sidsharma2002/DesignPatterns

StoryNotes from the book "HeadFirst Design Pattern". Helps you prepare for LLD interviews.

DesignPatterns [WIP + Contributions Accepted]

This repo is made as short notes to Head First Design Patterns Book. Follow StoryNotes package and learn real software engineering.

Design Principles

----------- [DESIGN PRINCIPLE 1 ] -----------

" Identify the aspects of your application that vary and separate them from what stays the same. "

All patterns are used for this only.
example : here, as for in one step , default fly behaviour can change. Hence we encapsulated it (separated it).

----------- [DESIGN PRINCIPLE 2 ] -----------

" Program to an interface, not an implementation. "

We already did that, eg : FlyBehaviour is an interface not an exact implementation, and we now can set different
behaviours dynamically. another example is LoginRepository and TestDoubleRepository.

Also this doesn't mean a java interface, it just means,  " program to a supertype. ".
So an abstract class can also be used instead of an interface.

eg : instead of,
        Dog dog = new Dog();
        dog.bark();

     use,
        Animal animal = new Dog();
        animal.makeSound();

     more better,
        Animal animal = getAnimal();  // delegate the responsibility to some another class/method.
        animal.makeSound();

----------- [DESIGN PRINCIPLE 3 ] -----------

" Favour Composition over Inheritance. " or " Favour HAS-A over IS-A"

eg : class Manager implements Attitude, DressCode { // Manager "IS-A" Attitude
     }

     instead use,

     class Manager {
        Attitude attitude;  // Manager "HAS-A" Attitude
        DressCode dressCode;

        public void setAttitude() {
        }

        public void setDressCode() {
        }
     }

     This helps in dynamic change of attitude, dressCode. Also logically these interfaces are set of algorithms,
     and Manager class implementing this creates a sense that Manager is also an algorithm of this family.
     Instead of Manager becoming itself an algorithm it should use these.