Design Pattern Essay

289 words - 2 pages


Observer pattern:

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems. The Observer pattern is also a key part in the familiar Model View Controller (MVC) architectural pattern.

Singleton pattern:
In software engineering, the ...view middle of the document...

That is you only want one object and use it in multiple places.

there are two ways of implement singleton pattern, create database connection and login file can be a very good example of singleton pattern:
here we will look at create database connection:

old ways of implementation: (lazy loading)
private static Database instanceOld;
public static Database getInstanceOld(){
if(instanceOld == null){
instanceOld = new Database();
return instanceOld;
since the old way of singleton implementation has thread safe problem, for example: there are several thread invoke getInstanceOld() method and there will be several instance created.

new way: (early loadings)
private static Database instance = new Database();
private Database(){//private constructor

public static Database getInstance(){
return instance;

Observer Pattern:
decorator pattern (also known as Wrapper, an alternative naming shared with the Adapter pattern) is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class

Momento pattern:
The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).

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