Java Solutions - New Java Platforms or Frameworks for Enterprise Solutions

Java is widely and high competitive words on Google or any other search engine. Today for online business application, java is the best option. When you find the sentence like we offer java solutions then you confuse obsessively why not. Java solutions mean use java and its latest technologies for web development, software development, Enterprise management software development, and mobile application development. However, you have one questions how all above development is possible through java. Not worry its can developed using latest java technology tools and frameworks like eclipse, struts, J2ME, J2EE, J2SE, JDBC etc. Java is widely used for web application development like e-commerce application, online shopping cart, dynamic website, blog etc. Web application using java is more secure, reliable, and robust then PHP and.net solutions. Today many big business owners or company would like to make their e-commerce web application through they sell their products or services across the world. However, security is main point for any e-commerce application because if you have popular then its chance to theft your transaction, important details for customer, products etc. So as per my way java is the best option for e-commerce application. Many outsource software development company offers java solutions e-commerce applications. They use latest frameworks or tools for e-commerce applications. Here I am listed downside some of the famous java e-commerce platforms with its creative features and benefits. Apache OF Biz (The Apache Open for Business Project) IT is one of the famous open sourced enterprise automation software under open source license. Its provides following functionality * Products catalog management * Business promotion & products pricing management * Sales & Purchase order management * Customer management (part of general party management) * Data warehouse management * Auto stock moves, batched pick, pack & ship * Content management (for product content, web sites, general content, blogging, forums, etc) * Point of sales module using a rich client interface It is specially build on Java designed for ERP, e-commerce, CRM, SCM, and CMMS/EAM development. Broad Leaf Commerce It is open source customizable e-commerce framework provides core functionally and business management tools. It is easily integrates with your business-exiting database and easily integrate with any other e-commerce platforms without any price. Its include some creative features like numerous bug fixes, Faster getting started - launch and view the sample demo and admin in under 30 seconds, Simplified creating and launching the demo and admin when compiling from source and many more. Konakart Widely known as java based shopping cart e commerce software with including professional and support services packages. It is one type o f free community edition for small business. It provides to customer all information and fictionally for orders any products from online shopping store. This framework is most suitable for retail business also provides enterprise level e Commerce solution that you can easily customize to match the requirements of your customers. Other famous java e commerce frameworks are Avetti, Soft Slate, Elastic Path, Jada site used for java solutions. You can chose as per your requirement. He has been working as business developers in India. He would like to provide information about software development using java technologies. Also, share information about Microsoft solutions with.net technology Article Source: http://EzineArticles.com/5677890

What is Java Object Initialization?

The concept of object in a broad sense can be described as a collection of data along with the methods to manipulate the corresponding data. The Objects are be loaded into the memory before processing. In main memory the object has a state determined by its instance variables and their values, which can be processed and evolved throughout its lifetime. The newly-created object should have a newly-allocated memory( heap) to accommodate the object's instance variables, this is done by Java virtual machine (JVM).

Object initialization is very important in programming, as it is a common source of bugs. The Java Programming language provides different built-in mechanisms to ensure proper initialization of the memory occupied by a newly-created object, there by helping the objects to get a valid initial state.

The Java Programming language offers three mechanisms to ensure proper initialization of newly created objects.

* Constructors.
* Instance variable initialization.
* Instance initialization.

When we are creating an object using new operator or the new Instance() method of class, JVM allocates memory for it, then Java virtual machine will run the initialization code before we use the newly-allocated memory.

Constructors

Constructors are methods, with a set of parameters and a body of code and has the same name as the class but has no return type. The constructors can be overloaded by varying the number, types, and order of parameters. When we are creating new instance using new keyword we often provide a constructor that initialize the Object. If we are using class that has no constructor defined, the compiler will automatically generate a default constructor for the class. This constructor has no parameter and has empty body, this ensures that each class has at least one constructor.

class Demo

{

int a;

public Demo()

{

// the initialization code for instance variables

a=0;

}

}

Instance variable initialization

In an instance variable initialization method java uses an equals sign and expression technique to initialize the instance variable.

class Demo

{

private int a = 1; //this evaluate a with a value of 1

}

Instance initialization

Instance initialization is also known as instance initialization block, which is one of the alternatives to Instance variable initialization. It is very useful in following situations

* Initializer code need to catch exceptions.
* Initializer code need to do calculations that can't be expressed with an instance variable initializer.
* If class has multiple constructors - no need to repeat the same initializing code in all constructors.
* When we use anonymous inner classes, which normally do not have constructors.

class Demo

{

private int a;

//intializer

{

a=2;

}

}

One of the main properties of initializers is they cant make forward reference, as the initializers are executed in textual order.You have to make sure that the referring of instance variable should be only after it is initialized.

Default initial values

If we are not providing any explicit initialization method to variables including instance and class variables, they will be initialized according to their type. Each data type has its own default initial values. The local variable must be initialized explicitly before use.

e.g boolean-false

int - 0

Java's initialization mechanisms help the programmer to ensure that objects we design have a valid and predictable state.



Article Source: http://EzineArticles.com/4564513

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New to Java

Can I learn java quickly? Yes now it's your cup of tea.

The new beginners section "New to Java technology" on Roseindia.net, can help the dummies to learn java from basic to masters level.

This new to java tutorial briefs you on following topics.

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2. Understanding the Java Technology

3. Why Java Technology is so important?

4. Different Editions of Java Technology

5. Components of each edition

· JSE Components:

· JEE - Components: etc... etc...

How to create and run my first java program, to what extension java file should be saved, java compiler, java and jdbc database connectivity or java with database management system. These are the very common problems raised while learning java.
Here in this new to java section you will find all the required solution to step ahead on next level of java technology.

The Additional features "Java Examples" of New to java tutorial can help you in learning java quickly and easily. As example codes are already written, you only have to copy and paste it to execute and run the program. In this way you can save yourself from writing code and fixing the bugs.

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Article Source: http://EzineArticles.com/562728

Introduction to Java Concurrent Programming

When computers didn't have operating systems yet, they executed only one program at the time. The program would run from the beginning to the end. Running only one program was the only way back then, and this is now seen as very inefficient. Because the program had access to all the resources in the computer at any time, programming was a lot simpler. But because the machine (computer) was only running once program at the time, the machine's resources where not used efficiently, resulting in an inefficient use of the computers capabilities. Nowadays, machines are able to run multiple programs. The OS (operating system) can even run multiple programs simultaneously, and one single program can even be divided into multiple concurrent threads. All these threads together do all the work that used to be done by just one program. Threads can also communicate with each other, with for instance; message passing and shared memory concurrency.

