Design patterns become popular after publishing of the book Design Patterns Elements of Reusable Object-Oriented Software book at 1995
This book was written by four editors so it was known as Gang of Four design patterns
Target was finding solutions for OO design problems
What is meant by a pattern :
The keyword pattern was built by the architect Cristopher alexander he defined it to solve a common problems
Language and domain independent for solving OOP design problems
patterns deliver different ways to solve recurring issue providing flexibility and reusability
developers can use these patterns as suggestions to solve their problems
[Note] A pattern give a generic solution and can have different implementation
Design pattern catalogue
01- Creational patterns
These patterns provide various object creation mechanisms, which increase flexibility and reuse of existing code.
02- Structural patterns
These patterns explain how to assemble objects and classes into larger structures while keeping these structures flexible and efficient
03- Behavioral patterns
These patterns are concerned with algorithms and the assignment of responsibilities between objects.
UML Class diagram
To understand the pattern and how interaction occurs UML understanding is needed
What mean by UML : UML stands for unified machine language and consists of
01 - class
Each class consists of a class name , attributes and the methods that class contains
example
To understand how classes interact with each others main types of relation lines understanding is needed
Is used when there is a parent class and child class and we need to show the relation between each others that the child Savings and Current classes is inherited from Account as shown
Note Generalization arrow is empty and points to the most general class to inherit from
The name of abstract class is shown in italics and contains virtual method and must be implemented by child classes and also it uses generalization arrow
A composition show part of for example Button is part of window and when window the container is destroyed the button will be destroyed one of container elements also and we can represent that using composition arrow
The need for aggregation appears when we need to destroy the container and keep it's elements and can be represented with hollo diamond a
Appears when a class need to use a feature of other class as shown here the driver use the car association relationship represented with a arrow
5-Note notation
This notation is needed when extra notes is needed and not mentioned in the diagram and known as a comment as shown PI is a constant 
