Code like a Pro | Courtesy: blogs.warwick.ac.uk<\/p>\n
We all know how important design patterns are, right? Well Just to bring you to perspective. Imagine, you compiled a code of say 10,000 lines without any definite structure. How difficult would it be for you to modify & restructure the code if required. Even if someone else wants to work on your code if would be a challenge without a definite Blue Print.<\/p>\n
Design Patterns are nothing but this blue print & are a popular topic in software development arena. A design pattern is a common, well-described solution to a common software problem. So the sensible use of the right design patterns can result in not only increased code maintainability, design patterns can also be recognized by other developers, thus reducing the learning curve when dealing with a particular piece of code.<\/p>\n
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A design pattern<\/b> describes the classes and interacting objects used to solve a general design problem in a specific context.<\/span><\/i><\/p>\n There are many design patterns available, some of the important ones that we will be discussing are:<\/p>\n 1.Singleton Pattern So let us start with the Singleton Pattern & cover the common know hows on how to implement it.<\/p>\n Definition:<\/i><\/b> The Singleton pattern ensures that a class has only one instance and provide a global point of access to that instance. It is named after the singleton set, which is defined to be a set containing one element.<\/i><\/p>\n An Example would be the office of the President of India, it\u2019s is a Singleton. The Indian Constitution specifies the means by which a president is elected, limit the term of office and defines the order of succession. As a result, there can be at most one active President at any given time. Regardless of the personal identity of the active President, the title \u201cThe President of India\u201d is a global point of access that identifies the person in the office.<\/p>\n <\/p>\n Here are some real situations where the singleton is used:<\/p>\n <\/p>\n How to Construct a Singleton<\/p>\n Nowadays Java web applications use ORM (Object Relational Mapping)<\/a> for database like Hibernate. In Hibernate as you know, SessionFactory is a long live multi threaded object. Usually one session factory should be created for one database. If you assume the scenario that you are using one database called mysql in your application then the following is a way to create the SessionFactory object, the java code snippets are:<\/p>\n <\/p>\n When we are use more than one database in our application then we use the HibernateUtil class which is implemented based on singleton design pattern, which insures that one and only one SessionFactory object will be created for the entire application.<\/span><\/p>\n <\/p>\n Usually in mobile, many applications run concurrently in the screen stack or in the background. Mobile operating systems do not support large databases like Oracle, Mysql etc. instead it supports light weight databases e.g., SQLite<\/a>. The reason is the issue of memory. Suppose one application is running on screen which stores the user data in the database(SQLite) and there are four applications that are running in the background which updates some data when a particular event occurs. In this case all applications can be woken up on the same time and this will create many instances of the database which is expensive and results may be OutOfMemory or StackOverFlow! If you use a Singleton multithreaded database instance however, you can run apps smoothly. Following is an Android code snippet which uses Singleton:<\/p>\n
\n2.Decorator Pattern
\n3.Factory Pattern
\n4.Abstract Factory Pattern<\/p>\nWhy do we use these Singleton Patterns?<\/h3>\n
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How to use a Singleton J2EE Environment for the Creations of Heavy Database Instances?<\/h3>\n
public<\/span> <\/span>class<\/span> <\/span>HibernateUtil<\/span> <\/span>{<\/span>\n <\/span>private<\/span> <\/span>static<\/span> <\/span>final<\/span> <\/span>SessionFactory<\/span> sessionFactory<\/span>;<\/span>\n <\/span>static<\/span> <\/span>{<\/span>\n <\/span>try<\/span> <\/span>{<\/span>\n sessionFactory <\/span>=<\/span> <\/span>new<\/span> <\/span>Configuration<\/span>().<\/span>configure<\/span>()<\/span>\n <\/span>.<\/span>buildSessionFactory<\/span>();<\/span>\n <\/span>}<\/span> <\/span>catch<\/span> <\/span>(<\/span>Throwable<\/span> ex<\/span>)<\/span> <\/span>{<\/span>\n <\/span>System<\/span>.<\/span>err<\/span>.<\/span>println<\/span>(<\/span>\"Initial SessionFactory creation failed.\"<\/span> <\/span>+<\/span> ex<\/span>);<\/span>\n <\/span>throw<\/span> <\/span>new<\/span> <\/span>ExceptionInInitializerError<\/span>(<\/span>ex<\/span>);<\/span>\n <\/span>}<\/span>\n <\/span>}<\/span>\n\n <\/span>public<\/span> <\/span>static<\/span> <\/span>SessionFactory<\/span> getSessionFactory<\/span>()<\/span> <\/span>{<\/span>\n <\/span>return<\/span> sessionFactory<\/span>;<\/span>\n <\/span>}<\/span>\n<\/span>}<\/span><\/pre>\nHow would we use Singleton in a Mobile Application Development?<\/h3>\n