Algoritmiek

Variables and MethodsGraphics

Hoorcollege 10 - Ma. 6 nov. 2006L.M. Bosveld-de Smet

Variables: associatie (1)

Variables: associatie (2)

Objects: instance variablesinstance methods

Voorbeeld Object: letter ‘a’

Classes en Objects

Variables: kenmerken

Naam Data Type

8 primitive types onbeperkt aantal object of reference types

Bereik (Scope) Levensduur (Lifetime) Initialisatie Declaratie

Variables: verschillende soorten

Local variables

Parameters

Instance variables (non-static fields)

Class variables (static fields)

Variables: overzichtkenmerk local variable /

parameterinstance variable

class variable

plaats van declaratie

vòòr gebruik ergens binnen method , constructor of block; in method-declaratie

ergens binnen class, maar buiten methods;bij voorkeur private

ergens binnen class, maar buiten methods;static en bij voorkeur final

gebruik waarde slechts van belang voor method , constructor of block

waarde van belang voor object’s state en meerdere methods

meestal gebruikt voor constanten

zichtbaarheid onzichtbaar buiten method , constructor of block

zichtbaar voor alle methods in de class; onzichtbaar voor andere classes

zichtbaar voor alle methods in de class; ook zichtbaar buiten de class, als public

levensduur start – exit uitvoer van method , constructor of block

object-aanmaak met new – verdwijnen van referentie naar object

start – exit uitvoer programma

public class variable: voorbeeld

Constante van de Math class (java API)public static final double PI = 3.1415926535…;

public class PrintPI {

public static void main (String[] args){

System.out.println(Math.PI);

}

}

Instance variable: voorbeeld

public class Bicycle {… … …private int speed;… … …public Bicycle (int startSpeed) {

speed = startSpeed;}… … …public int getSpeed (){

return speed;}… … …

}

instance variable

constructor:

Bicycle myBicycle = new Bicycle (10);

getter method:

myBicycle.getSpeed()

public class method: voorbeelden

Methods van de Math class (java API)

public static int round (float a)

public static double sqrt (double a)

public static double random ()

Math.round(5.5)

private class variable / public class method: voorbeelden

public class Bicycle {… … …private int speed;private int id;private static int numberOfBicycles = 0;… … …public Bicycle (int startSpeed) {

speed = startSpeed;id = ++numberOfBicycles;

}… … …public static int getNumberOFBicycles(){

return numberOfBicycles;}… … …

}

class variable

class method:

Bicycle.getNumberOfBicycles

local variables: voorbeeld (1)public class Adding {

public static void main (String[] args) {int next=1;int total = 0;while (next > 0) {

next = Integer.parseInt(SimpleIO.readLine());total = total + next;

}System. out.println("Total is : " + total);

}}

local variables: voorbeeld (2)

while (workToDo (table)) {for (int r = 0; r < table.length; r++) {

for (int c = 0; c < table [0].length; c++) {if (table [r] [c] > 3) {

stamps += 4; table [r] [c] -= 4;

parameter: voorbeeld

public void setFee (double newFee) {

fee = newFee;

}

parameter

doc.setFee (150.00);

Array parameterspublic class WordLists { public static void main(String[] args) String[] wordList1 = {"aap", "hond", "varken", "luipaard",

"bever"}; String[] wordList2 = {"hortensia", "roos", "lelie", "geranium",

"magnolia"}; System.out.println ("Longest word list 1: " +

wordList1[longestWord(wordList1)]); System.out.println ("Longest word list 2: " +

wordList2[longestWord(wordList2)]); }

private static int longestWord(String[] list) { int indexLongestWord = 0; for (int i = 1; i < list.length; i++) { if (list[i].length() > list[indexLongestWord].length()) { indexLongestWord = i; } } return indexLongestWord; }

}

Instance methodspublic class Person {

private String name;

public Person () { name = "no name yet."; }

public Person (String initialName) { name = initialName; }

public void setName (String newName) { name = newName; }

public String getName () { return name; }

public void writeOutput () { System.out.println ("Name: " + name); }

public boolean sameName (Person otherPerson) { return (this.name.equalsIgnoreCase (otherPerson.name)) }

}

Class methods: examples

public static void callByName (String name) { system.out.print (name);}

private static int updateScore (int currentScore, int points) {

return currentScore + points;}

Class var./method: gebruik in class

public class <class_name>

<declaratie class variable(s)>

public static void main (String [] args) {

<main body>

}

<declaratie class method(s)>

}

“helpers”

Class methods als “helpers” “cohesion” “stepwise refinement” “top-down decomposition”

Verdeling van taken

Quiz-programma:

Stel een vraag aan de gebruiker Lees het antwoord van de gebruiker Controleer of het antwoord correct is Geef adequate feedback Hou de score bij