The last few years, multicore processors have become less expensive. Most desktop computers and laptops are using multicore chips and developers are programming more and more with multiple concurrent threads. A downside is that the bug reports related to threading are also increasing every day; this shows that multicore programming is not the easiest thing to do. In Java, concurrency bugs are one of the most painful problems and in many occasions they are highly unpredictable. One of the biggest issues regarding Java concurrent programming is the mind-set of the programmers that doesn't match the concurrency features offered in Java. Programmers need to start thinking about concurrency and parallelism in the foundation of their programs. The Java language provides low level tools for concurrency such as condition waits, synchronization, message passing and resource sharing. The problem is that these tools need to be implemented at application level consistently. The conclusion? It can already be hard to write correct programs, but writing correct concurrent programs is even harder. The right mind-set is needed. The reason behind the high difficulty level is that there are simply more levels where things can go wrong with Java concurrent programming. The reason developers still choose to use concurrency on their programs is because threads are an inevitable feature of the Java language and the easiest way to use the multicore power we see almost everywhere these days. When looking are the future, the only conclusion that can be made is that concurrency programming will become increasingly important with the rising number of cores.

For more information regarding Java concurrent programming, please visit www.ateji.com.



Article Source: http://EzineArticles.com/6401289

Understand The Java Classpath

This article is about a small simple collection of batch files which can be used to test the behavior of the javac compiler and java class launcher for different classpath scenarios. The system is called ClasspathTester. Download instructions are included at the end of this article.

When you are in the middle of trying to debug a java build it is a bad time to try to boil everything down to a simple test case, so basically the intent of my test collection is to do this for you ahead of time. If you can't find the precise test you need, it is easy to modify the existing tests or create a new test to suit your needs.

How ClasspathTester Works

As previously stated, I did everything with batch files, because I wanted the tests to be as uncomplicated, portable (within the Windows environment), and simple as possible. There are a few helper batch files to do things like create formatted output and build a directory structure, but otherwise nearly all of the work is done in the RunAllTests.bat file.

What RunAllTests.bat consists of is tests and sub-tests. If you haven't downloaded it yet, I suggest you download it and take a look. Each test consists of some sort of a setup. Setup normally involves the following:

1. Test environment is cleaned up of old directories, files, and classpath.

2. New directory structure is created, new files are created and placed in correct directories, and the classpath is set if necessary.

After setup, the tests are run. This consists of running either the javac.exe compiler, the java.exe launcher, or both. That's all there is to it.

A Simple Example

To understand what ClasspathTester, does, open the RunAllTests.bat file in your text editor and look at TEST1. Each test is a collection of sub-tests which are loosely grouped around some kind of a central testing theme, and TEST1 is one of the simplest.

So what happens in Test1? First it calls banner.bat to add a header to the results file. Then it creates the Hello.java and MyLibClass.java files, by copying them from their corresponding avaj files (the reason for using avaj files is discussed later in this article). Now everything is set up to run some subtests.

TEST1A is very simple. According to its description, " The Hello.java file is located in the current directory, and the Hello.class file will be written to the current directory and run from the current directory". As you can imagine, TEST1A operates flawlessly.

TEST1B and TEST1C are designed to show different ways that using java.exe to launch the Hello.class file can fail. In TEST1B the classpath is been set to an empty directory. In TEST1C the file Hello.class is erased. Interestingly, these two tests fail in exactly the same way.

Note that all tests results are stored in the results.log file.

How the Tests are Kept Safe

By "safe" I mean safe to your computer, safe to your other data. Since the tests involve batch files that are creating and destroying files and directories, care must be taken to avoid accidentally destroying or writing over other unrelated files. This is accomplished in the cleanup.bat file by avoiding the use of statements like "erase /S *.*" or "rmdir /S *". Instead, all erasures are kept fairly specific.

In fact, the cleanup.bat file consists of the following statements:

set classpath=

rmdir /S /Q aq

rmdir /S /Q EmptyDirectory

erase Hello*.java

erase Hello*.class

erase MyLibClass*.java

erase MyLibClass*.class

This ensures that there will be no tragic accidental loss of unrelated data.

Why Use .avaj Suffix Files

Part of the operation of ClasspathTester involves completely wiping the base directory clean of all .java files, .class files, and subdirectories; therefore, all the suffix of all permanent java files is mangled to avaj. For any test, the appropriate avaj files are copied to java files. The available files are:

Hello.avaj

Hello1.avaj

Hello2.avaj

MyLibClass.avaj

MyLibClass1.avaj

MyLibClass2.avaj

Download Information

You can find and download a zipped copy of the ClasspathTester can be on my Article Support Page

Summary

ClasspathTester is simple, portable, unobtrusive, customizable and solves an annoying development problem, the need to quickly test special classpath situations with javac.exe, java.exe, or both.

Article Source: http://EzineArticles.com/505451

Difference Between Java and WordPress

ava and WordPress are very much different, this is an attempt to compare and see where they intersect each other.

Java

Java is an enterprise Language, what it means it is used to build enterprise applications, what do we mean by that?

· A variety of clients can interact with applications like browsers, smart tablets, B2B applications, .NET and other language apps.

· High Security to support the standards.

· Highly Scalable to support the growing traffic.

· Performance - Begin compile time performance is high.

· E.g applications are Gaming, ECommerce websites, Billing, Retail, CRM and tons of others




Java can be used to create blogging CMS like WordPress. There are CMS's like alfresco, Plone, JRoller whodo to attempt to that, but none has been able so popular as WordPress.

WordPress

Very specialized CMS/blogging engine build on top of PHP.

· It is very easy to learn software, compare it to learning MS Word.

· You don't need to know PHP/programming to be WordPress website developer.

· It has a themes concept, which allows a developer to configure website pages with easy.

· Supports thousands of plugins, almost easy to find any kind of functionality a website needs.

· Installs on Apache Server with PHP engine.

· Many hosting sites support 1 click install.

· Uses MySQL as the backend engine.

As you see, WordPress and Java cannot be compared as one is a language where another is a software built on PHP language.




