gistfile1.java

/*The Great Dependency Injection.*/

//This is psuedo code. Screw the syntax rules!

//No dependency injection.
class User {

    private Database db;

    private String email;

    private String password;


    public User (String email,  Sting password) {

     
     this.db = Database.getNewConnection(); //This might throw an exception if we can't connect!
   
     this.email = email;

     this.password = password;
     


    }

    public Boolean login () {

         return this.db.attemptLogin(this.email, this.password);

    }


}

//This is messy. If we wanted to change how we connect to the database or how we handle potential errors encountered upon doing 
//so, we would have to modify the User class' constructor.
// We could change the constructor body to this:
 

public User (String email,  Sting password) {
 
try {
    this.db = Database.getNewConnection();
 }catch (Exception e){
  
     (new Logger).goTellOnThisMethod(e);     

 }

    this.email = email;

    this.password = password;

}

//But what's the purpose of the User class? To connect to the database? No! It's to log in the user (hence the login method). So why 
//should we force our User class to care about connecting to the database?
//Enter Dependecy injection.
//Clearly, the User class has a dependecy on the Database class, as it uses it in its login() method. That's about it though so really that's
//the only place we should have any code relating to the Database class.
//So let's try that again:

//Constructor 
  public User (String email,  Sting password) {
 
   
     this.email = email;

     this.password = password;
     
     //No more code for here!
     

    }
    
    public Boolean login(Database db) { //Note we take a database object as an argument now. This is the injection of the dependency.
      
      return db.attemptLogin(this.email, this.password);
      
    } //Done
    
    
    //This way all our User class does is login and any time we have to make changes to it, they are going to be directly related to logging in.
    //That's it! We have now made the User class more maintainable because it is simpler.
    //Now as for all that fancy database connection code, you can put that somewhere else where it makes sense. Best to use a factory method in
    //my opinion.
    
    user = new User ('apple@sauce.com', 'secretmonkey' );
    
    result = user.login((new DatabaseFactory).connect());
    
    if(result) {
      
      //you are logged
    } else {
      
      //Call the NSA!
      
    }