How can you enhance software reusability with composition and delegation?

1 What is composition?

Composition is an OOD technique that involves creating complex objects by combining simpler objects as parts or attributes. For example, a Car object can be composed of an Engine object, a Wheel object, a Door object, and so on. Composition allows you to reuse existing code by aggregating it into larger units of functionality, without changing the original code or creating new subclasses. Composition also follows the principle of loose coupling, which means that the composed objects are independent and can be replaced or modified without affecting the whole system.

2 What is delegation?

Delegation is an OOD technique that involves delegating some of the behavior or responsibility of an object to another object, instead of implementing it directly or inheriting from a superclass. For example, a Printer object can delegate the printing task to a Paper object, a Ink object, or a Network object, depending on the type of printer. Delegation allows you to reuse existing code by delegating it to other objects that have the same interface or contract, without changing the original code or creating new subclasses. Delegation also follows the principle of high cohesion, which means that the delegating objects have a clear and focused purpose, and avoid doing too much or too little.

3 How can composition and delegation enhance software reusability?

Composition and delegation can enhance software reusability by allowing you to reuse existing code in different ways, without modifying it or inheriting from it. By using composition, you can create complex objects by combining simpler objects that already exist, and avoid creating unnecessary subclasses or duplicating code. By using delegation, you can delegate some of the behavior or responsibility of an object to another object that already exists, and avoid implementing it directly or inheriting from a superclass. Both techniques can help you achieve software reusability by following the OOD principles of loose coupling and high cohesion, which make your code more modular, flexible, and easy to maintain.

4 How can you apply composition and delegation in your code?

To apply composition and delegation in your code, you need to identify the objects that you want to reuse, and the relationships that you want to establish between them. You also need to design the interfaces or contracts that define how the objects communicate with each other, and how they expose their functionality to the outside world. For example, if you want to create a Car object that is composed of an Engine object and a Wheel object, and that delegates the start and stop methods to the Engine object, you can use the following pseudo-code:

class Car {
  // Composition: Car has an Engine and a Wheel as attributes
  Engine engine;
  Wheel[] wheels;
  // Constructor: initialize the attributes with the given parameters
  Car(Engine e, Wheel[] w) {
    engine = e;
    wheels = w;
  }
  // Delegation: Car delegates the start and stop methods to the Engine object
  void start() {
    engine.start();
  }
  void stop() {
    engine.stop();
  }
}        

5 What are the benefits and challenges of composition and delegation?

Composition and delegation have many benefits for software reusability, such as reducing code duplication and complexity, increasing code flexibility and adaptability, improving code quality and consistency, and facilitating code testing and debugging. However, these techniques also have some challenges, such as requiring careful design and planning, depending on the availability and compatibility of the existing code, and involving some performance and memory overhead. To reap the advantages of composition and delegation, one must identify the objects to be reused, establish relationships between them, ensure they have the necessary functionality and interface, and manage multiple objects and their interactions.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?