Introduction to Java Programming

Introduction to Computer Science using Java

Java is a simple and yet powerful object oriented programming language and it is in many respects similar to C++. Java originated at Sun Microsystems, Inc. in 1991. It was conceived by James Gosling, Patrick Naughton, Chris Warth, Ed Frank, and Mike Sheridan at Sun Microsystems, Inc. It was developed to provide a platform-independent programming language. This site gives you an Introduction to Java Programming accompanied with many java examples. Its a complete course in java programming for beginners to advanced java.

Platform independent

Unlike many other programming languages including C and C++ when Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by virtual Machine (JVM) on whichever platform it is being run.

Java Virtual Machine

What is the Java Virtual Machine? What  is its role?
Java was designed with a concept of ‘write once and run everywhere’. Java Virtual Machine plays the central role in this concept. The JVM is the environment in which Java programs execute. It is a software that is implemented on top of real hardware and operating system. When the source code (.java files) is compiled, it is translated into byte codes and then placed into (.class) files. The JVM executes these bytecodes. So Java byte codes can be thought of as the machine language of the JVM. A JVM can either interpret the bytecode one instruction at a time or the bytecode can be compiled further for the real microprocessor using what is called a just-in-time compiler. The JVM must be implemented on a particular platform before compiled programs can run on that platform.



<br /><font size=-1>

Object Oriented Programming

Since Java is an object oriented programming language it has following features:
  • Reusability of Code
  • Emphasis on data rather than procedure
  • Data is hidden and cannot be accessed by external functions
  • Objects can communicate with each other through functions
  • New data and functions can be easily addedJava has powerful features. The following are some of them:-

    Simple
    Reusable
    Portable (Platform Independent)
    Distributed
    Robust
    Secure
    High Performance
    Dynamic
    Threaded
    Interpreted
Object Oriented Programming is a method of implementation in which programs are organized as cooperative collection of objects, each of which represents an instance of a class, and whose classes are all members of a hierarchy of classes united via inheritance relationships.
OOP Concepts
Four principles of Object Oriented Programming are
Abstraction
Encapsulation
Inheritance
Polymorphism
Abstraction
Abstraction denotes the essential characteristics of an object that distinguish it from all other kinds of objects and thus provide crisply defined conceptual boundaries, relative to the perspective of the viewer.
Encapsulation

Encapsulation is the process of compartmentalizing the elements of an abstraction that constitute its structure and behavior ; encapsulation serves to separate the contractual interface of an abstraction and its implementation.
Encapsulation
* Hides the implementation details of a class.
* Forces the user to use an interface to access data
* Makes the code more maintainable.
Inheritance
Inheritance is the process by which one object acquires the properties of another object.
Polymorphism
Polymorphism is the existence of the classes or methods in different forms or single name denoting different
implementations.

Java is Distributed

With extensive set of routines to handle TCP/IP protocols like HTTP and FTP java can open and access the objects across net via URLs.


Java is Multithreaded

One of the powerful aspects of the Java language is that it allows multiple threads of execution to run concurrently within the same program A single Java program can have many different threads executing independently and continuously. Multiple Java applets can run on the browser at the same time sharing the CPU time.

Java is Secure

Java was designed to allow secure execution of code across network. To make Java secure many of the features of C and C++ were eliminated. Java does not use Pointers. Java programs cannot access arbitrary addresses in memory.

Garbage collection

Automatic garbage collection is another great feature of Java with which it prevents inadvertent corruption of memory. Similar to C++, Java has a new operator to allocate memory on the heap for a new object. But it does not use delete operator to free the memory as it is done in C++ to free the memory if the object is no longer needed. It is done automatically with garbage collector.

Java Applications

Java has evolved from a simple language providing interactive dynamic content for web pages to a predominant enterprise-enabled programming language suitable for developing significant and critical applications. Today, It is used for many types of applications including Web based applications, Financial applications, Gaming applications, embedded systems, Distributed enterprise applications, mobile applications, Image processors, desktop applications and many more. This site outlines the building blocks of java by stating few java examples along with some java tutorials.

