Java - What, Where and Why?

Java - What, Where and Why?

1. Java - What, Where and Why?
2. What is Java
3. Where Java is used
4. Java Applications

Java technology is wide used currently. Let's start learning of java from basic questions like what is java, where it is used, what type of applications are created in java and why use java?

What is Java?

Java is a programming language and a platform.

Platform Any hardware or software environment in which a program runs, known as a platform. Since Java has its own Runtime Environment (JRE) and API, it is called platform.

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Where it is used?

According to Sun, 3 billion devices run java. There are many devices where java is currently used. Some of them are as follows:

1. Desktop Applications such as acrobat reader, media player, antivirus etc.
2. Web Applications such as irctc.co.in, javatpoint.com etc.
3. Enterprise Applications such as banking applications.
4. Mobile
5. Embedded System
6. Smart Card
7. Robotics
8. Games etc.

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Types of Java Applications

There are mainly 4 type of applications that can be created using java:

1) Standalone Application

It is also known as desktop application or window-based application. An application that we need to install on every machine such as media player, antivirus etc. AWT and Swing are used in java for creating standalone applications.

2) Web Application

An application that runs on the server side and creates dynamic page, is called web application. Currently, servlet, jsp, struts, jsf etc. technologies are used for creating web applications in java.

3) Enterprise Application

An application that is distributed in nature, such as banking applications etc. It has the advantage of high level security, load balancing and clustering. In java, EJB is used for creating enterprise applications.

4) Mobile Application

An application that is created for mobile devices. Currently Android and Java ME are used for creating mobile applications.

What we will learn in Basics of Java ?

·         History of java ·         Features of java ·         Simple Program of Java ·         Internal of Hello Java Program ·         How to set path in Windows OS ·         Difference between JDK, JRE and JVM ·         Internal Details of JVM ·         Variable and Data Type ·         Unicode System ·         Operators

History of Java

1. Brief history of Java
2. Java Version History

Java history is interesting to know. Java team members (also known as Green Team), initiated a revolutionary task to develop a language for digital devices such as set-top boxes, televisions etc.

At that time, it was a advanced concept for the green team. But, it was good for internet programming. Later, Netscape Navigator incorporated Java technology.

James Gosling

Currently, Java is used in internet programming, mobile devices, games, e-business solutions etc. Let's see the major points that describes the history of java.

1) James Gosling, Mike Sheridan, and Patrick Naughton initiated the Java language project in June 1991. The small team of sun engineers called Green Team.

2) Originally designed for small, embedded systems in electronic appliances like set-top boxes.

3) Firstly, it was called "Greentalk" by James Gosling and file extension was .gt.

4) After that, it was called Oak and was developed as a part of the Green project.

5) Why Oak? Oak is a symbol of strength and choosen as a national tree of many countries like U.S.A., France, Germany, Romania etc.

6) In 1995, Oak was renamed as "Java" because it was already a trademark by Oak Technologies.

7) Why they choosed java name for java language? The team gathered to choose a new name. The suggested words were "dynamic", "revolutionary", "Silk", "jolt", "DNA" etc.

Java Version History

There are many java versions that has been released.

1. JDK Alpha and Beta (1995)
2. JDK 1.0 (23rd Jan, 1996)
3. JDK 1.1 (19th Feb, 1997)
4. J2SE 1.2 (8th Dec, 1998)
5. J2SE 1.3 (8th May, 2000)
6. J2SE 1.4 (6th Feb, 2002)
7. J2SE 5.0 (30th Sep, 2004)
8. Java SE 6 (11th Dec, 2006)
9. Java SE 7 (28th July, 2011)

Features of Java

1. Features of Java

1.    Simple

2.    Object-Oriented

3.    Platform Independent

4.    secured

5.    Robust

6.    Architecture Neutral

7.    Portable

8.    High Performance

9.    Distributed

10. Multi-threaded

There is given many features of java. They are also called java buzzwords.

1.Simple

2.Object-oriented

3.Platform independent

4.Secured

5.Robust

6.Architecture neutral

7.Portable

8.Dynamic

9.Interpreted

10.High Performance

11.Multithreaded

12.Distributed

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Simple

Java is simple in the sense that:

syntax is based on C++ (so easier for programmers to learn it after C++).

removed many confusing and/or rarely-used features e.g., explicit pointers, operator overloading etc.

No need to remove unreferenced objects because there is Automatic Garbage Collection in java.

