Although hello.java is simple, but it displays important basic structure of JAVA program.

After the first 2 classes, I assume you already knew that

1. A program needs a class definition , like class className.

2. A main method in a class is the start point for program.

3. File should be save as className.java.

4. Statement should be ended with ;.

Variables

Let's us count how many cars you and your little brothers have. Then calculate how many cars in total.

class car_count
{
public static void main(String[] args)
{
int His_Cars=30;
int My_Cars=40;
int TotallCars=His_Cars+My_Cars;
System.out.println("Total books we have="+TotallCars);
}
}

Look at the above code for a while. You may notice something new: int

I use variables to store information. They are called variables because the information stored there might change during program execution. His_Cars and My_Cars are variables.

Just imagine I have a lot of pockets(variables) for you to use. First you should name a pocket (Computer guru says: define a variable) and then you put a something into that pocket(Computer guru says: assign value to variable).

Just like pockets may have different shapes, variables have different kinds too. In this tutorial, we are gonna learn 4 different variables: int ,double, boolean and String.

When you want to use variables, please remember the following rules:

1. First character of variable must be a letter, an underscore(_) or a dollar sign($).

2. You cannot use a word reserved by JAVA (for example, you can not have a variable named main or one named System).

Compile the above code, how many cars in total?

Effective Java Programming Language Guide

Professional Java, JDK 5 Edition

Ivor Horton's Beginning Java 2, JDK 5 Edition