1 The First Step Learning objectives write Java programs that display text on the screen. distinguish between the eight built-in scalar types of Java; declare and assign values to variables; create constant values with the keyword final; join messages and values in output commands by using the concatenation (+) operator; use the input methods of the Scanner class to get data from the keyboard; design the functionality of a method using pseudocode.

2 public class Hello { public static void main(String[] args) { System.out.println("Hello world"); } } Hello World Your first program public class Hello { } public static void main(String[] args) { } System.out.println("Hello world");

3 Adding comments to a program // this is a short comment, so we use the first method public class Hello { public static void main(String[] args) { System.out.println("Hello world"); } /* this is the second method of including comments – it is more convenient to use this method here, because the comment is longer and goes over more than one line */ }

4 Simple data types in Java Interesting programs will have to store data in order to give interesting results; Types of value used within a program are referred to as data types; Price : for example £4.75 Total sold: for example 187 in Java the simple types are referred to as the scalar types or the primitive types A real number An integer

5 The scalar types of Java Java typeAllows forRange of values byte very small integers-128 to 127 short small integers to int big integers to long very big integers to float real numbers +/- 1.4 * to 3.4 * double very big real numbers +/- 4.9 * to 1.8 * char charactersUnicode character set boolean true or falsenot applicable

6 Variables A variable is a name given to a piece of data in your program You can choose almost any name for your variables as long as the name does not have spaces in it the name is not already used in Java (such as ‘class’ or ‘static’) ticket   cinema ticket cinemaTicket cinema_ticket void

7 Declaring variables in Java A variable is declared as follows dataType variableName ; For example to declare a variable suitable for holding the score in a computer game: int score;

8 The effect of declaring a variable on the computer's memory int score ; Computer MemoryJava Instruction score

9 Declaring many variables You may declare as many variables as you need int score; char level; Several variables can be declared on a single line if they are all of the same type: int score, hits; // two variables declared at once char level ; // this has to be declared separately

10 The effect of declaring many variables in Java int score, hits; char level ; Java InstructionsComputer Memory hits score level

11 Assignments in Java assignments allow values to be put into variables; variableName = value; they are written in Java with the use of the equality symbol (=); this symbol is known as the assignment operator. simple assignments take the following form: score 0

12 Assignments: some examples score = 0; Can combine the assignment statement with a variable declaration to put an initial value into a variable: int score = 0; this is equivalent to: int score; score = 0;

13 Spot the error! int score = 2.5 ; A real number cannot be placed into an integer variable!

14 Assigning a character to a variable char level = 'A'; when assigning a value to a character variable, you must enclose the value in single quotes:

15 Creating constants there are occasions where data items in a program have values that do not change EXAMPLES the maximum score in an exam (100); the number of hours in a day (24); the mathematical value of  ( ). constants are declared much like variables in Java except they are preceded by the keyword final, they are always initialized to their fixed value. final int HOURS = 24;

16 The arithmetic operators of Java OperationJava operator addition+ subtraction- multiplication* division/ remainder%

17 Carrying out calculations int x; x = ; At the end of these instructions ‘x’ holds the number 35 terms on the right-hand side of assignment operators are referred to as expressions.

18 Calculations: another example Cost = 500 Sales tax = 17.5% double cost; cost = 500 * ( /100);

19 Examples of the modulus operator in Java ExpressionValue % 9 6 % 8 40 % % 2

20 Modulus: An example 4 chairs per table30 people How many tables of four are required, and how many people will be left over? int tablesOfFour, peopleLeftOver; tablesOfFour = 30/4; peopleLeftOver = 30%4; 7 2 ?

21 Expressions in Java The expression on the right-hand side of an assignment statement can itself contain variable names; AN EXAMPLE: revisiting the cost of a product double price, tax, cost; price = 500; tax = 17.5; cost = price * (1 + tax/100);

22 Using variables on both sides of the assignment double price, tax; price = 500; tax = 17.5; price = price * (1 + tax/100); This will store original and final price The new value of ‘price’ The old value of ‘price’

23 A special shorthand x = x + 1;x++; increment (add one to an integer) decrement (take one off an integer) x = x - 1; x--;

24 A complete program RUN public class FindCost { } public static void main (String [] args) { } double price, tax; price = 500; tax = 17.5; price = price * (1 + tax/100);

25 Output in Java To display a message in Java: System.out.println(message to be printed on screen); For example System.out.println("Hello world"); Hello world _ System.out.print ("Hello world"); cursor on next line Hello world _ cursor on same line

26 Examples of ‘println’ and ‘print’ public class Hello { public static void main(String[] args) { System.out.println("Hello world"); System.out.println("Hello world again!"); } } Hello world Hello world again! RUN System.out.print("Hello world"); Hello worldHello world again!