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Object-oriented

Object-oriented means we organize our software as a combination of different types of objects that incorporates both data and behaviour.

Object-oriented programming(OOPs) is a methodology that simplify software development and maintenace by providing some rulues.

Basic concepts of OOPs are:

1.    Object 2.    Class 3.    Inheritance 4.    Polymorphism 5.    Abstraction 6.    Encapsulation

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Platform Independent

A platform is the hardware or software environment in which a program runs. There are two types of platforms software-based and hardware-based. Java provides software-based platform. The Java platform differs from most other platforms in the sense that it's a software-based platform that runs on top of other hardware-based platforms.It has two components: 1.    Runtime Environment 2.    API(Application Programming Interface)

Java code can be run on multiple platforms e.g.Windows,Linux,Sun Solaris,Mac/OS etc. Java code is compiled by the compiler and converted into bytecode.This bytecode is a platform independent code because it can be run on multiple platforms i.e. Write Once and Run Anywhere(WORA).

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Secured

Java is secured because:

·         No explicit pointer ·         Programs run inside virtual machine sandbox.

·         Classloader- adds security by separating the package for the classes of the local file system from those that are imported from network sources. ·         Bytecode Verifier- checks the code fragments for illegal code that can violate accesss right to objects. ·         Security Manager- determines what resources a class can access such as reading and writing to the local disk.

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Hello Java Example

1. Requirements for creating Hello Java Example
2. Creating Hello Java Example
3. Resolving javac not recognized exception

In this page, we will learn how to write the hello java program. Creating hello java example is too easy. Here, we have created a class named Simple that contains only main method and prints a message hello java. It is the simple program of java.

Requirement for Hello Java Example

For executing any java program, you need to ·         create the java program. ·         install the JDK if you don't have installed it, download the JDK and install it. ·         set path of the bin directory under jdk. ·         compile and execute the program.

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Creating hello java example

Let's create the hello java program:

1.    class Simple{  

2.        public static void main(String args[]){  

3.         System.out.println("Hello Java")  

4.        }  

5.    }  

save this file as Simple.java

To compile:	javac Simple.java
To execute:	java Simple

Output:Hello Java

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Understanding first java program

Let's see what is the meaning of class, public, static, void, main, String[], System.out.println().

• class is used to declare a class in java.
• public is an access modifier which represents visibility, it means it is visible to all.
• static is a keyword, if we declare any method as static, it is known as static method. The core advantage of static method is that there is no need to create object to invoke the static method. The main method is executed by the JVM, so it doesn't require to create object to invoke the main method. So it saves memory.
• void is the return type of the method, it means it doesn't return any value.
• main represents startup of the program.
• String[] args is used for command line argument. We will learn it later.
• System.out.println() is used print statement.

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To write the simple program, open notepad and write simple program as displayed below:

As displayed in the above diagram, write the simple program of java in notepad and saved it as Simple.java. To compile and run this program, you need to open command prompt by start -> All Programs -> Accessories -> command prompt.

To compile and run the above program, go to your current directory first; my current directory is c:\new . Write here:

To compile:	javac Simple.java
To execute:	java Simple

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Resolving an exception "javac is not recongnized as an internal or external command" ?

If there occurs a problem like displayed in the below figure, you need to set path. Since DOS doesn't know javac or java, we need to set path. Path is not required in such a case if you save your program inside the jdk/bin folder. But its good approach to set path. Click here for How to set path in java.

Internal Details of Hello Java Program

1. Internal Details of Hello Java

In the previous page, we have learned about the first program, how to compile and how to run the first java program. Here, we are going to learn, what happens while compiling and running the java program. Moreover, we will see some questions based on the first program.

What happens at compile time?

At compile time, java file is compiled by Java Compiler (It does not interact with OS) and converts the java code into bytecode.

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What happens at runtime?

At runtime, following steps are performed:

Classloader: is the subsystem of JVM that is used to load class files.

Bytecode Verifier: checks the code fragments for illegal code that can violate access right to objects.

Interpreter: read bytecode stream then execute the instructions.

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Q)Can you save a java source file by other name than the class name?

Yes, like the figure given below illustrates:

To compile:	javac Hard.java
To execute:	java Simple

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Q)Can you have multiple classes in a java source file?