Had WordPress been written in Java

As a Java Developer, I do wish WordPress was built on Java, it would have given

1. Java applications a web flair, we would have to see all together new set of applications /widgets/ plugins.

2. Designers would have not been scared of the language at all.

3. Applications could have used the WordPress database, which has a very simple schema.

WordPress And Java Together

Resin Quercus

Resin is a Java Application server, but with a twist, it has a Quercus engine which allow PHP applications to installed as Java applications.

What does this mean, if a WordPress can be installed on Resin will run as a Java application and also take advantage of libraries like Spring, Hibernate,SOA.

Also, will definitely perform way better as it will be compiled only once and not be interpreted with every request like it does now

To have this integration working has its own challenges but good news is there is a way.

Summary

While WordPress has it own niche, creating CMS websites with blog functionality, it is the DeFacto Platform.

But when it comes to creating highly scalable enterprise Applications Java is the DeFacto Platform.

Makarand Bhatamrekar is a enterprise Java RIA Career Coach. He has been coaching since 2007 on Java technologies to help trainees achieve their java career goals. http://careerInJava.com is a dedicated the effort to give all the resources needed to succeed.
To be informed of the latest update, do subscribe the Newsletter at http://www.careerInJava.com

WordPress is the easiest CMS out here, which allows to create your website with no knowledge of PHP. Makarand was able to develop coaching website on WordPress with no prior knowledge of the same, hence this post comparing the two.

Article Source: http://EzineArticles.com/6811651

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Why Java RDBMS?

It is a well known fact that Java as a programming language set off a new paradigm in the software industry. Suddenly, every software programmer worth his salt was amidst software jargons like 'Platform-Independence', 'Cross-Platform-Deployment' and 'The Java Virtual Machine'. In fact, it did not take long for Java to usurp the 'most sought after status' from many software languages, and become the most preferred tool for creating software; especially software for the web. As the recent trends in the industry show, Java is set to achieve an undeniable position as the most preferred software programming language for a long time to come. It is indeed Java's credit that many prominent vendors who tried to emulate the capacities of Java, failed miserably in the endeavor.

The importance of Java in the software scenario has led to another major trend. Software vendors are either rewriting their existing products in Java, or are creating new products exclusively or partially in Java. This has led many analysts to question the need to port already existing applications to Java. While the prospects of porting small or medium sized software to Java may not attract concern, it assumes significance when we consider porting existing databases that could be handling millions of sensitive blocks of information. Similarly, experts also have to decide between deploying reputed databases, and new databases that have been entirely developed in Java, in their organizations. Analysts have to answer many pertinent questions like: What are the advantages of a Java RDBMS that would compel my company to switch to it? Would it be cost effective to change to an RDBMS that has all the advantages of Java? Would the costs involved in porting millions of existing records justify the perceived performance and scalability features of the Java RDBMS? How secure would be the new RDBMS? What is the learning curve that has to be tackled by the staff who may be assigned to this database? In fact, there are a host of questions that a concerned technical head may ask himself before taking that all important decision to switch to a Java RDBMS. Let's try finding out some of the answers.

What are the advantages of a Java RDBMS?

OK, so we all know that Java is set to capture, or has already captured a sizeable chunk of the software market. However, how does that justify the use of a Java Database? After all, is not Java theoretically capable of integrating with any database? What advantages can be afforded by a Java RDBMS?

Well the answer is quite simple. Java RDBMS has, or supports, the much sought after qualities of Java, which is the most important reason to switch to it. Some of the Java RDBMS already available in the markets answers the immediate concerns of the Technical Lead in the most persuasive manner. For example, Daffodil DB, an RDBMS written in Java, considers the following as its core strengths:

Multiple Platform Portability

Small Size

Java Stored Procedures

Transparent to End User

Zero Administration Efforts

Some of these features are worth a second look.

One RDBMS, multiple avatars
Java RDBMS not only offers the advantages of Java, but also introduces new concepts that can revolutionize database programming techniques. For a start, the much famed portability of Java is the core strength of a Java RDBMS. Now, you don't have to buy different RDBMS for different platforms within the same enterprise. Other than the cost factors, the biggest advantage such an arrangement can give is that in-house code need not be redesigned for different platforms. By standardizing the database that is used, programs working across platforms can interact transparently and effectively. Project managers would vouch that portability, synchronization and customization effort for software running on different platforms are some of the most time consuming and tension filled activities in the office. Now all these activities can be easily implemented with the Java RDBMS in its rightful place.

Store and use those procedures with abandon

Security buffs within the enterprise would always throw a tantrum when dealing with stored procedures. Traditionally, the implementation of stored procedures had been the responsibility of the specific application vendor, and most of them could crash the whole database due to internal errors, rendering precious time and money waste. Java stored procedures with its inbuilt security features is the new fad of security administrators who are finding it powerful, fast, and very safe. Java does not implement Pointers, and so is a safe bet for developers to write secure code and procedures. In addition, with the JVM overlooking all the activities (the sandbox approach), there is no scope for external access that could jeopardize mission critical enterprise applications.

Transparent to End User

Java technologies have always been in the forefront to give us seamless transparent applications with inbuilt security features being given the utmost importance. In a Java RDBMS, database components can be embedded in applications, and they can perform complex tasks without user intervention. In addition, the user need not know the complexities of the database because the embedded database components will do all the work for him without him even knowing about it. This allows the user to concentrate on his tasks rather than configuring databases, which is an auxiliary requirement. In addition, other routine activities like starting the server and repetitive configuring of the database can also be avoided. Perhaps the biggest advantage that Java RDBMS offers is that the user need not weigh the pros and cons about choosing a database vendor. This is because Java RDBMS being extremely portable can seamlessly integrate with almost all platforms, and the embedded components will handle all the chores of integrating with different vendor software. Java RDBMS can also be seamlessly operated across multiple platforms without the need to configure the database for each and every operating system.

Zero Administration Efforts

Java RDBMS comes in with zero administration efforts, which means that once installed and configured, your clients need not change their settings to suit new changes that you make in your database. Java RDBMS also does away with, or considerably reduces, the role of the Database Administrator because of its self-managed features, and its extremely scalable mode of operation.

If you have not thought of it yet, it is a good time to think about Java RDBMS. The time is not far away when the databases industry will be greatly influenced by the power and functionality of Java RDBMS.


