Introduction to programming in the Java programming language

The first Java program

Consider the following simple Java program:

/* Print a greeting message in the console window */ public class Hello { public static void main(String[] args) { // The next statement prints a message to the console window System.out.print("Hello World!"); } }

We will use this simple program to illustrate some concepts

Class: container for program units

A Java program is an well-organized entity
(If your program gets messy, it's very hard to understand what the program is doing)
Every Java program is contained in a container called a class

The syntax to express a class is as follows:

public class CLASSNAME { PROGRAM UNITS }

Notes:

The words public and class are keywords in the Java language

Keywords have special meaning in a programming language:

We will have to delay the explanation of the keyword public for later (you do not have enough knowledge on Java to understand this keyword)
The keyword class introduces a new class (container)

A word CLASSNAME following the keyword class is the name of the new class (container)
The CLASSNAME is a name chosen by the programmer
The CLASSNAME is a name used to identify the new class

Identifiers

Identifier:
There are some rules in Java that you must follow to form the identifer name

Rules to form the name of an identifer in Java:

An identifier consists of a number of characters
There is no limit on the number of characters in the identifier (but do not try using identfiers that are too long because you will have to type it yourself...)
The first character of an identifer must be one of the following:
The subsequent characters of an identifer must be one of the following:
Identifiers are case-sensitive !

Examples of correct identifiers:

age Age (is different from age) greaterCommonDivisor R2D2 radius_of_the_circle

Examples of illegal identifiers:

3cpo (cannot start with a digit) radius-of-the-circle (cannot have minus sign in an identifier)