Programming with Java

Jussi PohjolainenTampere University of Applied Sciences

HELLOWORLD

HelloWorld.java

public class HelloWorld { public static void main(String [] args) { System.out.println("Hello World"); }}

BOOLEAN ALGEBRA AND CONDITIONS

About Conditions

• Conditions are used in while and in if – sentences– if(condition)• do something

• Condition is a statement that is either true or false

AND

• if(it is raining AND car does not work)– go to work by bus

• In Java, the AND is marked with &&– if(x >= 4 && x<=10)• do something

AND

A B A && B1 1 11 0 00 1 00 0 0

OR

• if(it is raining OR car does not work)– Go to work by bus

• In Java OR is marked with ||– if(x == 3 || x == 10)• do something

OR

A B A && B1 1 11 0 10 1 10 0 0

Negation

• Negation turns true to false and wiseversa• In Java, negation is marked with !• if(!rains)– go to work by bicycly

• if(!(x < 3))– do something

Negation

A !A1 00 1

Combining Conditions

• if(!rains && (temperature > 20C))– Walk with your t-shirt on

• Demos– Conditions.java– BooleanAlgebra.java

PRIMITIVE TYPES

About Variables

• Simple calculator with pseudocode– print "Give number"– a := readInput()– print "Give another number"– b := readInput()– sum := a + b– print sum

• Variables? a, b and sum!

Declaring Variables

• In Java you have to declare a variable before using it

• Declaring?– What is the variable's name?– What is the variable's type?

• Type?– What kind of information will be stored into the

variable?

Declaring Variables in Pseudocode

• Integer age• print "Your age?"• age := readInput()• print age;

Types

• Java has two kind of types– Primitive Types• int, byte, short, long, double, float, boolean, char

– Class Types• Everything else, for example

– String, Scanner, Arrays, Vector, JButton, JCheckBox

Primitive Types in Java

type size Example value

byte 8 bit 5

short 16 bit 10000

int 32 bit 200000

long 64 bit 30000000

float 32 bit 1.1234

double 64 bit 1.23487367

boolean 1 bit (undefined) true or false

char 16 bit 'a'

Declaring Variables with Java

• Examples– int number;– float weight;– char mycharacter;

• You can declare and set the variable– char mycharacter = 'a';

• You can assign a different value to variable after declaring with the =

Declaring Variables with Java

• Declare variable only once!– int x = 5;– x = 10;– System.out.println(x); // prints 10

• This is wrong!– int x = 5;– int x = 10; // Variable already declared!– System.out.println(x);

Final Variable

• Final variable is a special variable which value cannot be assigned later– final double PI = 3.14;– PI = 5.0; // Does not work!

Examples

• int age, shoeSize;• boolean gender;• char myCharacter = 'k';• double average = 7.7;

TYPE CASTING

Type Casting?class MyApp { public static void main(String [] args) { int a = 5; short b = a; System.out.println(b); }}

TB308POHJUS-L-2:temp pohjus$ javac MyApp.javaMyApp.java:4: possible loss of precisionfound : intrequired: short short b = a; ^1 error

Solution

class MyApp { public static void main(String [] args) {

int a = 5; short b = (short) a; System.out.println(b); }}

Why?class MyApp { public static void main(String [] args) { int a = 5; long b = 5; int result = a * b; System.out.println(result); }}MyApp.java:5: possible loss of precisionfound : longrequired: int int result = a * b; ^1 error

Why?

int a = 5;long b = 5;int result = a * b;

int * long -> long!

Example Result of different Calculations

Operand Operator Operand Result

int + / * - int int

long + / * - int, short, long, byte

long

double + / * - float double

double + / * - double double

float + / * - float float

double + / * - int, short, long, byte

double

What is the result?

double a = 5;int b = 5;double result = a / b;

double / int -> double

What is the result?

int a = 5;int b = 5;double result = a / b;

int / int -> int !!!

Solution

int a = 5;int b = 5;double result = (double) a / b;

VISIBILITY OF VARIABLES

What is the problem?import java.util.Scanner;

class MyApp { public static void main(String

[] args) { Scanner input = new

Scanner(System.in); int inputVariable; inputVariable =

input.nextInt(); if(inputVariable == 7) { int myVariable = 80; }

System.out.println(myVariable); }}

TB308POHJUS-L-2:temp pohjus$ javac MyApp.java

MyApp.java:15: cannot find symbolsymbol : variable myVariablelocation: class MyApp System.out.println(myVariable); ^1 errorTB308POHJUS-L-2:temp pohjus$

Braces and Variable visibility• Variable is visible in it's section(braces) and it's child sections• Variable is not visible outside of it's section.• This works:

int a = 1; if(true) { System.out.println(a); }

• This does not: if(true) { int b = 1;}System.out.println(b);

Braces?