Article Source: http://EzineArticles.com/3103
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BASIC CONCEPTS OF JAVA

Now that already we have seen large characteristics what Java can offer us, and before entering to bag in the generation of our first code Java, we are going to glance over the language Java in itself. The basic thing relative to the ones will result very that have know-how of C/C+ +. The programmers with experience in other languages procedurales will recognize most of the constructions. We expect that this chapter result not too intense, nevertheless, yes should been present, because more than one time we will resort to him as reference. In subsequent chapters we will deepen on aspects of the programming in Java by the ones that here we will pass on tiptoe and we will go presenting examples of code of each one of those aspects of the programming in Java.

If really it wants to dominate Java, is not sufficient to know the code. You should begin to think as a programmer, and in this new edition of his guide of best-seller, Java Concepts, programming Cay Horstmann guru shows him how. It brought up to date to integrate Java 6, Concepts of Java, fifth edition is an introduction to the techniques of fundamental programming and the abilities of design that are crucial to learn to plan. Basing on its many years of experience as expert programmer and professor, Horstmann shows the most important elements of the data processing, troubleshooting and the design of the program. Thanks to the formula tested Horstmann, you will be able to complete this book with a solid base in the concepts of programming and troubleshooting and to arrive quickly to the velocity to write program efficient and successful. You Can find the Best Java Knowledge Concepts here exclusive on javaequation.blogspot.com

Characteristics of the fifth new edition: * The focus "gradual Objects" conducts you to step oriented to objects thinking to step, the use of classes, the application of simple methods, until coming to design its own programs oriented to objects. * A strong emphasis in the development based on tests invites to consider the results as is written the code of programming so that a better design, that can be utilized more you program * unique Focus goes beyond the syntax of the language to be concentrate on concepts of data processing and troubleshooting, that encourage to think like a solucionador of problems * New and learning teaching tools in WileyPLUS -
including a corrector of unique allocation that will permit you to put to test your problems of programming in line before submitting them to a degree * Useful "the tests in trail" he introduces the techniques and tools step by step, what guarantees a teacher before passing to the following one * The graphics are themes developed gradually along the text, conveniently noticeable in different sections by color * The cover of updating is completely compatible with Java 5 and includes a discussion of the last characteristics of Java 6.

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My Aim Language Java

These annotations of the language Java are the hyphen structured of a course of Introduction to the Language Java. They are not a reference exahustiva of the language but they intend to provide a progressive approximation, leaving from it more elementary and introducing the concepts in an orderly fashion, being supported in the previous thing.

They are structured in the shape of brief lessons, that cover all the aspects of the language in a basic-medium level. Itself they are not assumed know-how of other programming languages, although obviously to be familiarized with concepts relating to the programming as variable, algorithms, objects, etc. facilitates a lot of the task. Nevertheless, the reference has been avoided deliberately or the comparison with other languages, especially with C/C + + that so present is in many texts of Java.

The first part of these annotations covers all the aspects of the language (Classes, Interfaces, inheritance, control of flow, types of data, etc.). The second part, at present in elaboration, treats matters of the API of Java, grouping them for themes (Entrance/Exit, Threads, applets, etc.).

Access Control

When writing new classes, it’s a good idea to pay attention to the issue of access
control. Recall that making a member of a class public makes it accessible from anywhere, including from other classes. On the other hand, a private member can
only be used in the class where it is defined.
In the opinion of many programmers, almost all member variables should be declared
private. This gives you complete control over what can be done with the
variable. Even if the variable itself is private, you can allow other classes to find out
what its value is by providing a public accessor method that returns the value of
the variable. For example, if your class contains a private member variable, title,
of type String, you can provide a method
public String getTitle() { return title; }
that returns the value of title. By convention, the name of an accessor method for
a variable is obtained by capitalizing the name of variable and adding “get” in front
of the name. So, for the variable title, we get an accessor method named “get” +
“Title”, or getTitle(). Because of this naming convention, accessor methods are
more often referred to as getter methods. A getter method provides “read access” to
a variable.
You might also want to allow “write access” to a private variable. That is, you
might want to make it possible for other classes to specify a new value for the variable.
This is done with a setter method. (If you don’t like simple, Anglo-Saxon
words, you can use the fancier term mutator method.) The name of a setter method
should consist of “set” followed by a capitalized copy of the variable’s name, and it
should have a parameter with the same type as the variable. A setter method for the
variable title could be written
public void setTitle( String newTitle ) { title = newTitle; }
It is actually very common to provide both a getter and a setter method for a
private member variable. Since this allows other classes both to see and to change
the value of the variable, you might wonder why not just make the variable public?
The reason is that getters and setters are not restricted to simply reading and writing
the variable’s value. In fact, they can take any action at all. For example, a getter
method might keep track of the number of times that the variable has been accessed:
public String getTitle() {
titleAccessCount++; / / Increment member v a r i a b l e t i t leAc ces sCount .
return title;
}
and a setter method might check that the value that is being assigned to the variable
is legal:
public void setTitle( String newTitle ) {
if ( newTitle == null ) / / Don ’ t al low n u l l s t r i n g s as t i t l e s !
title = " ( Unt i t led ) "; / / Use an appropr iate d e f a u l t value instead .
else
title = newTitle; }
Even if you can’t think of any extra chores to do in a getter or setter method, you
might change your mind in the future when you redesign and improve your class. If
you’ve used a getter and setter from the beginning, you can make the modification
to your class without affecting any of the classes that use your class. The private
member variable is not part of the public interface of your class; only the public
getter and setter methods are. If you haven’t used get and set from the beginning,
you’ll have to contact everyone who uses your class and tell them, “Sorry guys, you’ll
have to track down every use that you’ve made of this variable and change your code.”

