The document discusses Java programming concepts like comments, classes, methods, and user input. It provides explanations and examples of: 1) How comments are used in Java and the different types of comments. 2) The basic structure of a Java class including the main method that acts as the entry point. 3) How the System.out.println method prints output and how user input can be accepted using the Scanner class.

1. Java program explanation
• /* The Welcome class implements an application
that simply prints "Hello World..!" to standard
output. */
• class Welcome {
• public static void main(String[] args) {
System.out.println("Hello World..!"); // Display
the string.
• }
• }

2. Comments
• Comments are ignored by the compiler but are useful
to other programmers. The Java programming
language supports three kinds of comments:
• /* text */ The compiler ignores everything from /* to
*/.
• /** documentation */ This indicates a documentation
comment (doc comment, for short). The compiler
ignores this kind of comment, just like it ignores
comments that use /* and */. The javadoc tool uses
doc comments when preparing automatically
generated documentation.
• // text
• The compiler ignores everything from // to the end of
the line.

3. Java documentation
• Javadoc (originally cased as JavaDoc) is a
documentation generator created by Sun
Microsystems for the Java language (now
owned by Oracle Corporation) for generating
API documentation in HTML format from Java
source code.
• Some IDEs, such as Netbeans and Eclipse,
automatically generate Javadoc HTML.

4. Java documentation
• // import statements
• /**
• * @author Firstname Lastname <address @
example.com>
• * @version 1.6 (current version number of
program)
• * @since 2010-03-31 (the version of the package
this class was first added to)
• */
• class Welcome { // class body }

5. Javadoc tags
Tag & Parameter Usage
@author John Smith Describes an author.
@version version
Provides software version
entry. Max one per Class or
Interface.
@since since-text
Describes when this
functionality has first existed.
@see reference
Provides a link to other
element of documentation.
@param name description Describes a method parameter.
@return description Describes the return value.

6. class
• The most basic form of a class definition is:
class name { . . . }
• The keyword class begins the class definition
for a class named name
• The code for each class appears between the
opening and closing curly braces

7. public static void main(String[] args)
{…}
• public means that the method can be seen
outside of this class;
• static means that you don't have to create a
new object;
• void means it doesn't return a value

8. public static void main(String[] args)
{…}
• The keyword public is an access specifier.
• The keyword static is a kind of modifier.
• The keyword void means that the method main() does not
return any value.
• As we had seen before all the java program will start
execute by calling the main method.
• If we want to pass any information to a method will be
received by the variables declared within the parenthesis is
called parameters.
• In a main() method there is only one parameter ,String
args[] . args[] is a name of the parameter that is an array of
the objects of data type String.
• String store sequences of characters and args will receive
the command line arguments.

9. public static void main(String[] args)
{…}
• The modifiers public and static can be written in
either order (public static or static public), but
the convention is to use public static as shown
above.
• You can name the argument anything you want,
but most programmers choose "args" or "argv".
• The main method is similar to the main function
in C and C++; it's the entry point for your
application and will subsequently invoke all the
other methods required by your program.

10. System.out.println("Hello World!");
• Finally, the line:
• System.out.println("Hello World!");
• uses the System class from the core library to
print the "Hello World!" message to standard
output.
• This is included in Java.lang libraray.

11. static keyword
• The static keyword in Java means that the
variable or function is shared between all
instances of that class as it belongs to the
type, not the actual objects themselves.
• So if you have a variable:
• private static int i = 0;
• and you increment it (i++) in one instance, the
change will be reflected in all instances. i will
now be 1 in all instances.

13. Variable
• You can call them almost anything you like, but there are a few
rules:
• Variable names can't start with a number. So first_number is OK,
but not 1st_number. You can have numbers elsewhere in the
variable name, just not at the start.
• Variable names can't be the same as Java keywords. There are
quite a lot of these, like int above.
• You can't have spaces in your variable names. The variable
declaration int first number will get you an error. We've used the
underscore character, but it's common practise to have the first
word start with a lowercase letter and the second or subsequent
words in uppercase: firstNumber, myFirstNumber
• Variable names are case sensitive. So firstNumber and FirstNumber
are different variable names.

14. Java Keyword

15. Invalid Identifier

16. Accepting Input from a User
• One really useful class that handles input from
a user is called the Scanner class.
• The Scanner class can be found in the java.util
library.
• To use the Scanner class, you need to
reference it in your code. This is done with the
keyword import.
• import java.util.Scanner;

17. Accepting Input from a User
• To create a new Scanner object the code is this:
• Scanner user_input = new Scanner( System.in );
• So instead of setting up an int variable or a String
variable, we're setting up a Scanner variable.
We've called ours user_input.
• After an equals sign, we have the keyword new.
This is used to create new objects from a class.
The object we're creating is from the Scanner
class.
• In between round brackets we have to tell java
that this will be System Input (System.in).

18. Get the user input
• To get the user input, you can call into action
one of the many methods available to your
new Scanner object. One of these methods is
called next. This gets the next string of text
that a user types on the keyboard:
• String first_name;
first_name = user_input.next( );

19. Java Option Panes
• Another useful class for accepting user input, and
displaying results, is the JOptionPane class.
• This is located in the javax.swing library.
• The JOptionPane class allows you to have i/o
boxes like this one:

20. Java Option Panes
• import javax.swing.JOptionPane;
• This tells java that we want to use the
JOptionPane class, located in the javax.swing
library.

21. What Is a Package?
• A package is a namespace that organizes a set
of related classes and interfaces.
• The Java platform provides an enormous class
library (a set of packages) suitable for use in
your own applications. This library is known as
the "Application Programming Interface", or
"API" for short. Its packages represent the
tasks most commonly associated with general-
purpose programming.

22. How do I convert a String to an int
data type in Java?
• int i = Integer.parseInt(myString);