actions, like deposit and withdrawal.
• Hierarchy Cohesion: In a system with many objects, objects should still be closely related in what they do. This ensures everything is well-organized and makes the system easier to manage.

A good design keeps these levels of cohesion high so the software is easy to work with and understand.

3. Hierarchy and Factoring Guidelines

When designing the class hierarchy, it’s important not to make it too complicated. A base class (a starting point for other classes) shouldn’t have too many subclasses (classes that inherit from it). If a class has too many subclasses, it becomes difficult to understand and manage the design.

A good rule to follow is to have no more than 7±2 subclasses under one base class. This ensures that the design stays clear and manageable, without becoming overwhelming.

4. Keeping Message Protocols Simple

In Object-Oriented Design, objects communicate with each other by sending messages. If an object has to send a message with too many parameters (pieces of information), it means the design might be too complicated.

Good designs have simple, clear messages. For example, a message should not have more than three parameters. If it does, it’s a sign that the system is too tightly connected and needs to be simplified.

5. Number of Methods

Each object should have a reasonable number of methods. If an object has too many methods, it can become very specific to one task, which makes it difficult to understand or reuse in other parts of the program.

A good object design usually limits a class to having no more than 7 methods. This keeps things simple and organized, making it easier to understand and maintain.

6. Depth of the Inheritance Tree

In OOD, we often create a system where one class can inherit characteristics from another class. This creates a hierarchy. However, the hierarchy shouldn’t be too deep. If a class is too far down in the hierarchy, it might inherit too many behaviors, which can make the system complex and hard to manage.

It’s best to keep the depth of the class hierarchy shallow so that it’s easier to understand and maintain.

7. Number of Messages Per Use Case

In good OOD, a use case refers to a specific action that the software should perform. When an action is triggered, it shouldn’t cause too many messages to be sent between objects. If many objects are involved in responding to one action, it can be difficult to test and debug the system.

Try to limit the number of messages each use case generates to keep things simple and easier to test.

8. Response for a Class

When an action occurs, it can cause an object to call various methods. Response refers to how many different methods an object calls in response to an event. If an object calls too many methods (more than seven), it can become difficult to debug and maintain.

Try to keep the response from an object to a manageable number of method calls. This helps ensure that your object does not become too complex and hard to understand.

Must Read:

• Introduction to Object Oriented Design
• Differnce between object oriented design and function oriented design

Conclusion

Object-Oriented Design is all about organizing software into objects that work together to solve problems. Good OOD makes software easier to maintain, extend, and reuse. By following simple guidelines—like keeping coupling low, ensuring cohesion, and limiting the complexity of message protocols—you can create software that is clear, manageable, and effective.

When you focus on keeping things simple and organized, your design will not only be easier to understand but also easier to change and improve in the future. Remember, the goal of good design is not just to make something work, but to make it work in a way that is easy to maintain and grow over time.