imperative programming nvon neumann’s computer model: computer = + memoryprocessor to store values...

Imperative programming

Von Neumann’s computer model:

computer=+

memory processor

to store values

to execute instructions

Instructions

Instruction:direction to modify memory

Program:long sequence of instructions,which are executed one by oneby the processor

instructions are stored in memory, too, but don’t modify themselves

Programming languages

Programminglanguage:notation for programs

since 1945:3000 different languages published

History of programming languages

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

Assembler

Mach.lang.

Fortran

Algol

Basic

Pascal

Simula

C

C++

Java

Cobol

SQL

Lisp

Prolog

Haskell

FunctionalProgramming

ImperativeProgramming

Databases(SCI251)

Memory capacity

1965s: 8 kB RAM punched cards 1970s: 16 kB RAM tape 1980s: 64 kB RAM floppy 1985s: 640 kB RAM 1990s: 4 MB RAM harddisk 1995s: 16 MB RAM CD 2000s: 128 MB RAM DVD

Problems associated with programming

in olden times:how can memory be utilized efficiently?

now:how can memory be understood in full?

Order out of chaos

person

club

league

federation

county

province

state

country

unionunion

employee

department

division

company

holding

lecturer

chair

department

faculty

university

Memory structure

named memory location

group of variablesbelonging together

variable

object

object group of objects

belonging together

Program structure

direction to change memory

group of instructions belonging together

statement

method

class group of methods

belonging together

Programming paradigms

Imperative

Procedural

Object- oriented

Fortran Basic

Pascal C

C++ Java

Declarative

Functional

Logic

Lisp

Haskell

Prolog

Assembler

Algol

Simula

Excel

Java why?

Imperative and procedural and object oriented

Fun to learn Extensive libraries

also for window programs Cool : programming for the Internet

Machine language

Instructions are specificfor a target processor

Translating 1/4: Assembler

An assembler translates machine-specific source code to object code,which is executed subsequently

assembler

assembler

Higher programming language

Single program can be translated for many different processors

Source code also intelligible for human writer/reader

Translating 2/4: Compiler

A compiler translates source code to machine specific object code,which is executed subsequently

compiler

compiler

Translating 3/4: Interpreter

A processor sepcific interpreterreads source code and executes it

interpreter

interpreter

Translating 4/4: Compiler+Interpreter

A universal compiler translates source code to byte code, which can easily be interpreted

compiler

interpreter

interpreter

Implement

Programming cycle

Edit

Compile

Run

Specify

Model

Java program types

Java Appletprogram “lives” on a WWW-page

Java Applicationprogram has window of its own

Javascriptprogram assists in web page construction

JavaScript is not Java!

Java versus JavaScript

Imperative Can be Internet based Object oriented Compiled byte code In fixed rectangle Extensive method

libraries For programmers

Imperative Internet based “Only” procedural Interpreted directly Influences browser Fixed number built-

in methods For web designers

Source code structure

Statements to modify memory

Statements are grouped in methods

Methods are grouped in classes

which might be calls to yet other methodsso “where am I” tracking might grow complicated!

Statement forms

Assignment -statement:modify memory

Call of another method:first execute statements of that method, then continue were you were

Object orientation

Object:part of memory that belongs together

Object oriented programming:methods involves (mostly) an objectmentioned at the call

Example Java program

one class

...with onemethod

...with onestatement

braces delimitclass and method

import java.awt.Graphics;import java.applet.Applet;

public class Hello extends Applet{ public void paint (Graphics g) { g.drawString(“Hello!”, 20, 20); }}

Class and method header

public:may be used from outside

name:determined byprogrammer



Statement: method call

statement:method call

method namealwaysa dot

otherdetails



object involved

Communication with browserclass is

extension oflibrary class

Applet

... which comesin useful!

browser callspaint method,

providing aGraphics object ...



Class libraries

library classesmay be used...

if they areimported,

mentioning theirpackage

import java.awt . Graphics;import java.applet . Applet;


we get a parameter

Method header and call

we providethree parameters

methodheadermethod

call



WWW-page with Applet

special HTML-tag

<APPLET>

byte code which resultsfrom compiling Hello.java

<HTML>Here is a simpleapplet: 

<APPLET code = Hello.class width = 100 height= 50 ></APPLET></HTML>

Embedding an Appletimport java.awt.Graphics;import java.applet.Applet;


Hello.java

<HTML>Here is a simpleapplet: 

<APPLET code = Hello.class width = 100 height= 50 ></APPLET></HTML>

Hello.html

compiler

Hello.class

SDK: Software Development Kit

Sun’s Java SDK

compiler:javac

bytecode-interpreter:

appletviewer

Integrated Development Environment: JCreator

JCreator: New File

JCreator: syntax highlighting editor

JCreator: Error messages

JCreator: Run applet

JCreator: set options

Run applet in Explorer

JCreator: JDK help

Summary

Imperative programming:program consists of statements,modifying memory

Object oriented programming:variables belonging togetherare grouped in objects

Summary

Statement :direction to modify memory

Method :named group of statements

Class :named group of methods

Summary

Translating higher language (source code)

to machine language (object code)

Assembler Interpreter Compiler Compiler - bytecode - interpreter

Summaryimport java.awt.Graphics;import java.applet.Applet;

public class Hallo extends Applet{ public void paint (Graphics g) { g.drawString(“Hallo!”, 20, 20); }}

Hello.java

<HTML>Hier is een simpelapplet: 

<APPLET code = Hallo.class width = 100 height= 50 ></APPLET></HTML>

Hello.html

compiler

Hello.class

we get a parameter

Summary

we providethree parameters

method-headermethod-

call


public class Hallo extends Applet{ public void paint (Graphics g) { g.drawString(“Hallo!”, 20, 20); }}

Class and method header

method fromclass Graphics

other methodsfrom Graphics



g.drawLine(10,20,50,60);g.fillRect (70,30,20,20);

Methods from Graphics

drawString drawLine drawRect drawOval fillRect fillOval setColor

hello

x

y

determines color of subsequent drawing

Classes and objects

Class describesobject capability

Object can do andremember things

Class also provides constants

... (Graphics g)

{ g.drawLine(10,20,50,60); g.fillRect (70,30,20,20); g.setColor( Color.blue ); g.fillRect (30,50,20,20);

g.setColor( Color.blue );}

method fromclass of object g

Colors

redgreenyellow

blue

cyan magenta

white

black

Color .

