Java Lesson -2

First Java Program:

Let us look at a simple code that would print the words Hello World.

public class MyFirstJavaProgram{

   /* 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

    }

}

About Java programs, it is very important to keep in mind the following points.

• 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 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.
  
  Example public void myMethodName()
• Program File Name - Name of the program file should exactly match the class name. 
  
  When saving the file you 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 : Assume 'MyFirstJavaProgram' is the class name. Then 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 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 : default, 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 a fresh juice 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.

Example:

class FreshJuice{

   enum FreshJuiceSize{ SIZE, MEDIUM, LARGE }

   FreshJuiceSize size;

}

public class FreshJuiceTest{

   public static void main(String args[]){

      FreshJuice juice = new FreshJuice();

      juice.size = FreshJuice. FreshJuiceSize.MEDIUM ;

   }

}

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