Class “helper” methods private static String readAnswer (String question) { SimpleIO.prompt (question); String userInput = SimpleIO.readLine(); return userInput.trim().toLowerCase(); }

private static boolean match (String userAnswer, String correctAnswer) { return (userAnswer.equals(correctAnswer)); }

private static void giveFeedback (boolean ok, String hurrah, String sorry) {

System.out.println (ok ? hurrah : sorry); }

private static void updateScore (boolean ok) { if (ok) { score += 10; } }

Gebruik van “helpers” in mainimport jpb.*;public class Quiz { private static int score; public static void main(String[] args) { score = 0; String userAnswer = ""; boolean isCorrect = false; userAnswer = readAnswer ("Waarvan is bit de afkorting?\n") ; isCorrect = match (userAnswer, "binary digit") ; giveFeedback (isCorrect, "Goed geantwoord!", "Sorry. Verkeerd

antwoord."); updateScore (isCorrect) ; userAnswer = readAnswer ("Hoeveel bytes komen overeen met 32

bits?\n") ; isCorrect = match (userAnswer, "4") ; giveFeedback (isCorrect, "Het gaat prima zo!", "Wat jammer nu!") ; updateScore (isCorrect) ; System.out.println ("Your total score is " + score + " points."); } <declaration helpers>}

Graphics

Graphics class of AWT package

DrawableFrame object en methoden

import java.awt.*;import jpb.*;

public class <class name> {public static void main (String[] args) {

// create drawable frameDrawableFrame df = new DrawableFrame(<frame title>);df.show() / df.setVisible(true);df.setSize(<width>,<height>);

// obtain graphics contextGraphics g = df.getGraphicsContext();

//make drawing<Graphics methods applied to Graphics object>;

// repaint framedf.repaint();

}}

DrawableFrame- en Graphics-objecten

graphics context

frame / window

line

DrawLine class// draws a line inside a frame

import java.awt.*;import jpb.*;

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

// create drawable frameDrawableFrame df = new DrawableFrame("Line");df.setVisible(true);df.setSize(200,200);

// obtain graphics contextGraphics g = df.getGraphicsContext();

// draw lineg.drawLine(50,50,150,150);

// repaint framedf.repaint();

}}

Vormen tekenen

Gele achtergrondvorm stoplicht

// Draw background of sign

g.setColor(Color.yellow)

int[] xBackground = {0, 100, 200, 100};

int[] yBackground = {100, 0, 100, 200}; g.fillPolygon(xBackground, yBackground,

xBackground.length);

X coordinaten van vorm

Y coordinaten van vorm

kleur van de vorm

5 geneste vierkanten// Draw five nested polygons to form the stripe on the// outer border of the sign

g.setColor(Color.black); int[] xPolygon1 = {5, 100, 195, 100}; int[] yPolygon1 = {100, 5, 100, 195}; g.drawPolygon(xPolygon1, yPolygon1, xPolygon1.length); int[] xPolygon2 = {6, 100, 194, 100}; int[] yPolygon2 = {100, 6, 100, 194}; g.drawPolygon(xPolygon2, yPolygon2, xPolygon2.length); int[] xPolygon3 = {7, 100, 193, 100}; int[] yPolygon3 = {100, 7, 100, 193}; g.drawPolygon(xPolygon3, yPolygon3, xPolygon3.length); int[] xPolygon4 = {8, 100, 192, 100}; int[] yPolygon4 = {100, 8, 100, 192}; g.drawPolygon(xPolygon4, yPolygon4, xPolygon4.length); int[] xPolygon5 = {9, 100, 191, 100}; int[] yPolygon5 = {100, 9, 100, 191}; g.drawPolygon(xPolygon5, yPolygon5, xPolygon5.length);

Tekenen van stoplicht

// Draw a black rectangle (the traffic signal)

g.fillRect(77, 35, 46, 130);

// Draw red,yellow,and green circles (the traffic

// lights)

g.setColor(Color.red);

g.fillOval(83, 43, 34, 34);

g.setColor(Color.yellow);

g.fillOval(83, 84, 34, 34);

g.setColor(Color.green);

g.fillOval(83, 125, 34, 34);

Tonen van tekst

Monospaced, Bold, 30

SansSerif, Italic, 30

Serif, Plain, 40

Toon tekst: programmaimport java.awt.*;import jpb.*;

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

// create drawable frameDrawableFrame df = new DrawableFrame("Wirth's RULE");df.setVisible(true);df.setSize(350,200);

// obtain graphics contextGraphics g = df.getGraphicsContext();

// display text in red monospaced bold fontg.setColor(Color.red);g.setFont(new Font("Monospaced", Font.BOLD, 30));g.drawString("DATA STRUCTURES + ", 15, 50);

// display text in green sansserif italic font fontg.setColor(Color.green);g.setFont(new Font("SansSerif", Font.ITALIC, 30));g.drawString("ALGORITHMS = ", 15, 100);

// display text in blueserif plain fontg.setColor(Color.blue);g.setFont(new Font("Serif", Font.PLAIN, 40));g.drawString("PROGRAMS", 15, 150);

// repaint framedf.repaint();

}}