SOLID Principles: Dependency Inversion Principle (DIP)

• High-level modules should not depend on low-level modules. Both should depend on abstractions.
• Abstractions should not depend on details. Details should depend on abstractions.
• components should depend on abstraction.

module need to manage high-level functionality with zero implementation details this allows the module to be used for any type of application and for all modules inside the application.

• Example:

we have an existing system which sends notifications to users by email

public class Email{
public void sendMail(){
// send mail 
}
}
public class Notification{
private Email _email;
public Notification(){
_email = new Email();
}
public void promotionalNotification(){
_email.sendMail();
}
}

And if we need to send a notification by SMS, we need to change email to SMS in this class, which violates DIP, because the parent class doesn’t depend on abstraction, it has a specific data tie to it.

we can solve it by:

1. create an interface.

public interface Messeneger{
void sendMessage();
}

2. create classes implements this interface.

public class Email implements Messenger{
void sendMessage(){
// send email
}
}
public class SMS implements Messenaer{
void sendMessage(){
// send SMS
}
}

3. build class Notification which has Messenger property.

public class Notification{
private Messenger _messenger;
public Notification(){
_messenger = new Email();
}
public void promotionalNotification(){
_messeager.sendMessage();
}
}

You can replace crate a new object in three ways:

1. Constructor injection:

passing data to Notification Constructor

public class Notification{
private Messenger _messenger;
public Notification(Messenger messenger){
_messenger= messenger;
}
public void promotionalNotification(){
_messenger.sendMessage();
}
}

2. Property injection:

use the setter to set a Property of Notification class

public class Notification{
private Messenger messenger;
public void setMessenger(Messenger messenger) {
    this.messenger= messenger;
  }
public void promotionalNotification(){
_messenger.sendMessage();
}
}

3. Method injection:

pass object data to the method we want to use directly in Notification class

public class Notification{
public void promotionalNotification(Messenger messenger){
messenger.sendMessage();
}
}

continuo to the rest of SOLID