Dependency Injection

updated 22 April 2022

At first, I was confused too, what is dependency injection and why is it important in programming?

Dependency Injection (DI) is very important as it will produce loosely-coupled codebase that can be tested without much hassle. It also achieves singleton pattern where you are sure that the object's state was not changed before you use that in another class. Subtle changes are actually hard to be found. Lastly, the control is not on any class but on one class only that is responsible for creating objects. It's Inversion of Control.

An Example

A.java

//the object to be injected
class A {
    //a sample variable
    int number = 10;
}

DependencyInjectionClass.java

class DependencyInjectionClass {
    
    private A a;    
    
    DependencyInjectionClass(A a) {
        //`this.a` is pointing to the same
        //a object of A class
        //given this situation, we can now
        //do things on A's object safely
        this.a = a;   
    }
    
    public void businessLogic() {
        System.out.println("@DIClassBusinessLogicMethod");
        System.out.print("Before `a.number` is changed, `a.number` is: ");
        System.out.println(a.number);
        System.out.print("`a.number` is changed in this class, "
                               + "`a.number` is now: ");        
        a.number = 15;        
        System.out.println(a.number);
    }
    
}

FactoryClass.java

//this is the only place where we should create objects 
class FactoryClass {
    private A a = new A();  

    public DependencyInjectionClass dependencyInjectionClass() {
        return new DependencyInjectionClass(a);
    }
    
    public int returnANumber() {
        return a.number;   
    }
}

Main.java

class Main {
    public static void main(String args[]) {
        FactoryClass fc = new FactoryClass();

        DependencyInjectionClass 
            dependencyInjectionClass = fc.dependencyInjectionClass();
        dependencyInjectionClass.businessLogic();
        
        //there is only one instance of A
        //so we are always sure about the state of
        //the object, 
        //to test that, we call `a.number` from the 
        //factory class, and see whether number
        //variable was actually changed
        System.out.println("@FactoryClass called in Main");
        System.out.print("Last call to a.number: ");
        System.out.println(fc.returnANumber());
    }
}

result:

> run Main
@DIClassBusinessLogicMethod
Before `a.number` is changed, `a.number` is: 10
`a.number` is changed in this class, `a.number` is now: 15
@FactoryClass called in Main
Last call to a.number: 15

Take note, there are two A references, one in the DependencyInjectionClass, the other one in FactoryClass. They are both declared as private. But, the object is only created in FactoryClass. For large-scale projects, you want to be sure that the object's state is what actually is expected. In testing, that is the test fixture. You want to make sure that you test your code on at least one object's state.

Frameworks like Spring does the job exactly.