2. Lecture Notes - Week 2
Lecture-1

3. Previous lecture Introduction Programming languages
How to write a simple program

4. Outline of Lecture Primitive data types Variables Constants
Assignments
Expressions
Operators
Input and output
To do list

5. A Simple Java Program
First Java program that will evolve gradually into a number of more complicated programs that incorporate various new features of Java as we learn them.
Listing 1.1
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!"); }

6. Creating, Compiling and Running Programs

7. Classes and Class Names
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!"); }
Every Java program must have at least one class. Each class has a class name, which starts with an uppercase letter.

8. Main Methods
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!"); }
Each program must have a main method in a class in order to run the program. The program is executed from the main method. 8

9. Statements
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!"); }
Each class must have at least one statement that describes an action or a sequence of actions to be executed. Every statement ends with a semicolon (;), called statement terminator. 9 9

10. Class Blocks
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
System.out.println("Learning Java is" +
"exciting!”); }
Each class has a class block enclosed in a pair of braces. 10 10 10

11. Method Blocks
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
System.out.println("Learning Java is" +
"exciting!”); }
Each method has a method block enclosed in a pair of braces, which can contain more or more statement. 11 11 11 11

12. Reserved Words or Keywords
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
System.out.println("Learning Java is" +
"exciting!”); }
A reserved word or keyword has a specific meaning to the compiler and cannot be used for other purposes in the program. 12 12 12 12 12

13. Comments
// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
System.out.println("Learning Java is" +
"exciting!”); }
A line comment starts with double forward slashes (//). Comments are for program readability and ignored by the compiler. Use a pair of /* and */ for multi-lines of comments. 13 13 13 13 13 13

14. Variables and Primitive Data Types
// This program prints Welcome to Java!
public class ComputerArea {
//main method
public static void main(String[] args) {
double radius;
double area;
//assign a value to radius
radius = 20;
//Computer the area given a radius and
//assign the result to area
area = radius * radius * ;
//Display result (i.e. the value of radius)
System.out.println(”The area for the” +
”circle of radius” + radius + ”is” + area); }
radius and area are variables of double data type
Each variable represents a value stored in the computer.
Each variable needs to be declared with a data type (e.g. double).
A variable starts with a lowercase letter by convention. 14 14 14 14 14 14 14

15. Primitive Data Types 15

16. Assignments and Expressions
// This program prints Welcome to Java!
public class ComputerArea {
//main method
public static void main(String[] args) {
double radius;
double area;
//assign a value to radius
radius = 20;
//Computer the area given a radius and
//assign the result to area
area = radius * radius * ;
//Display result (i.e. the value of radius)
System.out.println(”The area for the” +
”circle of radius” + radius + ”is” + area); }
An assignment statement assigns a value to a variable.
An assignment statement assigns the value of an expression to a variable.
The syntax for assignment statements is:
variable = expression; 16 16 16 16 16 16 16 16

17. Operators 17

18. Named Constants Named constants
// This program prints Welcome to Java!
public class ComputerArea {
//main method
public static void main(String[] args) {
final double PI = ;
double radius;
double area;
//assign a value to radius
radius = 20;
//Computer the area given a radius and
//assign the result to area
area = radius * radius * PI;
//Display result (i.e. the value of radius)
System.out.println(”The area for the” +
”circle of radius” + radius + ”is” + area); }
Named constants
A named constant is declared to represent a fixed value.
A named constant is used in an expression.
The syntax for named constants is:
final datatype CONSTANTNAME = value; 18 18 18 18 18 18 18 18 18

19. Input from Keyboard
A comment across lines can be enclosed in a pair of /* and */.
/* This program reads a radius from keyboard and displays the area on monitor. */
import java.util.Scanner;
public class ComputerAreaWithConsoleInput {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
double radius, area;
//Prompt the user to enter a radius
System.out.print("Enter a number for radius: ");
double radius = input.nextDouble();
area = radius * radius * ;
System.out.println(”The area for the” +
”circle of radius” + radius + ”is” + area); }
Import scanner class from java.util package.
Create a scanner object for the keyboard.
Read a number entered by the user
system.in and system.out refer to the keyboard and the monitor by default.
System.out is a predefined object but system.in is not so an object representing system.in needs to be defined.
More than one variable of the same data type can be declared in a statement.
Input can be from other sources, e.g., a file if the scanner object is linked to the file. 19 19 19 19 19 19 19 19 19 19

20. Read Different Data Types

22. Questions?