Class Members and Instance Members

A class definition is made of members or components. A class can define variables (or
fields) and methods. Variables and methods can be static or non-static i.e. they are
defined with or without the keyword static.
e.g.
static double lastStudentNumber; / / a s t a t i c member / v a r i a b l e / f i e l d
double studentNumber; / / a non−s t a t i c v a r i a b l e
static void printLastNumber() {...} / / a s t a t i c member /method
void printNumber() {...} / / a non−s t a t i c method
The non-static members of a class (variables and methods) are also known as
instance variables and methods while the non-static members are also known as class
variables and class methods. Each instance of a class (each object) gets its own copy of
all the instance variables defined in the class. When you create an instance of a class,
the system allocates enough memory for the object and all its instance variables.
In addition to instance variables, classes can declare class variables. A class variable
contains information that is shared by all instances (objects) of the class. If one
object changes the variable, it changes for all other objects of that type. e.g. A Student
number generator in a NewStudent class.
You can invoke a class method directly from the class, whereas you must invoke
instance methods on a particular instance. e.g. The methods in the Math class are
static and can be invoked without creating an instance of the Math class for e.g. we
can say Math.sqrt(x).
Consider a simple class whose job is to group together a few static member variables
for example a class could be used to store information about the person who is
using the program:
class UserData { static String name; static int age; }
In programs that use this class, there is one copy each of the variables UserData.name
and UserData.age. There can only be one “user,” since we only have memory space
to store data about one user. The class, UserData, and the variables it contains exist
as long as the program runs. Now, consider a similar class that includes non-static
variables:
class PlayerData { String name; int age; }
In this case, there is no such variable as PlayerData.name or PlayerData.age,
since name and age are not static members of PlayerData. There is nothing much in the class except the potential to create objects. But, it’s a lot of potential, since
it can be used to create any number of objects! Each object will have its own variables
called name and age. There can be many “players” because we can make new
objects to represent new players on demand. A program might use this class to store
information about multiple players in a game. Each player has a name and an age.
When a player joins the game, a new PlayerData object can be created to represent
that player. If a player leaves the game, the PlayerData object that represents that
player can be destroyed. A system of objects in the program is being used to dynamically
model what is happening in the game. You can’t do this with “static” variables!
An object that belongs to a class is said to be an instance of that class and the
variables that the object contains are called instance variables. The methods that
the object contains are called instance methods.
For example, if the PlayerData class, is used to create an object, then that object
is an instance of the PlayerData class, and name and age are instance variables in the
object. It is important to remember that the class of an object determines the types
of the instance variables; however, the actual data is contained inside the individual
objects, not the class. Thus, each object has its own set of data.
The source code for methods are defined in the class yet it’s better to think of the
instance methods as belonging to the object, not to the class. The non-static methods
in the class merely specify the instance methods that every object created from the
class will contain. For example a draw() method in two different objects do the same
thing in the sense that they both draw something. But there is a real difference
between the two methods—the things that they draw can be different. You might
say that the method definition in the class specifies what type of behavior the objects
will have, but the specific behavior can vary from object to object, depending on the
values of their instance variables.
The static and the non-static portions of a class are very different things and serve
very different purposes. Many classes contain only static members, or only non-static.
However, it is possible to mix static and non-static members in a single class. The
“static” definitions in the source code specify the things that are part of the class itself,
whereas the non-static definitions in the source code specify things that will become
part of every instance object that is created from the class. Static member variables
and static member methods in a class are sometimes called class variables and
class methods, since they belong to the class itself, rather than to instances of that
class.
So far, we’ve been talking mostly in generalities. Let’s now look at a specific
example to see how classes and objects work. Consider this extremely simplified
version of a Student class, which could be used to store information about students
taking a course:
public class Student {
public String name; / / Student ’ s name . p u b l i c double test1 ,
test2, test3; / / Grades on three t e s t s .
public double getAverage() { / / compute average t e s t grade r e t u r n
(test1 + test2 + test3) / 3; }
} / / end of class Student
None of the members of this class are declared to be static, so the class exists
only for creating objects. This class definition says that any object that is an instance of the Student class will include instance variables named name, test1, test2, and
test3, and it will include an instance method named getAverage(). The names
and tests in different objects will generally have different values. When called for
a particular student, the method getAverage() will compute an average using that
student’s test grades. Different students can have different averages. (Again, this is
what it means to say that an instance method belongs to an individual object, not to
the class.)
In JAVA, a class is a type, similar to the built-in types such as int and boolean.
So, a class name can be used to specify the type of a variable in a declaration statement,
the type of a formal parameter, or the return type of a method. For example, a
program could define a variable named std of type Student with the statement
Student std;
However, declaring a variable does not create an object! This is an important
point, which is related to this Very Important Fact:
In JAVA, no variable can ever hold an object. A variable can only hold a
reference to an object.
You should think of objects as floating around independently in the computer’s
memory. In fact, there is a special portion of memory called the heap where objects
live. Instead of holding an object itself, a variable holds the information necessary
to find the object in memory. This information is called a reference or pointer to the
object. In effect, a reference to an object is the address of the memory location where
the object is stored. When you use a variable of class type, the computer uses the
reference in the variable to find the actual object.
In a program, objects are created using an operator called new, which creates an
object and returns a reference to that object. For example, assuming that std is a
variable of type Student, declared as above, the assignment statement
std = new Student();
would create a new object which is an instance of the class Student, and it would
store a reference to that object in the variable std. The value of the variable is a
reference to the object, not the object itself. It is not quite true to say that the object
is the “value of the variable std”. It is certainly not at all true to say that the object
is “stored in the variable std.” The proper terminology is that “the variable std refers
to the object,”.
So, suppose that the variable std refers to an object belonging to the class Student.
That object has instance variables name, test1, test2, and test3. These instance
variables can be referred to as std.name, std.test1, std.test2, and std.test3.
This follows the usual naming convention that when B is part of A, then the full name
of B is A.B. For example, a program might include the lines
System.out.println(" Hello , " + std.name + " . Your tes t grades are : ");
System.out.println(std.test1);
System.out.println(std.test2);
System.out.println(std.test3);
This would output the name and test grades from the object to which std refers.
Similarly, std can be used to call the getAverage() instance method in the object by
saying std.getAverage(). To print out the student’s average, you could say:
System.out.println( " Your average i s " + std.getAverage() );
More generally, you could use std.name any place where a variable of type String
is legal. You can use it in expressions. You can assign a value to it. You can pass it
as a parameter to method. You can even use it to call methods from the String class.
For example, std.name.length() is the number of characters in the student’s name.
It is possible for a variable like std, whose type is given by a class, to refer to no
object at all. We say in this case that std holds a null reference. The null reference
is written in JAVA as “null”. You can store a null reference in the variable std by
saying “std = null;” and you could test whether the value of “std” is null by testing
“if (std == null) . . .”.
If the value of a variable is null, then it is, of course, illegal to refer to instance
variables or instance methods through that variable–since there is no object, and
hence no instance variables to refer to. For example, if the value of the variable st is
null, then it would be illegal to refer to std.test1. If your program attempts to use a
null reference illegally like this, the result is an error called a null pointer exception.
Let’s look at a sequence of statements that work with objects:
Student std, std1, / / Declare four v a r i a b l e s of
std2, std3; / / type Student .
std = new Student(); / / Create a new objec t belonging
/ / to the class Student , and
/ / s tore a reference to t h a t
/ / objec t i n the v a r i a b l e std .
std1 = new Student(); / / Create a second Student objec t
/ / and s tore a reference to
/ / i t i n the v a r i a b l e std1 .
std2 = std1; / / Copy the reference value i n std1
/ / i n t o the v a r i a b l e std2 .
std3 = null; / / Store a n u l l reference i n the
/ / v a r i a b l e std3 .
std.name = " John Smith "; / / Set values of some instance v a r i a b l e s .
std1.name = "Mary Jones ";
/ / ( Other instance v a r i a b l e s have d e f a u l t
/ / i n i t i a l values of zero . )
After the computer executes these statements, the situation in the computer’s
memory looks like this:
This picture shows variables as little boxes, labeled with the names of the variables.
Objects are shown as boxes with round corners. When a variable contains a
reference to an object, the value of that variable is shown as an arrow pointing to the
object. The variable std3, with a value of null, doesn’t point anywhere. The arrows
from std1 and std2 both point to the same object. This illustrates a Very Important
Point:
When one object variable is assigned to another, only a reference is copied.
The object referred to is not copied.
When the assignment “std2 = std1;” was executed, no new object was created.
Instead, std2 was set to refer to the very same object that std1 refers to. This has
some consequences that might be surprising. For example, std1.name and std2.name
are two different names for the same variable, namely the instance variable in the
object that both std1 and std2 refer to. After the string “Mary Jones” is assigned to
the variable std1.name, it is also be true that the value of std2.name is “Mary Jones”.
There is a potential for a lot of confusion here, but you can help protect yourself from
it if you keep telling yourself, “The object is not in the variable. The variable just
holds a pointer to the object.”
You can test objects for equality and inequality using the operators == and !=,
but here again, the semantics are different from what you are used to. The test
“if (std1 == std2)”, tests whether the values stored in std1 and std2 are the
same. But the values are references to objects, not objects. So, you are testing
whether std1 and std2 refer to the same object, that is, whether they point to the
same location in memory. This is fine, if its what you want to do. But sometimes,
what you want to check is whether the instance variables in the objects have the
same values. To do that, you would need to ask whether
std1.test1 == std2.test1 && std1.test2 == std2.test2 && std1.test3
== std2.test3 && std1.name.equals(std2.name)}
I’ve remarked previously that Strings are objects, and I’ve shown the strings
“Mary Jones” and “John Smith” as objects in the above illustration. A variable of type String can only hold a reference to a string, not the string itself. It could also
hold the value null, meaning that it does not refer to any string at all. This explains
why using the == operator to test strings for equality is not a good idea.
The fact that variables hold references to objects, not objects themselves, has a
couple of other consequences that you should be aware of. They follow logically, if
you just keep in mind the basic fact that the object is not stored in the variable. The
object is somewhere else; the variable points to it.
Suppose that a variable that refers to an object is declared to be final. This
means that the value stored in the variable can never be changed, once the variable
has been initialized. The value stored in the variable is a reference to the object. So
the variable will continue to refer to the same object as long as the variable exists.
However, this does not prevent the data in the object from changing. The variable
is final, not the object. It’s perfectly legal to say
final Student stu = new Student();
stu.name = " John Doe"; / / Change data i n the objec t ;
/ / The value stored i n stu i s not changed !
/ / I t s t i l l r e f e r s to the same objec t .
Next, suppose that obj is a variable that refers to an object. Let’s consider what
happens when obj is passed as an actual parameter to a method. The value of obj
is assigned to a formal parameter in the method, and the method is executed. The
method has no power to change the value stored in the variable, obj. It only has a
copy of that value. However, that value is a reference to an object. Since the method
has a reference to the object, it can change the data stored in the object. After the
method ends, obj still points to the same object, but the data stored in the object
might have changed. Suppose x is a variable of type int and stu is a variable of type
Student. Compare:
void dontChange(int z) { void change(Student s) {
z = 42; s.name = " Fred ";
} }
The lines: The lines:
x = 17; stu.name = " Jane";
dontChange(x); change(stu);
System.out.println(x); System.out.println(stu.name);
outputs the value 17. outputs the value " Fred ".
The value of x is not The value of stu is not changed ,
changed by the method, but stu.name is.
which is equivalent to This is equivalent to
z = x; s = stu;
z = 42; s.name = " Fred ";

