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Showing posts with label Java Programming. Show all posts
Showing posts with label Java Programming. Show all posts

Sunday, January 18, 2015

Java Basic Syntax

 Sumit Kar     January 18, 2015     Java Programming, Programming     No comments   

Java Basic Syntax


When we consider a Java program, it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what do class, object, methods and instance variables mean.
·         Object - Objects have states and behaviours. Example: A dog has states-colour, name, and breed as well as behaviours -wagging, barking, and eating. An object is an instance of a class.
·         Class - A class can be defined as a template/blue print that describes the behaviours/states that object of its type support.
·         Methods - A method is basically behaviour. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
·         Instance Variables - Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables.


First Java Program:
Let us look at a simple code that would print the words Hello World.


public class HelloWorld{
/* This is my first java program.
 * This will print 'Hello World' as the output
 */
     public static void main(String []args){
        System.out.println("Hello World");// prints Hello World
     }
}




Let's look at how to save the file, compile and run the program. Please follow the steps given below:
·         Open notepad and add the code as above.
·         Save the file as: HelloWorld.java.
·         Open a command prompt window and go to the directory where you saved the class. Assume it's D:\SK\
·         Type ' javac HelloWorld.java ' and press enter to compile your code. If there are no errors in your code, the command prompt will take you to the next line (Assumption : The path variable is set).


Simply Easy Learning
·         Now, type ' java HelloWorld ' to run your program.
·         You will be able to see ' Hello World ' printed on the window.
D:\SK>javac HelloWorld.java
D:\SK>java HelloWorld
Hello World




Basic Syntax:
·         Case Sensitivity - Java is case sensitive, which means identifier Hello and hello would have different meaning in Java.
·         Class Names - For all class names, the first letter should be in Upper Case. If several words are used to form a name of the class, each inner word's first letter should be in Upper Case.
Example class MyFirstJavaClass
·         Method Names - All method names should start with a Lower Case letter. If several words are used to form the name of the method, then each inner word's first letter should be in Upper Case. This is known as Camel convention.
Example public void myMethodName()
·         Program File Name - Name of the program file should exactly match the class name. When saving the file, one should save it using the class name (Remember Java is case sensitive) and append '.java' to the end of the name (if the file name and the class name do not match your program will not compile).
Example: Let us assume 'MyFirstJavaProgram' is the class name. The file should be saved as'MyFirstJavaProgram.java'
·         public static void main(String args[]) - Java program processing starts from the main() method, which is a mandatory part of every Java program.


Java Identifiers:
All Java components require names. Names used for classes, variables and methods are called identifiers. In Java, there are several points to remember about identifiers. They are as follows:
·         All identifiers should begin with a letter (A to Z or a to z), currency character ($) or an underscore (_).
·         After the first character, identifiers can have any combination of characters.
·         A keyword cannot be used as an identifier.
·         Most importantly identifiers are case sensitive.
·         Examples of legal identifiers: age, $salary, _value, __1_value
·         Examples of illegal identifiers: 123abc, -salary


Java Modifiers:
Like other languages, it is possible to modify classes, methods, etc., by using modifiers. There are two categories of modifiers:
·         Access Modifiers: default, public, protected, private
·         Non-access Modifiers: final, abstract, strictfp


Java Variables:
We would see following type of variables in Java:
·         Local Variables
·         Class Variables (Static Variables)
·         Instance Variables (Non-static variables)


Java Arrays:
Arrays are objects that store multiple variables of the same type. However, an array itself is an object on the heap.
We will look into how to declare, construct and initialize in the upcoming chapters.


Java Enums:
In Java 5.0 Enums were introduced. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated list are called enums. With the use of enums, it is possible to reduce the number of bugs in a code. For example, if we consider an application for a Coke shop, it would be possible to restrict the glass size to small, medium and large. This would make sure that it would not allow anyone to order any size other than the small, medium or large.


Note: enums can be declared as their own or inside a class. Methods, variables, constructors can be defined inside enums as well.










Example:
Class cokeShop{
enum cokeShopSize{ SMALL, MEDUIM, LARGE }
cokeShopSize size;
}
public class cokeShop{
  public static void main(String args[]){
  cokeShop juice =new cokeShop();
  juice.size =cokeShop.cokeShopSize.MEDUIM ;
 }
}




Java Keywords
The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.














abstract
assert
Boolean
break
byte
case
catch
char
class
const
continue
default
do
double
else
enum
extends
final
finally
float
for
goto
if
implements
import
instanceof
int
interface
long
native
new
package
private
protected
public
return
short
static
strictfp
super
switch
synchronized
this
throw
throws
transient
try
void
volatile
while






