Programming with Java: the Basics

Programming with Java Jussi Pohjolainen Tampere 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 && B 1 1 1 1 0 0 0 1 0 0 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 && B 1 1 1 1 0 1 0 1 1 0 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 !A 1 0 0 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 {

int a = 5;

short b = a;

System.out.println(b);

}

}

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

MyApp.java:4: possible loss of precision

found : int

required: short

short b = a;

^

1 error

Solution

class MyApp {

int a = 5;

short b = (short) a;

}

}

Why?

class MyApp {

int a = 5;

long b = 5;

int result = a * b;

System.out.println(result);

}

}

MyApp.java:5: possible loss of precision

found : long

required: 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 {

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 symbol

symbol : variable myVariable

location: class MyApp

System.out.println(myVariable);

^

1 error

TB308POHJUS-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;

}

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

= tabulator

= 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

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 Scanner

public class ScannerDemo {

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 {

// This prints 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

*/

// This prints Hello World

}

}

Result

More Examples

Javadoc slides

http://home.tamk.fi/~pohjus/java/lectures/javadoc.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/index.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;

doSomething2;

} else {

doSomething3;

}

if else if else if else

if(something1) {

doSomething1;

doSomething2;

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:

break;

case 2:

break;

}

Switch Case

switch(a)

{

case 1:

break;

case 2:

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) {

i = i - 1;

}

while to for

int i = 5;

while(i >= 0) {

i = i - 1;

}

=>

for(int i=5; i>=0; i = i – 1) {

}

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++) {

}

do-while

int i = 0;

do {

System.out.println("Hello");

i++;

} while(i < 3);

EXAMPLES

Programming with Java: the Basics

0 comments

Notes on slide 1

Favorites, Groups & Events