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1) @Module and @Provides: define classes and methods which provide dependencies
2) @inject: request dependencies. Can be used on a constructor, a field, or a method
3) @Component: enable selected modules and used for performing dependency injection
4) @Singleton: Single instance of this provided object is created and shared.

Dependancy injection is a software design pattern that implements object oriented programming for resolving dependencies. Dependency injection in application allow us to have Testable classes, Re-usable and interchangeable Components.


<b> without DI </b>

private final Heater heater;
private final Pump pump

        coffeMaker()  {
            this.heate = new ElectricHeater();
             this.pump = new  Thermosiphon(heater);
        }
      
  coffee makeCoffee(){/* _ */}

      public static void main(String[] args){
      Coffee coffee = new  CoffeeMaker().makeCoffee();
    
    }

private final Heater heater;
private final Pump pump;

CoffeeMaker(Heater heater,Pump pump){

      this.heater = checkNotNull(heater);
      this.pump = checkNOtNull(pump);
    
}

Coffee makeCoffee(){/* _ */}

class CoffeMain{

    public static void main(String[] args){
  
	      Heater heater = new ElectricHeater();
	      Pump pump = new Thermosiphon(heater);
	      Coffee coffee = new  CoffeeMaker(heater,pump).makeCoffee();
      
    }

- We need to identify the dependent objects and its dependencies.
- Create a class with the @Module annotation, using the @Provides annotation for every method that returns a dependency.
- Request dependencies in your dependent objects using the @Inject annotation.
- Create an interface using the @Component annotation and add the classes with the @Module annotation created in the second step.
- Create an object of the @Component interface to instantiate the dependent object with its dependencies.