Enums and for-each Loops

Java 5.0 introduces a new “enhanced” form of the for loop that is designed to be
convenient for processing data structures. A data structure is a collection of data
items, considered as a unit. For example, a list is a data structure that consists
simply of a sequence of items. The enhanced for loop makes it easy to apply the
same processing to every element of a list or other data structure. However, one of
the applications of the enhanced for loop is to enum types, and so we consider it
briefly here.
The enhanced for loop can be used to perform the same processing on each of the
enum constants that are the possible values of an enumerated type. The syntax for
doing this is:
for ( enum−type−name variable−name : enum−type−name.values() )
statement
or
for ( enum−type−name variable−name : enum−type−name.values() ) {
statements
}
If MyEnum is the name of any enumerated type, then MyEnum.values() is a method
call that returns a list containing all of the values of the enum. (values() is a static
member method in MyEnum and of any other enum.) For this enumerated type, the
for loop would have the form:
for ( MyEnum variable−name : MyEnum.values() )
statement
The intent of this is to execute the statement once for each of the possible values of
the MyEnum type. The variable-name is the loop control variable. In the statement,
it represents the enumerated type value that is currently being processed. This variable
should not be declared before the for loop; it is essentially being declared in the
loop itself.
To give a concrete example, suppose that the following enumerated type has been
defined to represent the days of the week:
enum Day { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }
Then we could write:
for ( Day d : Day.values() ) {
System.out.print( d );
System.out.print(" i s day number ");
System.out.println( d.ordinal() );
}
Day.values() represents the list containing the seven constants that make up the
enumerated type. The first time through this loop, the value of d would be the first
enumerated type value Day.MONDAY, which has ordinal number 0, so the output
would be “MONDAY is day number0”. The second time through the loop, the value
of d would be Day.TUESDAY, and so on through Day.SUNDAY. The body of the loop
is executed once for each item in the list Day.values(), with d taking on each of those
values in turn. The full output from this loop would be:
MONDAY is day number 0
TUESDAY is day number 1
WEDNESDAY is day number 2
THURSDAY is day number 3
FRIDAY is day number 4
SATURDAY is day number 5
SUNDAY is day number 6
Since the intent of the enhanced for loop is to do something “for each” item in a
data structure, it is often called a for-each loop. The syntax for this type of loop is
unfortunate. It would be better if it were written something like “foreach Day d in
Day.values()”, which conveys the meaning much better and is similar to the syntax
used in other programming languages for similar types of loops. It’s helpful to think
of the colon (:) in the loop as meaning “in.”