27 More output examples System.out.print(10*10); 100 System.out.print("cost = " + (30*7.5) ); cost = expressions calculated and displayed Expressions and Strings joined by the concatenation operator ‘+’ A collection of characters are called Strings in Java, they are always enclosed in quotes

28 Modifying the FindCost program public class FindCost2 { public static void main(String[] args) { double price, tax; price = 500; tax = 17.5; price = price * (1 + tax/100); System.out.println("*** Product Price Check ***"); System.out.println("Cost after tax = " + price); } } RUN *** Product Price Check *** Cost after tax = 587.5

29 Keyboard input The Scanner class has recently been added into Java to simplify keyboard input score 35

30 Input in Java: the Scanner class In order to have access to the Scanner class you have to place the following line at the beginning of your program: import java.util.*; You will need to write the following instruction in your program. Scanner sc = new Scanner(System.in); This instruction creates a Scanner object, ‘sc’ that can be used to input values from the keyboard

31 Input methods of the Scanner class if we want a user to type in an integer at the keyboard, into the variable ‘x’: x = sc.nextInt(); in the case of a double, y: y = sc.nextDouble(); in the case of a char, c: c = sc.next().charAt(0);

32 Revisiting the FindCost program import java.util.*; public class FindCost3 { public static void main(String[] args ) { Scanner sc = new Scanner(System.in); double price, tax; System.out.println("*** Product Price Check ***"); System.out.print("Enter initial price: "); price = sc.nextDouble(); System.out.print("Enter tax rate: "); tax = sc.nextDouble(); price = price * (1 + tax/100); System.out.println("Cost after tax = " + price); } } RUN *** Product Price Check *** Enter initial price: _ 1000 Enter tax rate: _12.5 Cost after tax =

33 Re-running the FindCost program import java.util.*; public class FindCost3 { public static void main(String[] args ) { Scanner sc = new Scanner(System.in); double price, tax; System.out.println("*** Product Price Check ***"); System.out.print("Enter initial price: "); price = sc.nextDouble(); System.out.print("Enter tax rate: "); tax = sc.nextDouble(); price = price * (1 + tax/100); System.out.println("Cost after tax = " + price); } RUN *** Product Price Check *** Enter initial price: _ 50 Enter tax rate: _17.5 Cost after tax = 58.75

34 More about strings A String is not a simple data type like an int or a char ; A String is a class (you learn about classes in week 6); You can declare a String in a similar way to variables of type int or char; String name; notice that String "type" has to start with a capital "S". Java allows you to use the normal assignment operator ( = ) with strings: name = “Dula"; To obtain a string from the keyboard you can use the next method of Scanner Your string should NOT contain spaces

35 Strings: A sample program import java.util.*; public class StringTest { public static void main(String[] args) { Scanner sc = new Scanner(System.in); String name; int age; System.out.print("What is your name? "); name = sc.next(); System.out.print("What is your age? "); age = sc.nextInt(); System.out.println(); System.out.println("Hello " + name); System.out.println ("When I was your age I was " +(age +1)); } } RUN What is your name? _Aaron What is your age? _15 Hello Aaron When I was your age I was 16

36 Program design Programmer thinking about how to build the software public class FindCost3 { public static void main(String[] args ) { Scanner …… } } Sketching out the solution as a program design Implementing the design by writing the code when you sketch out the code for your methods a general purpose "coding language" can be used. code expressed in this way is referred to as pseudocode.

37 Pseudocode for the FindCost program BEGIN DISPLAY program title DISPLAY prompt for price ENTER price DISPLAY prompt for tax ENTER tax SET price TO price * (1 + tax/100) DISPLAY new price END