03 data types1june 15 03 data types ce00858-1: fundamental programming techniques

03 Data types 118 Apr 2023

03 Data types

CE00858-1: Fundamental Programming Techniques


Objectives

In this session, we will:• look at data types and converting between types• increment and decrement variables• use constants• introduce classes and objects• use String objects• use Scanner objects to input data


Data types

• data type defines:• what data is stored

• int• double• boolean

• what operations can be performed on the data • "Hello" + " Cathy" concatenates two Strings• 4 + 5 adds two integers

• String is not a primitive data type in Java


Converting between types

• conversion between types• done automatically if no danger of losing information

• error reported if possible loss of precision

int i = 10;double x = i;

double x = 10.0;int i = x;


Casting

• can force conversion by using a cast• required type in brackets placed in front of value to

be converted

int x = (int) 5.0; //5int y = (int) 4.8; //4int answer = (int) (5.0 / 3.0); //1double answer2 = (double) 5 / 3; //1.6667


Incrementing

• incrementing is most common operation

count = count + 1

• current value of count is retrieved• 1 is added to this value• new value is then put back into count

23 24

count

23 + 1

count


Increment and decrement operators

• shortcut to increase or decrease integers by 1• prefix increment:

• postfix increment:

• avoid using in calculations

j = ++i; // increase i by 1 THEN store in j

j = i++; // store in j THEN increase i by 1

j = i++ + ++i; // syntactically correct, but...


Constants

• used for data that will never change during execution

• benefits:• can't inadvertently change it• refer to meaningful name, rather than number• only need to make single edit if different value

needed

• by convention, constants are in UPPER_CASE final double PI = 3.14159;


Classes and objects

• large programs are constructed using smaller building blocks called objects

• objects are built from classes • a class is a template that groups:

• data: how it appears• methods: what it can do

• to use an object need to know:• where the class can be found• how to create an object• what the object can do

• do not need to know how it works


String data

• groups characters together• used for:

• titles• headings• prompts• error messages• instructions• help facilities


String class

• String is a class not a primitive data type• differs slightly from most classes in the way it

is declared• String objects are created from the String class• to use a String object need to know:

• where String class is found• String class is part of the Java language

• how to create a String object• what a String object can do


How to create a String object

• declared in same way as primitive variables:

• variables of type String:• contain zero or more characters in sequence• are enclosed by double quotes• the first character is at location 0

• character is enclosed by a single quote

String greeting = "Hello world!";

H e l l o w o r l d !

0 1 2 3 4 6 7 8 9 105 11

char letter = 'a';


What a String object can do

• classes have methods that provide useful operations

• to get a single character from a string:

• get the position of the first occurrence of a character:

• convert a string to upper case:

System.out.println(greeting.charAt(4));// o

System.out.println(greeting.indexOf('l'));// 2

System.out.println(greeting.toUpperCase());// "HELLO WORLD!"


String methods continued

• get the length of a string:

• replace every occurrence of one character with another:

• get the substring starting from a position:

System.out.println(greeting.length()); // 12

System.out.println(greeting.substring(6));// "world!"

System.out.println(greeting.replace('l', '*'));// "He**o wor*d!"


Scanner class

• Scanner objects are used to read input from the keyboard

• to use a Scanner object need to know:• where Scanner class is found• how to create a Scanner object• what a Scanner object can do


Where Scanner class is found

• Scanner class is found in a package (library of classes) called java.util

• package must be imported into application so the compiler can find it

• import statement must appear before class headerimport java.util.*;public class ReadExample...


How to create a Scanner object

• uses new keyword• requires parameter to indicate where to get

input from • System.in is the console input stream

Scanner myKeyboard = new Scanner(System.in);


What Scanner object can do

• read an integer:

• read a double:

• read a word up to space or new line:

• read the rest of the current input line:

int i = myKeyboard.nextInt();

String s = myKeyboard.next();

double x = myKeyboard.nextDouble();

String l = myKeyboard.nextLine();


Using data in programs

• for each data item need to consider:• what is it called?• where does its value come from?

• user input, calculated, hard coded• if it is calculated, what is the formula?

• what type of data is it?• integer, double, boolean, etc.• considering the units of measurements can help

for example, cost of item in pounds (double) or pence (integer)


Input example – Rectangle with input

• problem:• input width and height of rectangle• calculate and output area of rectangle using input values

• analysis:• what data is used?

• width: positive integer input by user• height: positive integer input by user• area: integer calculated by program

• what operations are performed?• create Scanner to read in data• prompt user for width and height• input width and height• area = width * height• output area


// input width and height, calculate and output area of rectangle

import java.util.*;public class RectangleInput{ public static void main (String[] args) {

//create Scanner to read in dataScanner myKeyboard = new Scanner(System.in);

//prompt user for input – use print to leave cursor on line

System.out.print("Please enter width: ");int width = myKeyboard.nextInt();

System.out.print("Please enter height: ");int height = myKeyboard.nextInt();

//calculate and output area - unchanged}

}

RectangleInput.java


Testing calculations

• must check results produced by programs• can't assume that any results are correct• calculating volume and surface area of

rectangle• width: 2• height: 3• area: width * height = 6

• choose different values for each variable• simple values => simple calculations


Summary

In this session we have:• looked at data types and converting between them• introduced classes and objects• used Scanner objects to input data

In the next session we will:• analyse and implement a program that uses the

concepts introduced so far

03 data types1june 15 03 data types ce00858-1: fundamental programming techniques

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