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

2. 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!"); } b
Michael Fung, CS&E, The Chinese University of HK

3. 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!"); } b
Michael Fung, CS&E, The Chinese University of HK

4. 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! b
Michael Fung, CS&E, The Chinese University of HK

5. 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! b
Michael Fung, CS&E, The Chinese University of HK

6. 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 b
Michael Fung, CS&E, The Chinese University of HK

7. 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! b
Michael Fung, CS&E, The Chinese University of HK

8. 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!"); } b
Michael Fung, CS&E, The Chinese University of HK

9. 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! b
Michael Fung, CS&E, The Chinese University of HK

10. 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!"); } b
Michael Fung, CS&E, The Chinese University of HK

11. 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!"); } b
Michael Fung, CS&E, The Chinese University of HK

12. 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!"); } b
Michael Fung, CS&E, The Chinese University of HK

13. 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!"); } b
Michael Fung, CS&E, The Chinese University of HK

14. 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!"); } b
Michael Fung, CS&E, The Chinese University of HK

15. 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? b
Michael Fung, CS&E, The Chinese University of HK

16. Michael Fung, CS&E, The Chinese University of HK
The First Java Program
Welcome to Java!
Welcome
main
Java Virtual Machine
Message sender b
Michael Fung, CS&E, The Chinese University of HK

17. 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 b
Michael Fung, CS&E, The Chinese University of HK

18. 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. b
Michael Fung, CS&E, The Chinese University of HK

19. 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 b
Michael Fung, CS&E, The Chinese University of HK

20. Michael Fung, CS&E, The Chinese University of HK
End Note
Readings and References
Chapter 1 b
Michael Fung, CS&E, The Chinese University of HK