[Interview Question] Dependency Injection

Dependency injection

Dependency: a relationship in which an object uses another object. → Dependency injection: something that establishes the relationship

Why do we need dependency injection?

public class Product {

	private Apple apple;
    
    public Product() {
    	this.apple = new Apple();
	}

}

In this example, if a class called Product uses the Apple constructor, we are using it without injecting it. If Product is used, it can be understood as a dependency that comes with Apple. In this case, it's not a connection between objects, but a relationship between classes.

Also, if I wanted to use an additional class called Orange, I would have to make changes to the Product class constructor, which is inflexible.

In conclusion, to summarize why dependency injection is necessary

Strong coupling between classes + inflexible code

These problems can be solved with dependency injection, so I'd say it's necessary!

Application of the solution

public interface Fruit {

}

public class Apple implements Fruit {

}

public class Product {

	private Fruit fruit;
    
    public Product(Fruit fruit) {
    	this.fruit = fruit;
    }
    
}

You can see that each class is guaranteed to be independent.

1. adding a new Fruit doesn't change Product
2. relationships between objects are formed instead of class relationships