Drawing

g.setColor( Color.white );g.fillRect(0,0,200,100);g.setColor( Color.black );g.fillRect(10,0,10,100);g.fillRect(50,0,10,100);g.fillRect(90,0,10,100);g.fillRect(0,40,200,10);g.fillRect(0,70,200,10);g.setColor( Color.blue );g.fillRect(0,50,10,20);g.setColor( Color.red );g.fillRect(100,0,100,40);

Composition with blue and red (freely after Piet Mondrian)

Modifying the drawing

Correction of bar width

Correction of picture height

Correction of picture width

g.setColor( Color.white );g.fillRect(0,0,200,100);g.setColor( Color.black );g.fillRect(10,0,10,100);g.fillRect(50,0,10,100);g.fillRect(90,0,10,100);g.fillRect(0,40,200,10);g.fillRect(0,70,200,10);g.setColor( Color.blue );g.fillRect(0,50,10,20);g.setColor( Color.red );g.fillRect(100,0,100,40);

lots of work ...

Variables

g.fillRect(10, 0, 10, 100);g.fillRect(50, 0, 10, 100);g.fillRect(90, 0, 10, 100);g.fillRect(0, 40, 200, 10);g.fillRect(0, 70, 200, 10);

g.fillRect(10, 0 ,bar, height);g.fillRect(50, 0, bar, height);g.fillRect(90, 0, bar, height);g.fillRect(0, 40, width, bar);g.fillRect(0, 70, width, bar);

bar = 10;width = 200;height = 100;

int bar, width, height;

use of variables

assignmentstatements:variables get

a value

declaration:announcement

of variablesand their type

Method call

Assignment statement

Statement forms

object . method ( parameters ) ;

variable = value ;

becomes

Declarations

Declaration structure

zero or more

int: whole numbers double: floating point numbers class name : object variables

type variabele , variabele ;

Variables

g.fillRect(x1, 0 ,bar, height);g.fillRect(x2, 0, bar, height);g.fillRect(x3, 0, bar, height);g.fillRect(0, y1, width, bar);g.fillRect(0, y2, width, bar);

x1=10; x2=50; x3=90;y1=40; y2=70;

int x1, x2, x3, y1, y2;

g.setColor( Color.blue);g.fillRect(0, y1+bar, x1, y2-y1-bar );

x1 x2 x3

y1

y2

Expression:program phrase having a value

Use of expressions: Right hand side of assignment

statement

Method parameter

Expressions

x3 = x2+40 ;

g.fillRect(0, y1+bar, x1, y2-(y1+bar) ) ;

expressie

Expression structure

constant

variable

... operator ...

( ... )

5

hoogte

x + y

(y + 1)

+ 1

x -

expressionexpression

Operators

+ addition - subtraction * multiplication / division % remainder after division

division of int values:result is truncated!

14 / 3 == 4 14 == 4*3 + 214 % 3 == 2

Operator precedence

Multiplicationprecedes addition

Equal precedence:left to right

Want it otherwise?place parentheses!

1 + 2 * 3 == 7

and divisionand subtraction

10 - 5 - 2 == 3

(1+2) * 3 == 9

Program

asdimport java.awt.Graphics;import java.awt.Color;import java.applet.Applet;

public class Mondri extends Applet{ public void paint(Graphics g) { int breedte, hoogte, balk, x1, x2, x3, y1, y2; breedte = 200; x1 = 10; x2 = 50; x3 = 90; hoogte = 100; y1 = 40; y2 = 70; balk = 10; g.setColor(Color.white); g.fillRect(0, 0, breedte, hoogte);

g.setColor(Color.black); g.fillRect(x1, 0, balk, hoogte); g.fillRect(x2, 0, balk, hoogte); g.fillRect(x3, 0, balk, hoogte); g.fillRect(0, y1, breedte, balk); g.fillRect(0, y2, breedte, balk);

g.setColor(Color.blue); g.fillRect(0, y1+balk, x1, y2-(y1+balk) );

g.setColor(Color.red); g.fillRect(x3+balk, 0, breedte-(x3+balk), y1); }}

/* This applet displays a Mondrian like "composition with red and blue”*/

// position of lines

// background

// black bars

// colored fields

with comments

Comments

Intended for human readerignored by compiler

Two notations: from /* to */ from // to line end

Drawing

g.drawRect(20,60,40,40);g.drawLine(20,60,40,40);g.drawLine(40,40,60,60);

g.drawRect(70,60,40,40);g.drawLine(70,60,90,40);g.drawLine(90,40,110,60);g.drawRect(120,40,60,60);g.drawLine(120,40,150,10);g.drawLine(150,10,180,40);

public void paint(Graphics g){

}

big mess ofcoordinates!

