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:

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Java

brief explanation and Sample Code:

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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.

There are four common situations in which the Proxy pattern is applicable.

• A virtual proxy is a placeholder for “expensive to create” objects.

The real object is only created when a client first requests/accesses the object. Think of an ATM implementation, it will hold proxy objects for bank information that exists in the remote server. RMI is an example of proxy implmenetation for this type in java.

• A remote proxy provides a local representative for an object that resides in a different address space.

This is what the “stub” code in RPC and CORBA provides.In place of a complex or heavy object use a skeleton representation. When an underlying image is huge in size, just represent it using a virtual proxy object and on demand load the real object. You feel that the real object is expensive in terms of instantiation and so without the real need we are not going to use the real object. Until the need arises we will use the virtual proxy.

• A protective proxy controls access to a sensitive master object.

The “surrogate” object checks that the caller has the access permissions required prior to forwarding the request. You might be well aware of the proxy server that provides you internet. Saying more than provides, the right word is censores internet. The management feels its better to censor some content and provide only work related web pages. Proxy server does that job. This is a type of proxy design pattern. Lighter part of censor is, we search for something critical in Google and click the result and you get this page is blocked by proxy server. You never know why this page is blocked and you feel this is genuine. How do you overcome that, over a period you learn to live with it.

• A smart proxy interposes additional actions when an object is accessed. Typical uses include:
  • Counting the number of references to the real object so that it can be freed automatically when there are no more references (aka smart pointer),
  • Loading a persistent object into memory when it’s first referenced,
  • Checking that the real object is locked before it is accessed to ensure that no other object can change it.

Implementation

Ruby

brief explanation and Sample Code:

     type the code here

Java

brief explanation and Sample Code:

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Composite Pattern

brief overview

Implementation

Ruby

brief explanation and Sample Code:

     type the code here

Java

brief explanation and Sample Code:

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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