Java Overview

Our First Java Program: Hello Word
Here we will write our first program in Java, and yes, you might have guessed it, its Hello Word! I recommend Eclipse IDE to start coding with Java. If you are new to Eclipse, refer this article first.

So Lets Start:
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package com.codinguide.test;

public class TestMain {

public static void main(String[] args) {

System.out.println("Hello World!");
} }

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Like very programmer, We start our programming from C and move forward to other high level language but startup program remains same. Its Hello Word. Well, we have main method which is starting point of execution. The class that contains main method need to be public class. 
Program Constructs:

Enough about Hello Word! Let have some thing serious, 

When we consider a Java program it can be defined as a collection of objects that communicate via invoking each others methods. Let us now briefly look into what do class, object, methods and instant variables mean.

Object: Objects have states and behaviors. Example: A dog has states-color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.

Class: A class can be defined as a template/ blue print that describe the behaviors/states that object of its type support.

Methods: A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.

Instant Variables: Each object has its unique set of instant variables. An object.s state is created by the values assigned to these instant variables.

Basic Syntax:

Point To Remember: 

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 words first letter should be in Upper Case.

Example class MyFirstClass

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.

Example public void myMethodName()

Class File Name: Name of class file must be same as Class name. If class is public then its mandatory to have this else you can ignore it, but its better to keep file name as class name.

public static void main(String args[]): Java program processing starts from the main() method which is a mandatory part of every java program. The class that contains the main method must be public class.

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 key word 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 : defualt, public , protected, private

Non-access Modifiers : final, abstract, strictfp

We will be looking into more details about modifiers in the next section.

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:

Enums were introduced in java 5.0. 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 your code.

For example if we consider an application for Pizza then its better to have restricted type of pizza,

public enum PizzaType {

Margarita,

FarmFresh,

MushroomDelight
}
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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. 
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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

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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 comment
/*multi line comment*/

Using Blank Lines: A line containing only whitespace, possibly with a comment, is known as a blank line, and Java totally ignores it.In next chapter, we will discuss about class and objects.

Thanks,

Mohit Singh