New methods

Method:named group of statements

method call executesits body

private void drawHouse(…){ … . drawRect (…); … . drawLine (…); … . drawLine (…);}

public void paint(Graphics g){ … . drawHouse (…); … . drawHouse (…); … . drawhouse (…);}

private void drawHouse(…){ … . drawRect (…); … . drawLine (…); … . drawLine (…);}

public void paint(Graphics g){ … . drawHouse (…); … . drawHouse (…); … . drawHouse (…);}

Parameters

which Graphics objectis involved?

declaration of extra

parameter

private void drawHouse (Graphics gr, …){ gr . drawRect (…); gr . drawLine (…); gr . drawLine (…);}

pass extraparameter

public void paint(Graphics g){ … . drawHouse (g, …); … . drawHouse (g, …); … . drawHouse (g, …);}

public class Houses extends Applet{

}



The object this

which ??? objectis involved?

private void drawHouse (Graphics gr, …){ gr . drawRect (…); gr . drawLine (…); gr . drawLine (…);}public void paint(Graphics g){ this . drawHouse (g, …); this . drawHouse (g, …); this . drawHouse (g, …);}

which Houses objectis involved?

this: the objectalready involved

with paint

public void paint(Graphics g){ this . drawHouse (g, …); this . drawHouse (g, …); this . drawHouse (g, …);}

More parameters

draw threeslightly different

houses

even moreparameters!

public void paint(Graphics g){ this . drawHouse (g, 20,100, 40); this . drawHouse (g, 70,100, 40); this . drawHouse (g, 120,100, 60);}

Parameters

(x, y-w) (x+w,y-w)

(tx,ty)

(x,y)w

private void drawHouse (Graphics gr, … ){

gr . drawRect (…); gr . drawLine (…); gr . drawLine (…);}

private void drawHouse (Graphics gr, int x, int y, int w){ int tx, ty; tx = x + w/2; ty = y - w - w/2; gr . drawRect (x, y-w, w, w); gr . drawLine (x, y-w, tx, ty); gr . drawLine (tx, ty, x+w, y-w);}

private void drawHouse (Graphics gr, int x, int y, int w){ int tx, ty; tx = x + w; ty = y - w – w/2; gr . drawRect (x, y-w, w, w); gr . drawLine (x, y-w, tx, ty); gr . drawLine (…);}

private void drawHouse (Graphics gr, int x, int y, int w){

gr . drawRect (x, y-w, w, w); gr . drawLine (…); gr . drawLine (…);}

private void drawHouse (Graphics gr, int x, int y, int w){

gr . drawRect (…); gr . drawLine (…); gr . drawLine (…);}

Flexibility

The more parameters, the more flexible the method

(x,y)w

h

roofh

private void drawHouse(Graphics gr, int x, int y, int w, int h, int roofh, Color c, Color roofc, … ) {

and the morecumbersome to call...

Flexibility

Many parameters: hard to program

now easy to extend later

Few parameters: easy to prgram

now might be hard to

extend later

Communication among methods

Parameters :caller passes value to method

Method result :methods returnsvalue to caller

like amathematical

function

Methods with result

private int square (int x){ return x*x ;}

result type

value of resultin special return statement

Methods with result

first, somestatements

private int succesorsThirdPower (int x){ int s; s = x+1; return s*s*s ;}

Call of a void-methodis a statement

Method calling

g.drawString(“hello”, 10, 10) ;

x = this.square (5) ; … this.square (5) + 1 …

withoutresult

Call of a method with a resultis an expression

Expressions Constant Variable Operator expr. Expr. in parentheses

Call of methodwith a result

Statements Assignment

Call of void method

Return statement

Summary of constructs

x=5;

this.drawHouse (…);

return x*x;

5

xx+1

(x+1)

this.square(5)

Summary chapter 2

Statement :directions to change memory

Method :named group of statements

Class :named group of methods

Summary chapter 3+4

Calling methods in Graphics: drawLine, fillRect, setColor

Defining new methods e.g: drawHouse

Passing parameters to methods to make methods more flexible

Summary chapter 3+4

Variables to temporarily store values declaration states type, e.g. int, double

Expression calculation resulting in a value uses constants, variables, operators

Expression Constant Variable Operator expr. Expression in parenth.

Calling a method with a result

Statements Assignment

Calling a void method

Return statement

Summary chapter 3+4

x=5;

this.drawHouse (…);

return x*x;

5

xx+1

(x+1)

this.square(5)

Methods and objects

gr . drawString (“Hi”, 50, 50) ;

x = Math . sqrt(2.0) ;

method fromclass Graphics

Graphics objectinvolved

method fromclass Math

no object involved

static method

Parameters

not the parameter of paint !should always be a Graphics-object

Parameters make a method more flexible:it can be used in various situations

Parameters make an applet more flexible: it can be used in various situations

then what?

allowed: method of Greeting and Applet

Class extensions inherit methods

yielding a text String

public class Greeting extends Applet{ public void paint(Graphics g) { String person, greeting;

person = this . getParameter(“name”); greeting = “Hello ” + person + “!”; g.drawString(greeting, 50, 20 ); }}

public class Greeting extends Applet{ public void paint(Graphics g) {

this . getParameter(“name”);

}}

public class Greeting extends Applet{ public void paint(Graphics g) { String person;

person = this . getParameter(“name”);

}}


}}


this . ???