Getting Started with Java

Getting Java Basics quickly has become easy with Javabeginner.com. This site is for the java programmers who want to use the Java programming language to create applications using the Java basics. This site is for absolute beginners to advanced java programmers who do not require any prerequisite Java knowledge. For getting started with Java you’ll have to have some basic understanding of the concepts of programming.
After going through all the tutorials in this site, you would have learnt the essential concepts and features of the Java Programming Language which include exceptions, Swing GUI programming, Collections framework etc. A lot of code examples are used through out tutorial to make you understand the language better.
All the listings and programs in the website are compiled and run using the JDK 1.5.
Download : JDK and JRE 1.5

Java Architecture

The Java environment is composed of a number of system components. You use these components at compile time to create the Java program and at run time to execute the program. Java achieves its independence by creating programs designed to run on the Java Virtual Machine rather than any specific computer system.
  • After you write a Java program, you use a compiler that reads the statements in the program and translates them into a machine independent format called bytecode.
    • <br /><font size=-1>
  • Bytecode files, which are very compact, are easily transported through a distributed system like the Internet.
  • The compiled Java code (resulting byte code) will be executed at run time.
Java programs can be written and executed in two ways:
  • Stand-alone application (A Java Swing Application)
  • Applet which runs on a web browser (Example: Internet Explorer)


Java source code

A Java program is a collection of one or more java classes. A Java source file can contain more than one class definition and has a .java extension. Each class definition in a source file is compiled into a separate class file. The name of this compiled file is comprised of the name of the class with .class as an extension. Before we proceed further in this section, I would recommend you to go through the ‘Basic Language Elements’.
Below is a java sample code for the traditional Hello World program. Basically, the idea behind this Hello World program is to learn how to create a program, compile and run it. To create your java source code you can use any editor( Text pad/Edit plus are my favorites) or you can use an IDE like Eclipse.
public class HelloWorld {
	public static void main(String[] args) {
		System.out.println("Hello World");
	}//End of main
}//End of HelloWorld Class

Output
Hello World
ABOUT THE PROGRAM
I created a class named “HelloWorld” containing a simple main function within it. The keyword class specifies that we are defining a class. The name of a public class is spelled exactly as the name of the file (Case Sensitive). All java programs begin execution with the method named main(). main method that gets executed has the following signature : public static void main(String args[]).Declaring this method as public means that it is accessible from outside the class so that the JVM can find it when it looks for the program to start it. It is necessary that the method is declared with return type void (i.e. no arguments are returned from the method). The main method contains a String argument array that can contain the command line arguments. The brackets { and } mark the beginning and ending of the class. The program contains a line ‘System.out.println(”Hello World”);’ that tells the computer to print out on one line of text namely ‘Hello World’. The semi-colon ‘;’ ends the line of code. The double slashes ‘//’ are used for comments that can be used to describe what a source code is doing. Everything to the right of the slashes on the same line does not get compiled, as they are simply the comments in a program.
Java Main method Declarations
class MainExample1 {public static void main(String[] args) {}}
class MainExample2 {public static void main(String []args) {}}
class MainExample3 {public static void main(String args[]) {}}
All the 3 valid main method’s shown above accepts a single String array argument.

Compiling and Running an Application

To compile and run the program you need the JDK distributed by Sun Microsystems. The JDK contains documentation, examples, installation instructions, class libraries and packages, and tools. Download an editor like Textpad/EditPlus to type your code. You must save your source code with a .java extension. The name of the file must be the name of the public class contained in the file.
Steps for Saving, compiling and Running a Java
Step 1:Save the program With .java Extension.
Step 2:Compile the file from DOS prompt by typing javac <filename>.
Step 3:Successful Compilation, results in creation of .class containing byte code
Step 4:Execute the file by typing java <filename without extension>


Java Development Kit

The Java Developer’s Kit is distributed by Sun Microsystems. The JDK contains documentation, examples, installation instructions, class libraries and packages, and tools

javadoc

The javadoc tool provided by Sun is used to produce documentation for an application or program,


Jar Files

A jar file is used to group together related class files into a single file for more compact storage, distribution, and transmission.


