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IT151: Introduction to Programming. Introduction to Java Applications. // Text-printing program // Created by: Siua Fonua // Date: 14 Feb 2006 public class Welcome { // main method begins execution of Java Applications public static void main (String args[ ]) { - PowerPoint PPT Presentation
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IT151: Introduction to Programming
Introduction to Java Applications
1. // Text-printing program
2. // Created by: Siua Fonua
3. // Date: 14 Feb 2006
4.
5. public class Welcome
6. {
7. // main method begins execution of Java Applications
8. public static void main (String args[ ])
9. {
10. System.out.println(“Welcome to IT151”);
11. System.out.println(“Enjoy programming with Java”);
12.
13. } // end main method
14. } // end class Welcome
Line 1 – Line 3
CommentsBegins with //Comments are used to improve the program’s
readabilityJava compiler ignores all commentsTo ways of indicating a comment
// end-of-line comment /* … */ multiple-line comments
Line 4, 12
White space Includes
Blank lines Space characters Tab characters
Makes the program easier to read Ignored by the compiler
Line 5
Class declaration Every program in Java consists of at least one class The keyword class is used to declare a class,
followed by the name of the class The name of the class is called it’s identifier
NOTE: YOU MUST DECLARE EVERYTHING IN JAVA
Reserved Words
Also known as keywords These words are reserved for use by Java Examples include:
class Public, and more to come
Don’t use any of the Java keywords for your own use. IT WILL CONFUSE THE COMPILER
Class Names Rules for naming your class
Contains only Letters Digits Underscore ( _ ) Dollar sign ( $ )
Does not contain spaces Does not begin with a digit
Convention All class names begin with a capital letter Capitalise the first letter of each word they include
Java is case-sensitive
That is:Uppercase and lowercase letters are distinctExamples:
TIHE is different from Tihe or tihe
Not using the correct letters for an identifier causes a compilation error
Public classes
Every class you declared public: Must be saved in its own file Class name must be the same as the filename Must be ended with the .java extension
Failing to follow these rules, result in errors Sometimes your class will not be compiled by
the compiler
Line 6, 14
Left brace, { Begins the body of the class
Right brace, } Ends the body of the class
It is a syntax error if braces do not occur in matching pairs
Line 8
The starting point of every Java applicationsYour program must have this line, exactly as it
is shown here!Otherwise, your program will not be compiled
by the compiler
Methods
A block of code in the program that performs a specific task
Methods are able to perform a task and returns information when they complete execution
Some methods will not return any information when they complete execution
All methods will have Return type Method name List of parameters (inside a pair of parentheses)
void methods
When the keyword void appears in front of the method name, it indicates that this method will NOT return anything when it completes execution
We’ll discuss later, the methods that will return information after execution
Line 9, 13
Left brace, {Begins the body of the method
Right brace, }The end of the method’s body
Whatever is between these braces, will be the actions to be performed by the method
Line 10
Instructs the computer to perform an action
That is:- To print a string of characters contained
between the double quotation marksWhite-space characters in strings are not
ignored by the compiler
Standard output
System.out is called the standard output object Allows Java to display sets of characters in
the window from which Java executes
System.out.println
This method displays a line of text in the command windowNeeds an argument, which is a string of
charactersOutputs its argument to the command windowAfter completion, it positions the output cursor
in the next line
Line 11
What does this line do?
Statement
The entire line 10, is called a statementEach statement MUST end with a semicolonA method is typically composed by one or
more statements that performs the method’s task
NOTE: omitting the semicolon at the end of a statement, is a syntax errorh
1. // Text-printing program
2. // Created by: Siua Fonua
3. // Date: 14 Feb 2006
4.
5. public class Welcome
6. {
7. // main method begins execution of Java Applications
8. public static void main (String args[ ])
9. {
10. System.out.print(“Welcome to IT151”);
11. System.out.println(“Enjoy programming with Java”);
12.
13. } // end main method
14. } // end class Welcome
1. // Text-printing program
2. // Created by: Siua Fonua
3. // Date: 14 Feb 2006
4.
5. public class Welcome
6. {
7. // main method begins execution of Java Applications
8. public static void main (String args[ ])
9. {
10. System.out.println(“Welcome\n to\n IT151”);
11. System.out.println(“Enjoy programming with Java”);
12.
13. } // end main method
14. } // end class Welcome
Escape characters
The backslash (\) is called an escape character in Java \n \t \r \\ \”