}}

like this one:

Passing parameters to Applets

<APPLET code=Greeting.class width=100 height=100>

</APPLET>

<PARAM name=name value=Jeroen>

as specifiedwhen calling

getParameter

will be the resultof the call to

getParameter

String

String object contains text Ways to obtain a String object:

constant calling a method yielding a String

calling an operatoryielding a String

this.getParameter (“name”)

“Hello” + person

“Hello”

not addition,but concatenation

Numbers versus texts

int x; x = 37;

String s; s = “37”;

x = 12 + 5; s = “12” + “5”;

17 “125”

x = Integer.parseInt(s); s = “” + x ;

conversion methodconcatenation operator

converts second operand to a String

whenever the first is

Applet with int parameters

public void paint (Graphics gr){

}

String text1, text2;

text1 = this.getParameter(“length”);text2 = this.getParameter(“width”);

length = Integer.parseInt(text1);width = Integer.parseInt(text2);gr.drawString(“perimeter: ” + 2*(length+width), 0, 0 );

int length, width;

Passing parameters toan Applet

<APPLET code=Rectang.class width=100 height=100>

</APPLET>

<PARAM name=length value=12><PARAM name=width value=8>

<APPLET code=Rectang.class width=100 height=100>

</APPLET>

<PARAM name=length value=7><PARAM name=width value=3>

Primitive versus Objecttypes types

int i; double d;

String s; Color c;

i

d

s

c

i = 5; d = 3.1415926;

s = “hello”; c = Color.blue;

3.1415926

5

object variablescontain references

How to obtain objects

String constants Constants in a class Result of a method Result of an operator Home made

“hello”

Color.blue

this.getParameter(“a”);

“total: ” + sum

new Color(0,0,0)

constructor method

Constructor method

Call of a constructor-methode: makes new object in memory treats it immediately returns the newly created object

new Color(0,0,0)

calling a constructoris an expression

Color c;c = gr.setColor( );

Colors

(255,0,0)(0,255,0)(255,255,0)

(0,0,255)

(0,255,255) (255,0,255)

(255,255,255)

(0,0,0)

Color(...)

Mixing colors

new Color(255, 0, 0)

new Color(255, 255, 0)

new Color(255, 64, 0) new Color(255, 128, 0) new Color(255, 192, 0)

Creating new objects

Color c;c = new Color(255,128,0);

Button b;b = new Button(“press here”);

ScrollBar s;s = new ScrollBar(...);

objects for interactionwith the user

Methods of Applet

paint

getParameter

init

add repaint

are calledby the browser

may be called from within the program

redefine inyour program!

Applet’s init and paint

init:called by the browserwhen the applet starts first

paint:called by the browsereach time the applet needs painting

just after init , but alsowhen a window re-appears

Applet’s add and repaint

add:to add interaction objects

repaint:to force an extra paint

typically: from init

Example: color mixerScrollBar Button

Using add

public class Mixer extends Applet{

public void init( ) {

} public void paint(Graphics gr) {

}

s = new ScrollBar(...);this.add( s );

.... s . getValue() ....

ScrollBar s; object-variable:is part of

this object

and therefore may be used inevery method!

ScrollBar s ;

Reacting on user actions

Event: action of the user click button slide scrollbar enter text in textfield ...

Event listener: object that reacts upon that

adding an Event listener to an interaction object

public class Mixer extends Applet{ Scrollbar s;

public void init( ) { s = new Scrollbar(...); this.add( s );

}

s . addAdjustmentListener (...);

the object that will be notifiedwhen the user adjusts scrollbar s

public class ...{ public void adjustmentValueChanged ( AdjustmentEvent e ) { // statements which will be executed // in response to user actions }}

Notification mechanism

implements AdjustmentListener

promise that adjustmentValueChanged will indeed be defined

public class Mixer extends Applet

{ ScrollBar s;

public void init( ) { s = new ScrollBar(...); this.add( s ); s . addAdjustmentListener ( ); }

Which event listener to use?

public void adjustmentValueChanged (AdjustmentEvent e){

}

Mixer object isevent listener of

its own scrollbar!

implements AdjustmentListener

this

// here you may use s and this

ScrollBar

public void init ( ){ s = new ScrollBar(...); this . add( s );

}

public void adjustmentValueChanged (AdjustmentEvent e){ this . repaint( );} public void paint (Graphics gr){

gr . fillRect( 0, 0, 100, 100 );}

int x; x = s . getValue( );gr . setColor( new Color(x, ..., ...) );

s . addAdjustmentListener (this);

Reacting on events

Interaction objectswith event listeners

ScrollBar s;s.addAdjustmentListener(this);

implements AdjustmentListenerpublic void adjustmentValueChanged (AdjustmentEvent e){...}

Button b;b.addActionListener(this);

implements ActionListenerpublic void actionPerformed (ActionEvent e){...}

Complete Color mixer

public class Mixer extends Applet implements ActionListener, AdjustmentListener { Scrollbar red, green, blue; Button black;

public void init ( ) {...} public void paint (Graphics gr) {...}

public void adjustmentValueChanged (...) {...} public void actionPerformed (...) {...}}

class Mixer: outline

object variables

redefiningApplet-methods

fullfilling promisesmade by implements

public void init ( ){ red = new Scrollbar(0,0,1,0,255); green = new Scrollbar(0,0,1,0,255); blue = new Scrollbar(0,0,1,0,255); black = new Button(“Black”); this . add(red); this . add(green); this . add(blue); this . add(black); red . addAdjustmentListener (this); green . addAdjustmentListener (this); blue . addAdjustmentListener (this); black . addActionListener (this);}

Mixer’s method init

creation ofinteraction

objects

building theuser interface

addition ofevent listeners

Mixer’s method paint

local declarations

getscrollbarpositions

draw coloredrectangle

public void paint (Graphics gr){ int rv, gv, bv;

rv = red . getValue(); gv = green . getValue(); bv = blue . getValue();

gr . setColor ( new Color(rv,bv,gv) ); gr . drawRect ( 20,60,260,220 );}

gr . drawString( rw+“ ”+gw+“ ”+bw );

public void adjustmentValueChanged (AdjustmentEvent e){ this . repaint( );}

Mixer’s Event-listenters

reation toScrollBar

reaction toButton

public void actionPerformed (ActionEvent e){ red . setValue(0); green . setValue(0); blue . setValue(0); this . repaint( );}

