CSC/ECE 517 Fall 2012/ch2b 2w39 ka

Introduction

Describe what Design pattern is and what are we going to discuss in this article.

Decorator Pattern

brief overview

Implementation

Ruby

brief explanation and Sample Code:

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Java

brief explanation and Sample Code:

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Adapter Pattern

brief overview

Implementation

Ruby

brief explanation and Sample Code:

     type the code here

Java

brief explanation and Sample Code:

     type the code here

Proxy Pattern

Sometimes we need the ability to control the access to an object. For example if we need to use only a few methods of some costly objects we'll initialize those objects when we need them entirely. Until that point we can use some light objects exposing the same interface as the heavy objects. These light objects are called proxies and they will instantiate those heavy objects when they are really need and by then we'll use some light objects instead.

Intent

Provide a surrogate or placeholder for another object to control access to it” is the intent provided by GoF<ref>"Gang of four"</ref>

Problem

You need to support resource-hungry objects, and you do not want to instantiate such objects unless and until they are actually requested by the client.

Solution

Design a surrogate, or proxy, object that: instantiates the real object the first time the client makes a request of the proxy, remembers the identity of this real object, and forwards the instigating request to this real object. Then all subsequent requests are simply forwarded directly to the encapsulated real object.

Let's take a look at the diagram definition before we go into more detail.

As usual, when dealing with design patterns we code to interfaces. In this case, the interface that the client knows about is the Subject. Both the Proxy and RealSubject objects implement the Subject interface, but the client may not be able to access the RealSubject without going through the Proxy. It's quite common that the Proxy would handle the creation of the RealSubject object, but it will at least have a reference to it so that it can pass messages along.

Let's take a look at this in action with a sequence diagram.

As you can see it's quite simple - the Proxy is providing a barrier between the client and the real implementation.

There are many different flavours of Proxy, depending on it's purpose. You may have a protection proxy, to control access rights to an object. A virtual proxy handles the case where an object might be expensive to create, and a remote proxy controls access to a remote object.

You'll have noticed that this is very similar to the Adapter pattern. However, the main difference between bot is that the adapter will expose a different interface to allow interoperability. The Proxy exposes the same interface, but gets in the way to save processing time or memory.

Implementation

Ruby

brief explanation and Sample Code:

     type the code here

Java

brief explanation and Sample Code:

     type the code here

Composite Pattern

brief overview

Implementation

Ruby

brief explanation and Sample Code:

     type the code here

Java

brief explanation and Sample Code:

     type the code here

Comparison of the Patterns

Comparison	Decorator	Adapter	Proxy	Composite
Supports abc
Supports def
Supports ghi
Supports bla
Supports bla

Conclusion

This article makes an attempt to explain the concept of Reflection in Object Oriented Programming. The article mentions the different approaches to reflection in Ruby and other languages.It mentions the usage of Reflections and the advantages and disadvantages of using Reflection. A follow up to this article would be to study the concept of Metaprogramming.

References

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Additional Reading

• Java Reflection
• Java reflection basics
• A Tutorial on Behavioral Reflection and its Implementation
• Uses of reflection
• Java reflection official website
• Ruby Documentation
• Behavioral reflection
• C# Basics
• C# Reflection basics
• Introduction to C# Reflection
• C# Attributes and Reflection