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[ ]) System.out.println(“Welcome to IT151”); System.out.println(“Enjoy programming with Java”); } // end main method } // end class Welcome

Line 1 – Line 3 Comments Begins with // Comments are used to improve the program’s readability Java compiler ignores all comments To ways of indicating a comment // end-of-line comment /* … */ multiple-line comments

Line 4, 12 White space Includes Makes the program easier to read 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 Convention 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 distinct Examples: 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, { Right 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 applications Your 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, { Right 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 marks White-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 window Needs an argument, which is a string of characters Outputs its argument to the command window After 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 statement Each statement MUST end with a semicolon A 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

// 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[ ]) System.out.print(“Welcome to IT151”); System.out.println(“Enjoy programming with Java”); } // end main method } // end class Welcome

// 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[ ]) System.out.println(“Welcome\n to\n IT151”); System.out.println(“Enjoy programming with Java”); } // end main method } // end class Welcome

Escape characters The backslash (\) is called an escape character in Java \n \t \r \\ \”