Summary Declarationsobject variable

parameter

local variable

public class Mixer ...{ Scrollbar red;

public void paint ( Graphics gr ) { int rv; rv = red . getValue( ); }}

permanently,assigned to in init

for communication,assigned to at call

temporarilyassigned to with an

assignment

Applet init paint getParamet

er add repaint

Summary AWT-classes

Graphics drawString fillRect setColor

Color String

ScrollBar getValue setValue

Button

ActionListener actionPerformed

AdjustmentListener adjustmentValueChanged

Summary chapter 4

Objects existing: parameter or constant result of method or operator home-made using new

Methods “involve” an object except: static methods

Summary chapter 5

String constants: “hello” operator + concatenates string-objects conversion from and to int:

s = n + “”;n = Integer.parseInt(s);

Color constants: Color.blue home made: new Color(r,g,b)

Summary chapter 5

Parameters for the entire Applet:

class Greeting extends Applet{ void paint(Graphics gr) { person = this.getParameter(“name”); }}

<APPLET code=Greeting.class width=100 height=100> <PARAM name= name value=Jeroen></APPLET>

Summary chapter 6

Applet-methods are inherited byextension classes Intended to call

getParameterrepaint

Intended to re-defineinitpaint

for creation ofinteraction objects

Class challange

How to make interaction componentsand how to react on their use?

Write a program showing a Scrollbar,and beneath it a numerical representationof its value

now!approx 5 min.

peeking at ch. 6 is allowed,but try first without that!

public class Interact extends Applet

{

}

Interaction

public void init(){ s = new Scrollbar(0,0,1,0,100); this . add(s);}public void paint(Graphics gr){ gr.drawString( s.getValue()+””, 50,50);}public void adjustmentValueChanged(...){ this . repaint();}

Scrollbar s;

implements AdjustmenListener

s.addAdjustmentListener(this);

while ( x<1000 ) x = 2*x;

public void paint (Graphics gr){ int x; x = 1;

gr . drawString( “” + x, 10, 10);}

Repeating statements

the body isexecuted over

and over

while thecondition

holds...

X 12481632641282565121024

Repeating multiple statements

int x, t;x=1; t=0;while ( x<1000 ){ x = 2*x; t = t+1;}

return t;}

n

private static int twoLog (int n){

braces pack twostatements into

a compound one

counter countsthe numberof doublings

Repetition with a counter

public void paint (Graphics gr){ int t; t=0; while ( t<10 ) { gr.drawString( “:-)”, 0, 20*t ); t = t+1; }}

counter controlsnumber of iterations

and comes inuseful

Class challenge

Write a method power having two parameters base (real number) exponent (natural number)

calculating

baseexponent

while ( t<n ){ t = t+1 ;}

t = 0;

private static double power (double x, int n){

}

Power method

return result;

result = result * x ;

result = 1;int t; double result;

Comparison operators

< less than <= less than or equal > bigger than >= bigger than or equal == equal to != not equal to

x=5 x becomes 5 !x==5 is x equal to 5 ?

Expressions

Expression with numeric value

Expression with text value

Expression with truth value

2 * (length + width)

“Hello ” + person

counter < exponent

type int

type boolean

type String

primitive

object-

primitive

Boolean expressions

Comparing values

Combining other Boolean expressions using logic operators && and || or ! not

x <= y

x<0 && y>0

! (x==0 && y==0) x!=0 || y!=0

George Boole

(1815-1864)

Constants

Constants with numeric value

Constants with text value

Constants with truth value

2

“Hello”

true

-5 137

“@#$%” “123” “”

false

“true”

Using Boolean expressions

As condition of a while statement

As right hand side of an assignment

As method result

while (x<10) ...

boolean b; b = x<10 ;

return x<10 ;

Class challenge

Write a method divisible , which determines whether x is divisible by dprivate static boolean divisible (int x, int d){

}

x%d == 0return ; truth valueas a result

Class challenge

Write a method smallestDivisor , whichdetermines the smallest divisor of xprivate static int smallestDivisor (int x){

}

int d;d = 2;

while ( ! divisible(x,d) ) d = d+1;

return d ;

try them all,one at a time

Changing variables

Shorthand notations:

x = x * 2 ;

old valuebecomesnew value

x = x * 2 ; x *= 2 ;

x = x + 5 ; x += 5 ;

x = x + 1 ; x += 1 ; x ++ ;

is multiplied by

is incremented with

is incremented

Repetition with a counter

int t ;

t = 0;while ( t<x )

t ++ ;}

{ // do something // useful with t

for ( t=0 ; t<x ; t++ )for

for-statementinitialisation

int t ;for ( t=0 ; t<x ; t++ )

condition tocontinue

step to next

gr.drawString( “#”, 0, t*10 );

for ( t=1 ; t<=x ; t++ )

for ( t=x ; t>=0 ; t -- )

for ( t=2 ; !divisible(x,t) ; t++ )

Repeated repetition

Body of a for-statement is a statementwhich may be a for-statement itself!

int x, y;for (y=0; y<10; y++) for (x=0; x<10; x++) gr.drawString( “+”, 10*x, 10*y );

y

+ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + +

++ ++ + ++ + + ++ + + + ++ + + + + ++ + + + + + ++ + + + + + + ++ + + + + + + + +

Extreme cases

Zero repetitions

for (y=0; y<0; y++) gr.drawString(“hi”, 10, 10);

Infinitely many repetitions

while (true) audio.play( );

gr.drawString(“hi”, 10, 10); this statmentwill never be

executed!

Unintensional infinity

x=1;counter = 0;while (counter<10) x = x*2; counter = counter+1;{

}

forgot thebraces...

Compiler messages

Error messages variabele not declared missing semicolon ...