Yes, like the figure given below illustrates:

How to set path of JDK in Windows:

1. How to set path of JDK in Windows OS

1.    Setting Temporary Path of JDK

2.    Setting Permanent Path of JDK

2. How to set path of JDK in Linux OS

Path is required for using tools such as javac, java etc. If you are saving the java file in jdk/bin folder, path is not required.But If you are having your java file outside the jdk/bin folder, it is necessary to set path of JDK. There are two ways to set path of JDK: 1.    temporary 2.    permanent

1)Setting temporary Path of JDK in Windows:

For setting the temporary path of JDK, you need to follow these steps:

·         Open command prompt ·         copy the path of bin folder ·         write in command prompt: set path=copiedpath

For Example:

set path=C:\Program Files\Java\jdk1.6.0_23\bin

Let's see it in the figure given below:

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2)Setting Permanent Path of JDK in Windows:

For setting the permanent path of JDK, you need to follow these steps:

• Go to MyComputer properties -> advanced tab -> environment variables -> new tab of user variable -> write path in variable name -> write path of bin folder in variable value -> ok -> ok -> ok

For Example:

1)Go to MyComputer properties

2)click on advanced tab

3)click on environment variables

4)click on new tab of user variables

5)write path in variable name

6)Copy the path of bin folder

7)paste path of bin folder in variable value

8)click on ok button

9)click on ok button

Now your permanent path is set.You can now execute any program of java from any drive.

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Setting Path in Linux OS

Setting the path in Linux OS is same as setting the path in the Windows OS. But here we use export tool rather than set. Let's see how to set path in Linux OS:

export PATH=$PATH:/home/jdk1.6.01/bin/

Here, we have installed the JDK in the home directory under Root (/home).

Difference between JDK,JRE and JVM

1. Brief summary of JVM
2. Java Runtime Environment (JRE)
3. Java Development Kit (JDK)

Understanding the difference between JDK, JRE and JVM is important in Java. We will have brief overview of JVM here. If you want to gain the detailed knowledge of JVM, move to the next page. Firstly, let's see the basic differences between the JDK, JRE and JVM.

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JVM

JVM (Java Virtual Machine) is an abstract machine.It is a specification that provides runtime environment in which java bytecode can be executed.

JVMs are available for many hardware and software platforms (i.e.JVM is plateform dependent).

The JVM performs four main tasks: ·         Loads code ·         Verifies code ·         Executes code ·         Provides runtime environment

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JRE

JRE is an acronym for Java Runtime Environment.It is used to provide runtime environment.It is the implementation of JVM.It physically exists.It contains set of libraries + other files that JVM uses at runtime.

Implementation of JVMs are also actively released by other companies besides Sun Micro Systems.

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JDK

JDK is an acronym for Java Development Kit.It physically exists.It contains JRE + development tools.

JVM (Java Virtual Machine)

1. Java Virtual Machine
2. Internal Architecture of JVM
3. Classloader
4. Class Area
5. Heap Area
6. Stack Area
7. Program Counter Register
8. Native Method Stack
9. Execution Engine

JVM (Java Virtual Machine) is an abstract machine.It is a specification that provides runtime environment in which java bytecode can be executed.

JVMs are available for many hardware and softwareplatforms (i.e.JVM is plateform dependent).

The JVM performs four main tasks: ·         Loads code ·         Verifies code ·         Executes code ·         Provides runtime environment

JVM provides definitions for the: ·         Memory area ·         Class file format ·         Register set ·         Garbage-collected heap ·         Fatal error reporting etc.

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Internal Architecture of JVM

Let's understand the internal architecture of JVM. It contains classloader, memory area, execution engine etc.

 

1) Classloader:

Classloader is a subsystem of JVM that is used to load class files.

2) Class(Method) Area:

Class(Method) Area stores per-class structures such as the runtime constant pool, field and method data, the code for methods.

3) Heap:

It is the runtime data area in which objects are allocated.

4) Stack:

Java Stack stores frames.It holds local variables and partial results, and plays a part in method invocation and return.

Each thread has a private JVM stack, created at the same time as thread.

A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.

5) Program Counter Regiser:

PC (program counter) register. It contains the address of the Java virtual machine instruction currently being executed.

6) Native Method Stack:

It contains all the native methods used in the application.

7) Execution Engine:

It contains:

1) A virtual processor

2) Interpreter:Read bytecode stream then execute the instructions.

3) Just-In-Time(JIT) compiler:It is used to improve the performance.JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term ?compiler? refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.