Comments in Java
Java supports single-line and multi-line comments very similar to C and C++. All characters available inside any comment are ignored by Java compiler.
Single Line:     // ---
Multiline:
                        /* ---
                         */
Using Blank Lines:
A line containing only whitespace, possibly with a comment, is known as a blank line, and Java totally ignores it.
Inheritance:
Java classes can be derived from classes. Basically, if we need to create a new class and we already have a class that has some of the code we require; it is possible to derive a new class from the already existing code. This concept allows you to reuse the fields and methods of the existing class without having to rewrite the code in a new class. In this scenario, the existing class is called the superclass and the derived class is called the subclass.
Interfaces:
In Java language, an interface can be defined as a contract between objects on how to communicate with each other. Interfaces play a vital role when it comes to the concept of inheritance.
An interface defines the methods, a deriving class (subclass) should use. But the implementation of the methods is totally up to the subclass.



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Java Environment Setup

 Sumit Kar     January 18, 2015     Java Programming, Programming     No comments   



Java





Before
proceeding further, it is important to set up the Java environment correctly. Install
the JDK downloaded from the Oracle website. It is available free of cost.


Once
you installed Java on your machine, you would need to set environment variables
to point to correct installation directories:



Setting up the path for Windows 7/8/8.1/10:



Assuming
you have installed Java in
c:\Program Files (x86)\java\jdk  directory:


·        
Right-click
on 'Computer' and select 'Properties'.


·        
Click
‘Advanced System Settings’


·        
Click
on the 'Environment variables' button under the 'Advanced' tab.


·        
Now,
add (remember ADD not Replace) the
'Path' variable so that it also contains the path to the Java executable.
Example, if the path is currently set to 'C:\WINDOWS\SYSTEM32', then change
your path to 'C:\WINDOWS\SYSTEM32;c:\Program
Files (x86)\java\jdk\bin'.






Setting up the path for Windows 2000/XP:



Assuming
you have installed Java in
c:\Program Files\java\jdk  directory:


·        
Right-click
on 'My Computer' and select 'Properties'.


·        
Click
on the 'Environment variables' button under the 'Advanced' tab.


·        
Now,
add (remember ADD not Replace) the
'Path' variable so that it also contains the path to the Java executable.
Example, if the path is currently set to 'C:\WINDOWS\SYSTEM32', then change
your path to 'C:\WINDOWS\SYSTEM32;c:\Program
Files\java\jdk\bin'.



Setting up the path for windows
95/98/ME:



Assuming
you have installed Java in c:\Program Files\java\jdk directory:


·        
Edit
the 'C:\autoexec.bat' file and add the following line at the end:


'SET
PATH=%PATH%;C:\Program Files\java\jdk\bin'



Setting up the path for Linux, UNIX,
Solaris, FreeBSD:



Environment
variable PATH should be set to point to where the Java binaries have been
installed. Refer to your


shell
documentation if you have trouble doing this.


Example,
if you use bash as your shell, then you would add the following line to the end
of your


'.bashrc: export PATH=/path/to/java:$PATH'




















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Features of Java

 Sumit Kar     January 18, 2015     Java Programming, Programming     No comments   










Ø  Object
Oriented:
In
Java, everything is an Object. Java can be easily extended since it is based on
the Object model.





Ø  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.





Ø  Simple: Java is designed to be easy to
learn. If you understand the basic concept of OOP, Java would be easy to
master.





Ø  Secure: With Java's secure feature, it
enables to develop virus-free, tamper-free systems. Authentication techniques
are based on public-key encryption.





Ø  Robust: Java makes an effort to
eliminate error prone situations by emphasizing mainly on compile time error
checking and runtime checking.





Ø  Architectural-neutral: Java compiler generates an
architecture-neutral object file format, which makes the compiled code to be
executable on many processors, with the presence of Java runtime system.





Ø  Portable: Being architectural-neutral and
having no implementation dependent aspects of the specification makes Java
portable. Compiler inJava is written in ANSI C with a clean portability
boundary which is a POSIX subset.





Ø  Multithreaded: With Java's multithreaded
feature, it is possible to write programs that can do many tasks simultaneously.
This design feature allows developers to construct smoothly running interactive
applications.





Ø  Interpreted: Java byte code is translated on
the fly to native machine instructions and is not stored anywhere. The
development process is more rapid and analytical since the linking is an
incremental and lightweight process.





Ø  High
Performance:

With the use of Just-In-Time compilers, Java enables high performance.





Ø  Distributed: Java is designed for the
distributed environment of the internet.





Ø  Dynamic: Java is considered to be more
dynamic than C or C++ since it is designed to adapt to an evolving environment.
Java programs can carry extensive amount of run-time information that can be used
to verify and resolve accesses to objects on run-time.












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