Warnings unreachable code statement has no effect while-statement doesn’t terminate

this warning istheoreticallyimpossible!

The Halting-problem is unsolvable

boolean stops (String filename){ ....}

void clever (String filename){ while ( stops(filename) ) x++;}

Ha! What about this?

Not so!This method

solves it:

stops( “Clever.java” )

what is thevalue of:

?

The Halting-problem is unsolvable

void clever (String filename){ while ( stopt(filename) ) x++;}

stops( “Clever.java” )

Two possibilities: “Clever” does stop

“Clever” doesn’t stop

...so stops returns true

...so clever hangs!

...so stops returns false

...so clever stops immediately!

contradiction!

thus, stops is impossible to write

Other Halting -like problems

Will a statement be executed? Is a program a virus?

but what about existing virus scanners? either too strict: false alarms or too generous: some viruses passed

irritating

dangerous

Example: interst rates

TextField -objectenfor user interaction

TextField-objectspossess an

ActionListener

public class Interest extends Applet implements ActionListener{ TextField startT, renteT;

public void init ( ) {...} public void paint (Graphics gr) { ... }

public void actionPerformed (...) { this.repaint(); }}

Outline of class Interest

object-variabels

re-defining Applet-methods

fulfillingimplements promise

public void init ( ){ startT = new TextField(“100”, 8 ); rateT = new TextField(“5”, 4 );

this . add(startT); this . add(rateT);

startT . addActionListener (this); rateT . addActionListener (this);}

Interest ’s method init

creation ofinteraction

components

setup ofuser interface

addition ofevent listeners

public void paint (Graphics g){

}

Interest ’s method paint

startT.getText()rate.getText()

start = Integer.parseInt( );rate = Integer.parseInt( );

for (year=0; year<=10; year++){ gr.drawString( “year:”+year + “amount:”+captial, ... );

}

capital = start;

captial *= (1 + 0.01*rate) ;

int start, rate, year; double capital;

Class challenge

Write a method root , calculating the square root of a non-negative double

Without using Math.sqrt !

Use Newton approximation:when y approximates sqrt(x),then the average of y and x/yis a better approximation

private static double sqrt (double x){

}

Sqrt method

double y;y = 1;

return y;

y = (y + x/y) / 2 ;while ( y*y != x )

does it stop?

mathematician:no, equality hold only in limiting case

programmer:yes, precision islimited

computer scientist:no, roundof errorsprohibit reaching astable state

while ( Math.abs (y*y - x) <0.000001 )

safer:

Structure of repetition

Initialisation

Repeated improvement

Use of result

captial=start; y=1; total=0;

captial*=(1+0.01*rate); y=(y+x/y)/2; total+=x;

gr . drawString (“”+captial);

return y;

sb . setValue ( total );

DrawString or return?

gr.drawString (x, ...)result immediately visible

return x;result is returned to caller,who can decide what to do with it

more flexibleuseful for

for “simple” results

useful for “complicated” layout

Program constructions

Statements Assignment Call void-methode return-statement while- statement for- statement { ... } - grouping

Expressions Constant Variable Call methode Expression with

operators Expression with parenth. new-expression

to do to calculate

Program constructions

Declaration Local variable

for temporary use Object-variable

permanent part of object

Parameterfor passing values to methods

Type Primitive type

int double boolean

Object-type Color, String etc. TextField, Button etc. Applet, Hallo, etc.

states typeof variables

set of possible valuesof an expression

Summary ch.7

while-statement: repeating statements

while (x<1000) x = 2*x;

body is repeatedlyexecuted

while condition is valid

Summary ch.7

for-statement: repetition with a counter

int t ;

t = 0;while ( t<x )

t ++ ;}

{ // do something // useful with t

for ( t=0 ; t<x ; t++ )

Summary program constructions

Statements Assignment Call void-method return-statement while-statement for-statement { ... } - grouping

Expressies Constant Variable Call method Expression with

operators Expression with parenth. new-expression

can be done can be calculated

Summary program constructions

Declaration Local variable

for temporary use Object variable

permanently part ofthe object

Parameterto pass values to amethod

Type Primitive type

int double boolean

Object type Color, String etc. TextField, Button etc. Applet, Hello, etc.

indicates variable type

set of possible valuesof a variable

Executing statements conditionally

statement is only executedwhen condition holds

statement is executed over and overwhile condition holds

if (temperature<0) gr.drawString(“It’s freezing!”, 10, 10);

while (temperature<0) temperature += 5;

Two alternatives

statement is only excecutedwhen condition doesn’t hold

if (temperatuur<0) gr.drawString(“It’s freezing!”, 10, 10);else gr.drawString(“It’s thawing.”, 10, 10);

Choice is a statementas any other

if (n%3==0) gr.drawString(n + “divisible”, 10, 10 );else gr.drawString(n + “indivisible”, 10, 10 );

for (n=1; n<20; n++)

*n

*n

if-statements acts asbody of for-statement

one statementas a body,

so no bracesneeded

Multiple-statement body

braces group two statementsinto one

if (temperatuur<0) gr.drawString(“It’s freezing!”, 10, 10); gr.drawString(“Isnt’ that cold?”, 10, 20);

{

}

More than two alternatives

if (age<65) tf.setText(“Full fare”);else tf.setText(“Senior”);

if (age<12) tf.setText(“Railrunner”);

else

if (age<4) tf.setText(“Free”);

else

if-statement as a wholeis body of else-part

if-statement as a wholeis body of else-part

if (age<4) tf.setText(“Free”);else if (age<12)

tf.setText(“Railrunner”);else if (age<65)

tf.setText(“Full fare”);else tf.setText(“Senior”);

tf.setText(“Full fare”);else tf.setText(“Senior”);

tf.setText(“Railrunner”);else if (age<65)

exeption to ruleto indent the body

private String tariff(int age){

}


if (age<4) return “Free”;else if (age<12) return “Railrunner”;else if (age<65) return “Full fare”;else return “Senior”;

if (age<4) return “Free”;if (age<12) return “Railrunner”;if (age<65) return “Full fare”;return “Senior”;

return-statementends method body

Button b;public void init ( ){ radius = 100; b = new Button(“grow”); this.add(b); b.addActionListener(this);

