Dependency Injection vs Factory Pattern in Java

1. Introduction

In software engineering, Dependency Injection (DI) and the Factory Pattern are two design patterns used for creating objects. DI is a pattern that allows the creation and binding of dependencies externally from the actual components that use them. The Factory Pattern is a creational pattern that provides an interface for creating objects in a superclass but allows subclasses to alter the type of objects that will be created.

2. Key Points

1. DI decouples the usage of an object from its creation by having it provided externally.

2. The Factory Pattern encapsulates object creation in a single location, controlling the creation process.

3. DI typically relies on a container or framework to inject dependencies.

4. The Factory Pattern does not rely on a framework and typically does not manage the lifecycle or dependencies beyond creation.

3. Differences

Dependency Injection	Factory Pattern
Injects objects that a class depends on from the outside, often at runtime.	Encapsulates the instantiation of a class within a dedicated factory.
Managed by a container that controls the lifecycle of the objects.	The consumer usually controls the lifecycle of the objects created by the factory.
Can be implemented using various frameworks like Spring.	Does not need a framework and can be implemented in pure Java.

4. Example

// Example of Dependency Injection
public class Car {
    private Engine engine;

    // Dependency injection via constructor
    public Car(Engine engine) {
        this.engine = engine;
    }
}

// Usage
Engine engine = new Engine();
Car car = new Car(engine); // Injecting the dependency

// Example of Factory Pattern
public class CarFactory {
    // Factory method for creating instances
    public static Car createCar() {
        Engine engine = new Engine();
        return new Car(engine);
    }
}

// Usage
Car car = CarFactory.createCar(); // Creating an instance using the factory

Output:

// No specific output, it is a conceptual demonstration.

Explanation:

1. In the DI example, the Car class does not instantiate its own Engine. Instead, the Engine is created outside the Car and passed into its constructor.

2. In the Factory Pattern example, the CarFactory class has a static method createCar that creates an Engine and a Car and returns the car instance with its engine.

5. When to use?

- Use Dependency Injection when you want to increase the modularity and testability of your application, and you're okay with the overhead of using a DI framework.

- Use the Factory Pattern when you need a flexible and extensible way to create objects, allowing for new derived types without changing existing code that uses the factory.