Introduction to Enums

Ghar Builder

JAVA comes with eight built-in primitive types and a large set of types that are defined
by classes, such as String. But even this large collection of types is not sufficient
to cover all the possible situations that a programmer might have to deal with.
So, an essential part of JAVA, just like almost any other programming language, is the
ability to create new types. For the most part, this is done by defining new classes.
But we will look here at one particular case: the ability to define enums (short for
enumerated types). Enums are a recent addition to JAVA. They were only added in
Version 5.0. Many programming languages have something similar.

Technically, an enum is considered to be a special kind of class. In this section, we
will look at enums in a simplified form. In practice, most uses of enums will only need
the simplified form that is presented here.
An enum is a type that has a fixed list of possible values, which is specified
when the enum is created.
In some ways, an enum is similar to the boolean data type, which has true and false
as its only possible values. However, boolean is a primitive type, while an enum is
not.
The definition of an enum types has the (simplified) form:
enum enum−type−name { list−of−enum−values };
This definition cannot be inside a method. You can place it outside the main()
method of the program. The enum−type−name can be any simple identifier. This
identifier becomes the name of the enum type, in the same way that “boolean” is the
name of the boolean type and “String” is the name of the String type. Each value in
the list−of−enum−values must be a simple identifier, and the identifiers in the list
are separated by commas. For example, here is the definition of an enum type named
Season whose values are the names of the four seasons of the year:
enum Season { SPRING, SUMMER, AUTUMN, WINTER };
By convention, enum values are given names that are made up of upper case letters,
but that is a style guideline and not a syntax rule. Enum values are not variables.
Each value is a constant that always has the same value. In fact, the possible
values of an enum type are usually referred to as enum constants.
Note that the enum constants of type Season are considered to be “contained in”
Season, which means–following the convention that compound identifiers are used
for things that are contained in other things–the names that you actually use in your
program to refer to them are Season.SPRING, Season.SUMMER, Season.AUTUMN, and
Season.WINTER.
Once an enum type has been created, it can be used to declare variables in exactly
the same ways that other types are used. For example, you can declare a variable
named vacation of type Season with the statement:
Season vacation;
After declaring the variable, you can assign a value to it using an assignment
statement. The value on the right-hand side of the assignment can be one of the enum
constants of type Season. Remember to use the full name of the constant, including
“Season”! For example: vacation = Season.SUMMER;.
You can print an enum value with the statement: System.out.print(vacation).
The output value will be the name of the enum constant (without the “Season.”). In
this case, the output would be “SUMMER”.
Because an enum is technically a class, the enum values are technically objects.
As objects, they can contain methods. One of the methods in every enum value is
ordinal(). When used with an enum value it returns the ordinal number of the
value in the list of values of the enum. The ordinal number simply tells the position
of the value in the list. That is, Season.SPRING.ordinal() is the int value
0, Season.SUMMER.ordinal() is 1, while 2 is Season.AUTUMN.ordinal(), and 3 is
Season.WINTER.ordinal() is. You can use the ordinal() method with a variable of
type Season, such as vacation.ordinal() in our example.
You should appreciate enums as the first example of an important concept: creating
new types. Here is an example that shows enums being used in a complete
program:
public class EnumDemo {
/ / Define two enum types−−d e f i n i t i o n s go OUTSIDE The main ( ) r o u t i n e !
enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY }
enum Month { JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC }
public static void main(String[] args) {
Day tgif; / / Declare a v a r i a b l e of type Day .
Month libra; / / Declare a v a r i a b l e of type Month .
tgif = Day.FRIDAY; / / Assign a value of type Day to t g i f .
libra = Month.OCT; / / Assign a value of type Month to l i b r a .
System.out.print("My s ign i s l ibra , since I was born in ");
System.out.println(libra); / / Output value w i l l be : OCT
System.out.print(" That ’ s the ");
System.out.print( libra.ordinal() );
System.out.println("−th month of the year . ");
System.out.println(" (Counting from 0 , of course ! ) ");
System.out.print(" I sn ’ t i t nice to get to ");
System.out.println(tgif); / / Output value w i l l be : FRIDAY
System.out.println( tgif + " i s the " + tgif.ordinal()
+ "−th day of the week. "); / / Can concatenate enum values onto St r ings !
}
}

Types

JAVA, like most programming languages classifies values and expressions into types.
For e.g. String’s and int’s are types. A type basically specifies the allowed values
and allowed operations on values of that type.
Definition: A type is a set of values together with one or more operations
that can be applied uniformly to all these values.
A type system basically gives meaning to collections of bits. Because any value
simply consists of a set of bits in a computer, the hardware makes no distinction
between memory addresses, instruction code, characters, integers and floating-point
numbers. Types inform programs and programmers how they should treat those bits.
For example the integers are a type with values in the range −2, 147, 483, 648 to +
2, 147, 483, 647 and various allowed operations that include addition, subtraction, modulus
etc.
The use of types by a programming language has several advantages:
• Safety. Use of types may allow a compiler to detect meaningless or invalid code.
For example, we can identify an expression ”Hello, World” / 3 as invalid because
one cannot divide a string literal by an integer. Strong typing offers more safety.
• Optimization. Static type-checking may provide useful information to a compiler.
The compiler may then be able to generate more efficient code.
• Documentation. Types can serve as a form of documentation, since they can
illustrate the intent of the programmer. For instance, timestamps may be a
subtype of integers – but if a programmer declares a method as returning a
timestamp rather than merely an integer, this documents part of the meaning
of the method.
• Abstraction. Types allow programmers to think about programs at a higher
level, not bothering with low-level implementation. For example, programmers
can think of strings as values instead of as a mere array of bytes.
There are fundamentally two types in JAVA: primitive types and objects types i.e.
any variable you declare are either declared to be one of the primitive types or an
object type. int, double and char are the built-in, primitive types in JAVA.
The primitive types can be used in various combinations to create other, composite
types. Every time we define a class, we are actually defining a new type. For example,
the Student class defined above introduces a new type. We can now use this type like
any other type: we can declare variables to be of this type and we can use it as a type
for parameters of methods.
Before a variable can be used, it must be declared. A declaration gives a variable
a name, a type and an initial value for e.g. int x = 8 declares x to be of type int. All
objects that we declare also have to be of a specified type—the type of an object is the
class from which it is created. Thus, when we declare objects we state the type like
so: Student st = new Student();. This statement declares the variable st to be of
type Student. This statement creates a new object of the specified type and runs the
Student constructor. The constructor’s job is to properly initialize the object.
The String type is another example of an object type. Student and String are
composite types and give us the same advantages as the built-in types. The ability to
create our own types is a very powerful idea in modern languages.
When declaring variables, we can assign initial values. If you do not specify initial
values, the compiler automatically assigns one: Instance variables of numerical
type (int, double, etc.) are automatically initialized to zero; boolean variables are
initialized to false; and char variables, to the Unicode character with code number
zero. The default initial value of object types is null.