Drawing circlesclass Cirkel extends Applet{

}

public void paint(Graphics gr){ gr.fillOval(150-radius,150-radius,2*radius,2*radius);}

int radius;public void init ( ){ radius = 100;

}public void actionPerformed(ActionEvent e){ radius += 10; this.repaint();}

implements ActionListener

Drawing circlesclass Cirkel extends Applet{

int radius;Button small, big;

public void init ( ){ radius = 10; small = new Button(“shrink”); this.add(b); big = new Button(“grow”); this.add(b); small.addActionListener(this); big.addActionListener(this);}


Drawing circles

public void actionPerformed(ActionEvent e){ if ( e.getSource() == small && radius>10 ) radius -= 10; if ( e.getSource() == big && radius<150 ) radius += 10;

this . repaint();}

equality ofButton-object-references)

public void actionPerformed(ActionEvent e){ if ( e.getSource() == small ) radius -= 10; if ( e.getSource() == big ) radius += 10;

this . repaint();}

Secret drawingclass Secret extends Applet{

public void paint (Graphics gr){ gr.setColor(Color.green); gr.fillOval(50,50,100,100); gr.setColor(Color.blue); gr.fillOval(81,85,8,8); gr.fillOval(111,85,8,8); gr.drawArc(75,75,50,50,225,90);

}

boolean open;public void init (){ open = false;

}

if (open){

}

TextField pass;

pass = new TextField(20); this.add(pass); pass.addActionListener(this);}

Secret drawingclass Secret extends Applet implements ActionListener{ boolean open; TextField pass;

public void actionPerformed (ActionEvent e){

open = true; this . repaint();

}

if ( pass.getText() . equals (“geheim”) ){

}

pass . setVisible(false);

operator == or method equals ?

== tests variable equality

equals tests String-contents equality

5

5

x

y

s

thoi

s

t

hoi

hoi

s

thoi

s

t

hoi

hoi

method for

String-objects

Min./max. thermometerclass Thermo extends Applet

{ ScrollBar meter; Button reset;

public void init (){ meter = new ScrollBar(ScrollBar.HORIZONTAL,0,1,-50,50); reset = new Button(“reset”);

}

this.add(meter); this.add(reset); meter.addAdjustmentListener(this); reset.addActionListener(this);

int minimum, maximum;

minimum = 0; maximum = 0;

impelements AdjustmentListener, ActionListener

Min./max. thermometerpublic void paint (Graphics gr){ gr.drawString( “hoogste ” + maximum, 50,50); gr.drawString( “laagste ” + minimum, 50,70);}public void adjustmentValueChanged(AdjustmentEvent e){

this.repaint();}

maximum =

minimum =

if (waarde>maximum) waarde;if (waarde<minimum) waarde;

int waarde;waarde = meter.getValue();

Interaction components

ScrollBar sb; sb = new ScrollBar(

hv, value, step, range); sb.addAdjustmentListener(

...);

n = sb.getValue(); sb.setValue(n);

TextField tf; tf = new TextField(

width); tf.addActionListener(...)

;

s = tf.getText(); tf.setText(s); n = Integer.parseInt(s);

Conversion

From String to int

String s; int n;n = Integer.parseInt (s);

From String to double

String s; double d; d = Double.valueOf (s); Double dob; obd = dob.doubleValue ();

Graph-drawing program

graph of theparabola

solutions according toabc-formula

entered values abc

Design of class Parabola

declaration Objectvariables method init method actionPerformed method paint method zeroes method axes method graph method parabola

makes interface

converts a,b,c

“manages” drawing

do the actual

drawing

calculates ax2+bx+c

Parabola: initclass Para extends Applet implements ActionListener{

TextField abox, bbox, cbox;double a, b, c;public void init (){ a = 0.5; b = 2.0; c = -4.0;

}

abox = new TextField( “”+a, 8 );bbox = new TextField( “”+b, 8 );cbox = new TextField( “”+c, 8 );this.add(abox); this.add(bbox); this.add(cbox);abox.addActionListener(this);bbox.addActionListener(this);cbox.addActionListener(this);

Parabola: actionPerformed

public void actionPerformed(ActionEvent e){

}

a =b =c =this.repaint ();

abox.getText()bbox.getText()cbox.getText()

Double.parseDouble( ); Double.parseDouble( );Double.parseDouble( );

Parabola: paintpublic void paint (Graphics gr){

}

zeroes

axes

graph

this .

this .

this .

