1. Design Principles

2. Singleton Pattern

   • Eager Instantiation
   • Lazy Instantiation

3. Builder Pattern

4. Facade Pattern

5. Factory Pattern

6. Strategy Pattern

7. Template Method Pattern

8. Iterator Pattern

9. Decorator Pattern

• Keep it simple
• Keep it flexible
• Loose coupling
• Separation of concern
• Information hiding
• Principle of modularity
• DRY: Don’t repeat yourself
• Encapsulate what varies
• Solid
  • Single Responsibility Principle (SRP)
  • Open-Closed Principle (OCP)
  • Liskov Substitution Principle (LSP)
  • Interface Segregation Principle (ISP)
  • Dependency Inversion Principle (DIP)

Singleton is a very simple pattern, just have one object instance.

Implementation:

1. Make the constructor private.
2. Declare a private static instance of the class.
3. Add a static method to get an instance of the singleton class.

some example of singleton: Connection Pool, PrinterBuffer and Cache.

With singleton we often share the resources but making or instantiating shared resources , reading something from a property file, creating database connection takes a lot of time. so we two options with singleton 1. Eager Instantiation 2. Lazy Instantiation.

We create connection pool in the beginning before even calling static method .

public class ConnectionPool {
​//declare a private static instance of a class
private static ConnectionPool pool=new ConnectionPool();
//this is a pool with only one connection
private Connection connection=new Connection();

private ConnectionPool() {}
//add a static method to get an instance of a singleton class
public static ConnectionPool getPool() {
​return pool;
​}
public Connection getConnection() {
​return connection;
}
}

Whenever we call method than we create a connection pool. Most often this we use.

public class ConnectionPool {
private static ConnectionPool pool;
//this is a pool with only one connection
private Connection connection=new Connection();
private ConnectionPool() {}
//add a static method to get an instance of a singleton class
public static ConnectionPool getPool() {
​if(pool==null) {
​​pool=new ConnectionPool(); //lazy instantiation
​}
​}
public Connection getConnection() {
​return connection;
}

}

• Builds a complex object using step by step approach. This pattern is helpful while creating objects with many different parameters.
• If we want Expressive code and Immutable class
  • solution: Bulilder pattern

The facade pattern provides a unified interface to a complex set of classes. It hides the complexity from the clients.

A factory creates objects

• Encapsulation of the logic to create objects

• Simple factory method
  • Static or not static
• Factory method pattern
• Abstract factory pattern

The strategy pattern extracts algorithms (strategies) from a certain class (context class) and makes a different class for every single algorithm. This gives the following advantages

• We can easily add new algorithms without changing the context class
• The strategies are better reusable

The template method pattern defines the skeleton of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

• What problem does it solve?

  Whenever we have an algorithm with different steps that is used in different situations, then we can define this algorithm in one place (parent class) and let child classes implement the concrete steps. The template method can be used when you have a stable algorithm whose individual steps may vary.

Iterators are used to access the elements of an aggregate object sequentially without exposing its underlying implementation. An iterator object encapsulates the internal structure of how the iteration occurs.

• External iterator: The client controls the iteration
• Internal iterator: The iterator controls the iteration

Allows to dynamically add new behavior to an existing object. This pattern is useful when we need to enhance the behavior of a class while adhering to the Open-Closed Principle — that is, our code should be open for extension but closed for modification.