PATH and CLASSPATH

The following are the general programming errors, which I think every beginning java programmer would come across. Here is a solution on how to solve the problems when running on a Microsoft Windows Machine.
1. ‘javac’ is not recognized as an internal or external command, operable program or batch file
When you get this error, you should conclude that your operating system cannot find the compiler (javac). To solve this error you need to set the PATH variable.
How to set the PATH Variable?
Firstly the PATH variable is set so that we can compile and execute programs from any directory without having to type the full path of the command. To set the PATH of jdk on your system (Windows XP), add the full path of the jdk<version>\bin directory to the PATH variable. Set the PATH as follows on a Windows machine:
a. Click Start > Right Click “My Computer” and click on “Properties”
b. Click Advanced > Environment Variables.
c. Add the location of bin folder of JDK installation for PATH in User Variables and System Variables. A typical value for PATH is:
C:\jdk<version>\bin (jdk<version is nothing but the name of the directory where jdk is installed)
If there are already some entries in the PATH variable then you must add a semicolon and then add the above value (Version being replaced with the version of JDK). The new path takes effect in each new command prompt window you open after setting the PATH variable.

Basic Language Elements

Keywords

There are certain words with a specific meaning in java which tell (help) the compiler what the program is supposed to do. These Keywords cannot be used as variable names, class names, or method names. Keywords in java are case sensitive, all characters being lower case.
Keywords are reserved words that are predefined in the language; see the table below (Taken from Sun Java Site). All the keywords are in lowercase.
	abstract    default    if            private      this
	boolean     do         implements    protected    throw
	break       double     import        public       throws
	byte        else       instanceof    return       transient
	case        extends    int           short        try
	catch       final      interface     static       void
	char        finally    long          strictfp     volatile
	class       float      native        super        while
&lt;font size=-1&gt;

	const       for        new           switch
	continue    goto       package       synchronized

Comments

Comments are descriptions that are added to a program to make code easier to understand. The compiler ignores comments and hence its only for documentation of the program.
Java supports three comment styles.
Block style comments begin with /* and terminate with */ that spans multiple lines.
Line style comments begin with // and terminate at the end of the line. (Shown in the above program)
Documentation style comments begin with /** and terminate with */ that spans multiple lines. They are generally created using the automatic documentation generation tool, such as javadoc. (Shown in the above program)

Variable, Identifiers and Data Types

Variables are used for data that change during program execution. All variables have a name, a type, and a scope. The programmer assigns the names to variables, known as identifiers. An Identifier must be unique within a scope of the Java program. Variables have a data type, that indicates the kind of value they can store. Variables declared inside of a block or method are called local variables; They are not automatically initialized. The compiler will generate an error as a result of the attempt to access the local variables before a value has been assigned.
Java has four main primitive data types built into the language. We can also create our own data types.

  • Integer: byte, short, int, and long.

  • Floating Point: float and double

  • Character: char

  • Boolean: variable with a value of true or false.

  • Classes

    A class is nothing but a blueprint for creating different objects which defines its properties and behaviors. An object exhibits the properties and behaviors defined by its class. A class can contain fields and methods to describe the behavior of an object. Methods are nothing but members of a class that provide a service for an object or perform some business logic.

    Objects

    An object is an instance of a class created using a new operator. The new operator returns a reference to a new instance of a class. This reference can be assigned to a reference variable of the class. The process of creating objects from a class is called instantiation. An object reference provides a handle to an object that is created and stored in memory. In Java, objects can only be manipulated via references, which can be stored in variables.
     



    •  

     


     


     

    Subscribe to: Comments (Atom)