Inheritance or Composition: When Should You Choose Which?

Inheritance vs. Composition: Understanding the Distinctions

While the terms "inheritance" and "composition" are often used interchangeably, they represent fundamentally different relationships in programming.

Inheritance: The "Is-A" Relationship

Inheritance is an "is-a" relationship, where a child class inherits the properties and methods of its parent class. This implies a direct connection between the two classes, suggesting that the child "is-a" type of the parent.

Composition: The "Has-A" Relationship

Composition, on the other hand, is a "has-a" relationship, where one class contains instances of another class as fields. It does not imply a hierarchical connection but rather represents a dependency between the two classes. The containing class "has-a" reference to the referenced class.

Implementing Composition in Java

To implement composition in Java, simply create an instance of the referenced class as a field within the containing class. For example:

class Car {
    private Engine engine;

    public Car() {
        engine = new Engine();
    }
}

In this example, the Car class composes an instance of the Engine class, establishing a "has-a" relationship where Car has an engine.

Advantages of Composition Over Inheritance

Composition is often preferred over inheritance because it provides greater flexibility and avoid the pitfalls of inheritance. For instance:

• Reduced coupling: Composition allows for the replacement of components without affecting other parts of the system.
• Increased flexibility: Composition enables the creation of new classes by combining existing ones, allowing for more reusable and adaptable designs.
• Avoidance of the Diamond Problem: Unlike inheritance, composition avoids the issue of multiple inheritance, where a class can inherit from multiple parent classes with conflicting methods.

Reference Materials

For further insight into the differences between inheritance and composition, refer to the following resources:

• Effective Java 2nd Edition by Josh Bloch (Item 16 and 17)
• Composition versus Inheritance: https://www.thoughtworks.com/insights/blog/composition-versus-inheritance-comparative-look-two-fundamental-ways-relate-class

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