Classes

In object-oriented software, it’s possible to have many objects of the same kind that
share characteristics: rectangles, employee records, video clips, and so on. A class is
a software blueprint for objects. A class is used to manufacture or create objects.
The class declares the instance variables necessary to contain the state of every
object. The class would also declare and provide implementations for the instance
methods necessary to operate on the state of the object.
Definition: A class is a blueprint that defines the variables and the methods
common to all objects of a certain kind.

After you’ve created the class, you can create any number of objects from that
class.
A class is a kind of factory for constructing objects. The non-static parts of the
class specify, or describe, what variables and methods the objects will contain. This
is part of the explanation of how objects differ from classes: Objects are created and
destroyed as the program runs, and there can be many objects with the same structure,
if they are created using the same class.

Messages

Software objects interact and communicate with each other by sending messages to
each other. When object A wants object B to perform one of B’s methods, object A
sends a message to object B
There are three parts of a message: The three parts for the message
System.out.println{‘‘Hello World’’}; are:
• The object to which the message is addressed (System.out)
• The name of the method to perform (println)
• Any parameters needed by the method (“Hello World!”)

Encapsulation

Object diagrams show that an object’s variables make up the center, or nucleus, of
the object. Methods surround and hide the object’s nucleus from other objects in the
program. Packaging an object’s variables within the protective custody of its methods
is called encapsulation.

Encapsulating related variables and methods into a neat software bundle is a
simple yet powerful idea that provides two benefits to software developers:
• Modularity: The source code for an object can be written and maintained independently
of the source code for other objects. Also, an object can be easily
passed around in the system. You can give your bicycle to someone else, and it
will still work.
• Information-hiding: An object has a public interface that other objects can use
to communicate with it. The object can maintain private information and methods
that can be changed at any time without affecting other objects that depend
on it.

Fundamentals of Objects and Classes

We move now from the conceptual picture of objects and classes to a discussion of
software classes and objects.4
Objects are closely related to classes. A class can contain variables and methods.
If an object is also a collection of variables and methods, how do they differ from
classes?

Objects and Classes
Objects
In object-oriented programming we create software objects that model real world objects.
Software objects are modeled after real-world objects in that they too have
state and behavior. A software object maintains its state in one or more variables. A
variable is an item of data named by an identifier. A software object implements its
behavior with methods. A method is a function associated with an object.
Definition: An object is a software bundle of variables and related methods.
An object is also known as an instance. An instance refers to a particular object.
For e.g. Karuna’s bicycle is an instance of a bicycle—It refers to a particular bicycle.
Sandile Zuma is an instance of a Student.
The variables of an object are formally known as instance variables because they
contain the state for a particular object or instance. In a running program, there
may be many instances of an object. For e.g. there may be many Student objects.
Each of these objects will have their own instance variables and each object may have
different values stored in their instance variables. For e.g. each Student object will
have a different number stored in its StudentNumber variable.

Classes and Instances

The next important principle of object-oriented programming is
All objects are instances of a class. The method invoked by an object in
response to a message is determined by the class of the receiver. All objects
of a given class use the same method in response to similar messages.
Fred is an instance of a category or class of people i.e. Fred is an instance of a
class of florists. The term florist represents a class or category of all florists. Fred is
an object or instance of a class.
We interact with instances of a class but the class determines the behaviour of instances.
We can tell a lot about how Fred will behave by understanding how Florists
behave. We know, for example, that Fred, like all florists can arrange and deliver
flowers.
In the real world there is this distinction between classes and objects. Real-world
objects share two characteristics: They all have state and behavior. For example, dogs
have state (name, color, breed, hungry) and behavior (barking, fetching, wagging tail).
Students have state (name, student number, courses they are registered for, gender)
and behavior (take tests, attend courses, write tests, party).

Messages and Responsibilities

Members of an object-oriented community make requests of each other. The next
important principle explains the use of messages to initiate action:
Action is initiated in object-oriented programming by the transmission of a
message to an agent (an object) responsible for the actions. The message
encodes the request for an action and is accompanied by any additional
information (arguments/parameters) needed to carry out the request. The
receiver is the object to whom the message is sent. If the receiver accepts the message, it accepts responsibility to carry out the indicated action. In
response to a message, the receiver will perform some method to satisfy the
request.

There are some important issues to point out here:
• The client sending the request need not know the means by which the request
is carried out. In this we see the principle of information hiding.
• Another principle implicit in message passing is the idea of finding someone else
to do the work i.e. reusing components that may have been written by someone
else.
• The interpretation of the message is determined by the receiver and can vary
with different receivers. For example, if you sent the message “deliver flowers”
to a friend, she will probably have understood what was required and flowers
would still have been delivered but the method she used would have been very
different from that used by the florist.
• In object-oriented programming, behaviour is described in terms of responsibilities.
• Client’s requests for actions only indicates the desired outcome. The receivers
are free to pursue any technique that achieves the desired outcomes.
• Thinking in this way allows greater independence between objects.
• Thus, objects have responsibilities that they are willing to fulfill on request. The
collection of reponsibilities associated with an object is often called a protocol.