Design Pattern walkthrough

1. Proxy Pattern
- Proxy controls and manages access to the real subject.
- Same interface, so proxy can be substituted anywhere the subject can be used

• Remote proxy: act as local represtative (RMI)
• Virtual proxy: expensive to create, lazy initialization (Hibernate)
• Transaction proxy: wire transactional support (Spring)
• Security: protect

2. Command Pattern
- Encapsulate method invocation: store them, pass them around and invoke them when you need them
- It decouples the requester of an action from the oject that actually performs the action

• Order a meal: order slip
• Job queue: ControllerUpCmd, ApAddCmd
• Device import: commands passed around and executed in context (e.g., session establish, error handling), acts like closure, function pointer. It is like Template method pattern, but instead of subclassing, the method is passed as command objects here

3.Strategy Pattern
- Defines a family algorithms, encapsulates each one and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it
- Encapsulates the behaviour, take what varies and "encapsulate" it so it won't affect the rest of your code

4.Observer Pattern
- Defines a one-to-many dependency between objects so that when one object (subject) changes state, all of its dependents (observers) are notified and updated automatically
- Subjects and observers are *loosely* coupled
- A key part in the familiar Model View Controller (MVC) pattern