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Here are the guidelines that we follow to refactor the methods in one of the legacy Java application, simplified from Martin Fowler refactoring book.

Rename a method to server its purpose
Add a parameter to the method if it is required
Remove a parameter if it is not required
Separate query from modifier and let each method does it […]

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This is from the Refactoring book by Martin Fowler.
In our code we always need to check if a object is null, and depending on it, different actions are taken.
E.g. for the following code

1: if (subscriber == null) {
2: ratePlan = defaultRatePlan;
[…]

To create immutable Java object, we should use the Builder Pattern, as described in Effective Java. This is the pattern I normally used now for immutable class.

1: public class Person {
2: private final String name;
3: private final int […]

This is the sequel of my previous post on the AppFuse source code, a good MVC framework. Thanks to Matt Raible and the team for this wonderful framework.
By reading the code, I can understand the framework better in order to use it correctly.
For AppFuse, for the data access layer you can use Hibernate, iBATIS or […]

I have been using AppFuse in several projects. It is a excellent framework which implemented a proper MVC architecture.
In order to better understand the framework, I spent sometime browsing through the source code.
In Part I of this articles I am going to show you the model layer.
For all the model classes in AppFuse, it has […]

I am not totally against Singleton design pattern. However, based on experience, normally if there is Singleton design pattern in your application, you should carefully study your code. Usually there is always a better way of doing it other than using Singleton design pattern.
Consider the following code snippet I encountered for one of the legacy […]

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This is the design pattern I came across while browsing through my RSS feed reader. Not very confirmed now from where I read about this. It is quite hard to apply this to the data access layer since I use ORM tools but I am not sure how this can be applied to […]

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I was trying Apache Commons Chain when I was trying to design a simple workflow system for my back-end application using the Chain of Responsibility design pattern.
Chain of Responsibility is a popular technique for organizing the execution of complex processing flows. It is not difficult if you want to write it yourself […]

This is one of the applications that I developed some time back and I am going to use it to demonstrate some of the design patterns.
Basically this is a simplied share trading system. You can have a look at the E-R diagram here.

The MySQL script can be downloaded here.
MySQL Script
Using this application, I am […]

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Abstract Factory pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes.
E.g. I have a DocumentFactory class that either returns DefaultDocumentFactory or CustomDocumentFactory dependening on the environment variable.

using System;
using System.Collections.Generic;
using System.Text;

namespace AbstractFactory
{
abstract class DocumentFactory
{
[…]

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A Command pattern is an object behavioral pattern that allows us to achieve complete decoupling between the sender and the receiver. It allows you to encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests.
Here is a rewrite of the C# example provided in dofactory.com.
First […]

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This is Part II of the previously described Lazy Load pattern.
value holder
A value holder is used as a generic Lazy Load. It is an object that wraps some other object. To get the underlying object you ask the value holder for the value, and the first access will pull the data from […]

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Lazy Load is one of the object relational behavioral patterns. With lazy load, an object does not contain all of the data you need but knows how to get it when it is needed. This pattern is commonly found in most of the OR mappers, e.g. Hibernate.
E.g. when you load the Product table, […]

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Factory Method Pattern defines an interface for creating an object, but let subclasses decide which class to instantiate. It lets a class defer instantiation to subclasses.
E.g., I defined a abstract News class.

using System;
using System.Collections.Generic;
using System.Text;

namespace FactoryPattern
{
abstract class News
{
}
}

Different kinds of […]

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Facade Pattern provides a unified interface to a set of interfaces in a subsystem. It defines a higher-level interface that makes the subsystem easier to use.
E.g.
I have a Call Details Record (CDR) to be processed by different systems. It is defined in the CDR class

using System;
using System.Collections.Generic;
using System.Text;

namespace FacadePattern
{
class […]

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Decorator Pattern attaches additional responsibilities to an object dynamically and provides a flexible alternative to subclassing for extending functionality.
E.g.
BaseMessage is defined

using System;
using System.Collections.Generic;
using System.Text;

namespace DecoratorPattern
{
abstract class BaseMessage
{
private string sender;
[…]

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Proxy Pattern provides a surrogate or placeholder for another object to control access to it.
E.g.
I defined a IGreeting interface

using System;
using System.Collections.Generic;
using System.Text;

namespace ProxyPattern
{
interface IGreeting
{
void SayHello();
}
}

Greeting implements IGreeting

using System;
using System.Collections.Generic;
using System.Text;

namespace ProxyPattern
{

[…]

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Builder pattern is used to construct a complex object from simple objects step by step.
E.g.,
I have a Message class

public class Message {
private String sender;
private String recipient;
private String header;
private String body;

public String getSender() {
[…]

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NOTE: This is written for me to recap and relearn what I learnt before….
This article is written in order for me to recap what I have learnt and used before, though I know there are already many sites which talked about all these.
Visitor Pattern defines a new operation to deal with the […]