Design Patterns

Only one object instance should exist:

• Example

  public class Person {
  private final Logger LOG = LoggerFactory.getLogger(Person.class);
  Logger getLogger() {
  return LOG;
  }
  }

  In this case LOG is the same object even without static due to the way the LoggerFactory works.

Object creation & configuration is complex

• Example

  interface Product { float getPrice(); }public class Milk implements Product {
  final float price;
  public Milk(final float price) { this.price = price; } //constructor
  public float getPrice() { return price; } //getter
  }public class Sugar implements Product {
  final float price;
  public Sugar(final float price) { this.price = price; } //constructor
  public float getPrice () { return price; } //getter
  }public class Product Factory {
  public static Product createProduct(String what){
  //When sugar is requested, we return sugar:
  if (what.equals("Sugar")) { return new Sugar(1.49F) ; }
  // When milk is needed, we return milk:
  else if (what.equals("Milk")) { return new Milk(0.99F); }
  // Otherwise return at least sugar
  else { return new Sugar(1.49F); }
  }
  }

  

Achieving higher abstraction by grouping individual factories with a common theme

A group of individual factories that have a common theme

Two hierarchies → abstract AbstractFactory class provides interface

Client only knows abstract interface → Family may grow independently of the client

Need simplified access to a complex subsystem

Provides an abstracted interface of a subsystem

• Example

  public class SimpleMail {
  public static int sendMail(String address, String subject, String body) {
  int status = 0;
  ... //Complex mail sending operation
  return status;
  }
  }

Need to integrate incompatible external functionality

Translates (data transformation) into a compatible interface

Need to integrate further actions before intended method call

Extends concept of the delegation pattern

Implements interface and acts as a representative of the „original“ implementation

Used in: security, logging, caching

Trennung von Interface/Implementation (mehrere Implementationen möglich

Dependency Injection Systeme (Spring)

• Create (insert..()) • Read (get..(), search..()) • Update (update..(),save..()) • Delete(delete..())