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;

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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.

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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");

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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 */

}

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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

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The scalar types of Java

Java type Allows for Range of values

byte very small integers -128 to 127

short small integers -32768 to 32767

int big integers -2147483648 to 2147483647

long very big integers -9223372036854775808 to 9223372036854775807

float real numbers +/- 1.4 * 10-45 to 3.4 * 1038

double very big real numbers +/- 4.9 * 10-324 to 1.8 * 10308

char characters Unicode character set

boolean true or false not applicable

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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

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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;

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The effect of declaring a variable on the computer's memory

int score;

Computer Memory Java Instruction

score

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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

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The effect of declaring many variables in Java

int score, hits;

char level;

Java InstructionsComputer Memory

hitsscore

level

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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:

score0

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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;

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Spot the error!

int score = 2.5 ;

A real number cannot be placed into an integer

variable!

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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:

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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 (3.14176).

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;

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The arithmetic operators of Java

Operation Java operator

addition +

subtraction -

multiplication *

division /

remainder %

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Carrying out calculations

int x;x = 10 + 25;

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.

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Calculations: another example

Cost = 500

Sales tax = 17.5%

double cost; cost = 500 * (1 + 17.5/100);

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Examples of the modulus operator in Java

Expression Value

2

6

0

0

29 % 9

6 % 8

40 % 40

10 % 2

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Modulus: An example

4 chairs per table 30 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;

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2

?

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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);

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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’

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A special shorthand

x = x + 1; x++;

increment (add one to an integer)

decrement (take one off an integer)

x = x - 1; x--;

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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);

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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

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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!

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More output examples

System.out.print(10*10);

100

System.out.print("cost = " + (30*7.5) );

cost = 225.0

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

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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

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Keyboard input

The Scanner class has recently been added into Java to simplify keyboard input

score 35

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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

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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);

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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 = 1125.0

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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

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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

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Strings: A sample programimport 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

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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.

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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