Java Programming: Variables & Operators (PDF)

INTERNATIONAL & ACCESS FOUNDATION PROGRAMMES COMPUTER SCIENCE SEMESTER ONE Part 3 Java Programming Variables & Operators © Nevan Bermingham Main Principles of Java Coding In this section we are going to look at some of the main prin...

INTERNATIONAL & ACCESS FOUNDATION PROGRAMMES COMPUTER SCIENCE SEMESTER ONE Part 3 Java Programming Variables & Operators © Nevan Bermingham Main Principles of Java Coding In this section we are going to look at some of the main principles of Java, and how they are used: Variables Operators (, =, ==, !=, &&, ||,.....) Printing on screen and reading user input (generally referred to as input/output or I/O) First, lets look at a Program This is the classic “Hello World” Program in Java: class HelloWorldApp { public static void main(String[] args) { System.out.println("Hello World!"); // Display the string. } }  These are What does each part do? “Comment s” and are only for the ignores everything class HelloWorldApp from , and // public static void main(String[] args) { System.out.println("Hello World!"); // Display the string. } } What does each part do? This is called a enough to know that every class HelloWorldApp { application public static void main(String[] args) begins with a { class definition. System.out.println("Hello World!"); // Display the string. } } What does each part do? In Java every application class HelloWorldApp must contain a { main method. public static void main(String[] args) { System.out.println("Hello World!"); // Display the string. } } What does each part do? Uses the “System Class” the "Hello World!" class HelloWorldApp message to { standard output public static void main(String[] args) (normally the { Screen) System.out.println("Hello World!"); // Display the string. } } What does each part do? to bunch code together. We’ll class HelloWorldApp { be using these public static void main(String[] args) lot more later. { System.out.println("Hello World!"); // Display the string. } } What does each part do? ; All Java compiler the line has no class HelloWorldApp more code. { Leave it out by public static void main(String[] args) accident and { you’ll get an System.out.println("Hello World!"); // Display the string. error. } } Well...? Was that a little scary? Do not worry if you didn’t understand some or any of it, we will go over some basics first and then come back to the program. First, we’re going to cover off the basics of Variables and Operators – these are the first things you’ll need to understand before you can start coding. Variables Nearly all Applications (Programs) need to be able to store and manipulate data. For example:-  Numbers to be added or multiplied, and the answer.  A User Name  To count the occurrence of something Variables are places in the computer's memory where you store the data for a program. Each variable is given a unique name which you refer to it with, and can have many values => Hence the name Variable The opposite to a variable is a constant. Variables are like a box... “The Dublin Institute of Technology” Variable €34.56 Name 3.14159 42 265 Lets Look at an Example: To Multiply 2 numbers. First Number is stored in a variable x Second Number is stored in a variable called y The answer is stored in a variable called answer So:- answer = x MULTIPLIED BY y Note, this is Mathematically written: x x y = answer Or in computational mathematics: x * y = answer How would this program look? 1. Ask User to input first number. 2. Store this number in variable named x 3. Ask User to input second number. 4. Store this number in variable named y 5. Multiply x and y 6. Store this result in variable named answer 7. Print the value of answer to the screen Or in another way... Start Input First Number = x Input Second Number = y Output Answer = x * y Answer End Or in another way... Start In computing you have: Addition + Subtraction - Input First Number = x Multiplication * Division / These are called Input Second Number = y OPERATORS Output Answer = x * y Answer End Here’s the Java Program import java.util.Scanner; public class Multiply { // main method begins execution of Java application public static void main( String[] args ) { // create Scanner to obtain input from command window Scanner input = new Scanner( System.in ); int x; // first number int y; // second number int answer; // to store the answer of x * y System.out.println( "Enter first integer: " ); // prompt x = input.nextInt(); // read first number from user System.out.println( "Enter second integer: " ); // prompt y = input.nextInt(); // read second number from user answer = x * y; // multiply numbers System.out.printf( "Sum is %d\n", answer ); // display sum to screen } // end method main } // end class Multiplication Here’s the Java Program import java.util.Scanner; public class Multiply { // main method begins execution of Java application public static void main( String[] args ) { // create Scanner to obtain input from command window Scanner input = new Scanner( System.in ); int x; // first number int y; // second number Lets examine each part of int answer; // to store the answer of x * y System.out.println( "Enter first integer: " ); // prompt this Program.... x = input.nextInt(); // read first number from user System.out.println( "Enter second integer: " ); // prompt y = input.nextInt(); // read second number from user answer = x * y; // multiply numbers System.out.printf( "Sum is %d\n", answer ); // display sum to screen } // end method main } // end class Multiplication Comments public class Multiply { // main method begins execution of Java application public static void main(String[] args) { Comments are used to describe the Java code The compiler ignores everything from // to the end of the line Comments are good practice, and help you and others understand the flow of the code. Important: All your code should always contain comments. Operators answer = x * y; Operators perform a mathematical operation and return a result. Or, you could say another way:- They are used to manipulate data, data usually stored in variables. For a full list of Operators, see http://docs.oracle.com/javase/tutorial/jav a/nutsandbolts/opsummary.html Operators Assignment Operators = Arithmetic Operators - + * / % ++ -- Relational Operators > < >= < >= ) less than (=) less than or equal ( < >= y FALSE x >= y FALSE x