Hands-on Introduction to JAVA First Java Program 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program Type all carefully and save it to a file named Welcome.java class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program Java program source files (.java) contain definition of classes class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program Curly braces pair enclose a block of code, class Welcome here class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } Don’t miss me! 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program Curly braces pair enclose a block of code, method main( ) here class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } Don’t miss me! 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program This is a block of comments, for human, not for computer class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } It explains to you what happens 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program /* and */ pair encloses a comment block class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } Don’t miss me! 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program This is a method of the class Welcome, named main( ) class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program There MUST be a pair of parentheses following ALL method names class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } Don’t miss me! 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

Michael Fung, CS&E, The Chinese University of HK The First Java Program Let's leave these first. Let's leave these first. class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program Standard properties of the main( ) method class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program A statement (instruction) to display a message class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

The First Java Program After every statement, there must be a semi-colon! class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

Michael Fung, CS&E, The Chinese University of HK The First Java Program How to ask the computer to act according to the instructions in this program? class Welcome { /* The Welcome Program ------------------- Illustrates a simple program displaying a message. */ public static void main (String [ ] args) { System.out.println("Welcome to Java!"); } 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

Michael Fung, CS&E, The Chinese University of HK The First Java Program Change to the directory containing the file Welcome.java Type javac Welcome.java It generates a new file Welcome.class Type (without .class) java Welcome What’s the result? 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

Michael Fung, CS&E, The Chinese University of HK The First Java Program Welcome to Java! Welcome main Java Virtual Machine Message sender 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

What has happened? Java Program [Welcome.java] Compile Java Compiler [javac] Compile Java Byte Code [Welcome.class] native code Translate Java Virtual Machine (JVM) [java] It locates the class method main() from the class Welcome and starts execution from there 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

Big Program For Simple Task? Yes, it just displays a little message. When writing business letters, we conform to grammar and format. Likewise for programming! For more complicated tasks, such overheads are relatively trivial. 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

Software Life Cycle Computer programming is not just writing programs after learning a language. requirements specification program design conception design analysis 70% of the software cost is related to software maintenance in the operation phase. Well-designed and constructed software is easier to maintain. coding actual program operation debugging testing 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK

Michael Fung, CS&E, The Chinese University of HK End Note Readings and References Chapter 1 2008-2009 1b Michael Fung, CS&E, The Chinese University of HK