link: Object Oriented Programming , Relations Between Objects

Composition relationships

Content

Composition is a stringent form of aggregation in object-oriented programming that defines a “part-whole” relationship where parts cannot exist independently of the whole. This type of relationship is crucial for ensuring that objects are well-integrated within a system, enforcing encapsulation and enhancing the system’s robustness.

Composition is a specific kind of aggregation known for its strictness in part-whole relationships:

Important

Composition ensures that when the container (whole) is destroyed, its contents (parts) are also destroyed. This relationship is ideal for representing natural part-whole hierarchies in an application, such as graphical user interfaces or management systems where components must not exist without the whole.

Example

using System;
 
public class Engine
{
    public void Start() => Console.WriteLine("Engine started");
    public void Stop() => Console.WriteLine("Engine stopped");
}
 
public class Car
{
    private Engine engine = new Engine(); // Composition: Car "has-a" Engine
 
    public void Start()
    {
        engine.Start();
        Console.WriteLine("Car started");
    }
 
    public void Stop()
    {
        engine.Stop();
        Console.WriteLine("Car stopped");
    }
}
 
class Program
{
    static void Main()
    {
        Car myCar = new Car();
        myCar.Start();
        myCar.Stop();
    }
}
 

In this C# example:

• The Car class is composed of an Engine. The Car owns the Engine and controls its lifecycle.
• The Engine cannot exist independently of the Car, demonstrating a composition relationship.
• The methods Start and Stop in Car delegate to Engine, encapsulating its functionality within the Car.

References

Favor Composition Over Inheritance - design principle https://refactoring.guru/design-patterns