• If control statement (if, while, for) contains only one statement you do NOT have to use braces

if(something)doSomething

CLASS - TYPES

Types

• Java has two kind of types– Primitive Types• int, byte, short, long, double, float, boolean, char

– Class Types• Everything else, for example

– String, Scanner, Arrays, Vector, JButton, JCheckBox

Differences

Primitive Type• first letter lowercase

– int

• Initialized with value– int x = 0;

• Does not have methods

Class Type• first letter uppercase

– Scanner

• Initialized with new– Scanner x = new Scanner();

• Does have methods– x.nextInt();

About String

• String is a class type with lot of exceptions compared to other class types.

• Usually class types are initialized with new. In String you can initialize also with value– String example = new String("Hello World");– String example = "Hello World";

Class – type: String

String m = "hello";System.out.println(m);int length = m.length();String newVariable = m + " world";System.out.println(newVariable);

Special Characters

• \t = tabulator• \n = enter• \" = "• \' = '

INPUT AND OUTPUT JAVA

Output

• System.out is a stream that normally outputs the data you write to the console

• Has different methods: – print, without enter– println, with enter

• Usage– System.out.println("Hello World!");– System.out.print(5);– System.out.println('a');

Input

• System.in is an stream connected to keyboard input of console programs

• Problem with System.in is that it can only read one byte at a time from the console.

• If you want to read for example whole line of text, you have to use other classes..

Scanner and System.in• Scanner – class and System.in provides easy

access to keyboard input• You need to import the Scanner– import java.util.Scanner;

• You have to define to Scanner, what stream to be used when reading– Scanner sc = new Scanner(System.in);

• After creating the Scanner, you can read user input:– int i = sc.nextInt();

The use of Scannerimport java.util.Scanner;

public class ScannerDemo { public static void main(String [] args) { Scanner scanner = new Scanner(System.in); String name; int age; System.out.println("Your name: "); name = scanner.nextLine();

System.out.println("Your age: "); age = scanner.nextInt(); System.out.println("Your name is " + name); System.out.println("Your age is " + age); }}

Scanner methods

• Scanner reader = new Scanner(System.in);• int i = reader.nextInt();• double d = reader.nextDouble();• boolean b = reader.nextBoolean();• String line = reader.nextLine();

COMMENTING CODE

Commenting code

• Comments in code are intended for other programmers

• Three different kind of comments– One liner– Multiple lines– Javadoc

Example/*

This is my beautiful hello world application.

Made by Jussi

*/class MyApp { public static void main(String [] args) { // This prints Hello World System.out.println("Hello World!"); }}

Javadoc

• Javadoc is a tool for creating documentation from comments.

• Javadoc comments start with /** and the comments may have special attributes

Javadoc Example/*** Class that provides functionality for printing* the "Hello World" String to the console.** @author Jussi Pohjolainen* @version 2009-10-26*/public class MyApp { /** * Starting point for the app * * @param args command line arguments */ public static void main(String [] args) { // This prints Hello World System.out.println("Hello World!"); }}

Result

More Examples

• Javadoc slides– http://home.tamk.fi/~pohjus/java/lectures/javado

c.html• Java ME Project Works– http://koti.tamk.fi/~t4hheina/mobiili1/– http://koti.tamk.fi/~c5msalo/scorchedtamk/– http://koti.tamk.fi/~c6tkoris/mobile/project/– http://koti.tamk.fi/~c7msorvo/TsunamiGame/ind

ex.html

IF, SWITCH, WHILE, DO-WHILE, FOR

If

if(something) { doSomething;}

if else

if(something) { doSomething;} else { doSomethingElse;}

if else if

if(something1) { doSomething1;} else if(something2) { doSomething2;}

if else if else

if(something1) { doSomething1;} else if(something2) { doSomething2;} else { doSomething3;}

if else if else if else

if(something1) { doSomething1;} else if(something2) { doSomething2;} else if(something3) { doSomething3} else { doSomething4;}

Intro to Switch Case

int a = 1; if(a == 1) { System.out.println("you gave one");

} else if(a == 2) { System.out.println("you gave two");

}

Switch Case (same than previous)

switch(a) { case 1: System.out.println("you gave one");

break; case 2: System.out.println("you gave two");

break; }

Switch Case switch(a) { case 1: System.out.println("you gave one"); break; case 2: System.out.println("you gave two"); break; default: System.out.println("You did NOT give 1 or

2"); }

Switch Case switch(a) { case 1: case 2: System.out.println("you gave one or two"); break; default: System.out.println("You did NOT give 1 or

2"); }

while

int i = 0;while(i < 5) { System.out.println(i); i = i + 1;}

while

int i = 5;while(i >= 0) { System.out.println(i); i = i - 1;}

while to for

int i = 5;while(i >= 0) { System.out.println(i); i = i - 1;}=>for(int i=5; i>=0; i = i – 1) { System.out.println(i);}

Incremental

• i = i + 1;– i++;

• i = i – 1;– i--;

• i = i + 2;– i += 2;

• i = i – 2;– i -= 2;

while to for

for(int i=0; i<5; i++) { System.out.println(i);}

do-while

int i = 0;do { System.out.println("Hello"); i++;} while(i < 3);

EXAMPLES