Programming with Java: the Basics

Programming with Java Jussi Pohjolainen Tampere University of Applied Sciences

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

Combining Conditions if(!rains && (temperature > 20C)) Walk with your t-shirt on Demos Conditions.java BooleanAlgebra.java

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? 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.java MyApp.java:4: possible loss of precision found : int required: 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 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;

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 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; } 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

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 \" = " \' = '

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 Scanner import 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 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!"); } }

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; } 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);

Programming with Java: the Basics

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Programming with Java: the Basics