(gr);

(gr);

(gr);

gr . setColor( Color.black );

gr . setColor( Color.red );

gr . setColor( Color.blue );

private void axes (Graphics gr){ gr.drawLine(0, 250, 500, 250); gr.drawLine(250, 0, 250, 500);}

Parabola: zeroespublic void zeroes (Graphics gr){

}

gr.drawString( (-b -root)/denomin + “ and ” +(-b+root)/denomin , 50, 50 );

root = Math.sqrt(discriminant);

discriminant = b*b - 4*a*c;denomin = 2*a;

if (discriminant<0) gr.drawString(“geen nulpunten”);else{

}

-b ± b2-4ac

2a

Parabola: graphpublic void graph (Graphics gr){

}

for ( x=0 ; x<500 ; x++ ){

}

y = this.parabool( x );

gr.drawLine( , , x, y );x-1 oldyoldy = y;

oldy = 0;

if (x>0)

scale = 0.03;

xw = scale * x ; w wy = yw / scale ;

( -250)

(int) (250-( ))

int x, y,oldy; double xw, yw, scale;

Summary lecture 5

if-statement if-else-statement

compare int with compare String with

x==y

s.equals(t)

Summary ch. 8

if-statement if-else-statement

compare integers with compare Strings with

x==y

s.equals(t)

Min./max. thermometerclass Thermo extends Applet

{ ScrollBar meter; Button reset;

public void init (){ meter = new Scrollbar(ScrollBar.HORIZONTAL,0,1,-50,50); reset = new Button(“reset”);

}

this.add(meter); this.add(reset); meter.addAdjustmentListener(this); reset.addActionListener(this);

int minimum, maximum;

minimum = 0; maximum = 0;

implements AdjustmentListener, ActionListener

Min./max. thermometerpublic void paint (Graphics gr){ gr.drawString( “hoogste ” + maximum, 50,50); gr.drawString( “laagste ” + minimum, 50,70);}public void adjustmentValueChanged(AdjustmentEvent e){

this.repaint();}

maximum =

minimum =

if (waarde>maximum) waarde;if (waarde<minimum) waarde;

int waarde;waarde = meter.getValue();

All aboutobjects and classes,

in one picture...

Program

statements

methods

class

variables

and Memory

objects

classclass

grouped in

grouped in

grouped in

have as type

change

treat

Class-definition

Description of methodseach consisting ofstatements

Description of objectsby declaration ofvariables

treat

class Counter ...{

... init(){

}

... actionPerformed(){

}

... paint(...){}

}

Button b;int t;

b = new Button();t = 0;

g.drawString(t...);

t = t+1;this.repaint();

x

Declaration

Declaration allocates space in memory For a primitive value

For an object reference

int x;

x = 5;

ScrollBar s;s = new ScrollBar(...);

5

s

as describedin class

Scrollbar

Creation of new objects

Objects having a library class as type

Scrollbar s;s = new Scrollbar(...);

type ofdesired object

Objects having ahome-made class as type

Thermo t; t = new Thermo( );

this is doneby the browser

t . init ( );t . paint (...);

Creation of objects

Structure of the objectis described in the class

meter

min max

Thermo

browser’scurrentapplet

inherited

declared in the class

class Thermo

{

}

Scrollbar meter;int min, max;

// methods...

extends Applet

meter

min max

Thermo


Call of init

reset

this

Scrollbar

Button

00

public void init ( ){

}

Button reset;min = 0; max = 0;meter = new ScrollBar(...);reset = new Button(...);

Call of init

meter

min max

Thermo


reset

this

Scrollbar

Button

00

}

this . add(meter);this . add(reset);reset . addActionListener(this);meter . addAdjListener(this);

added

adjListener

actListener

0 value

reset = new Button(...);

Situation aftercompletion of init

meter

min max

Thermo


Scrollbar

Button

00

added

adjListener

actListener

0 value

Classes and objects

Class definition in libraryObject creation with new

Class definition in programObject creation by browser

Class definition in programObject creation with new

Thermo

Scrollbar

Space

Example: simulation of moving particles in a spaceSimulation -

object

Button -objects

Space -objects

Particle -objects

Class Simulationpublic class Simulation extends Applet{

Button step, auto;Space r1, r2, r3;

public void init ( ){ stap = new Button (“step”); auto = new Button (“start”); r1 = new Space(...); r2 = new Space(...); r3 = new Space(...); ...}

}

ClassSpace

yet to be defined!

Class Space

private class Space

{

}

Space is an extension of an

existinginteraction objectextends

Canvas

Particle d1, d2, d3;

// methods to be defined

every Space objecthas three Particleobjects of its own

Class Particle

private class Particle

{

}

Particle is not extending

anything

int x, y;

// method to be defined

position ofthe Particle

int dx, dy; Color col;

velocity of the Particle

browser’s currentapplet

Objects

Simulation

r1 r2 r3

stap

auto

d1 d2 d3

Space

d1 d2 d3

Space

x

y

dx

dy

col

Particle

x

y

dx

dy

col

Particle

x

y

dx

dy

col

Particle

x

y

dx

dy

col

Particle

x

y

dx

dy

col

Particle

x

y

dx

dy

col

Particle

d1 d2 d3

Space

Button

Button

x

y

dx

dy

col

Particle

x

y

dx

dy

col

Particle

x

y

dx

dy

col

Particle

inherited form Applet

declared in the class

inherited from Canvas

Construction of a new object

Allocate memory

Call constructor method

new expression does two things:

value is reference to the new object

r1

d1 d2 d3

Space

100 196

which can be stored

r1 = new Space ( ... ) ;

private Space ( ){

}

Constructor method

private class Space extends Canvas { Particle d1, d2, d3;

}

int width, int height

name is sameto that of the class

no resulttype!

inherited from

Canvas

this . setBackground (Color.gray);d1 = new Particle (...);d2 = new Particle (...);d3 = new Paticle (...);

this . setSize (width,height);

private class Particle {

}

Class Particle

int x, y, dx, dy; Color col;

public Particle (...) {...}public void setPos (...) {...}public void setVelo (...) {...}public void doStep ( ) {...}public void draw (Graphics g )

{...}


}

Methods of class Particle


public void setPos (int x0, int y0){ x = x0; y = y0;}

public void draw ( Graphics gr){ gr . setcolor (col); gr . fillOval( x-3, y-3, 7, 7 );}

}


int x, y, dx, dy; Color col;public void doStep ( ){ x += dx; y += dy;

if (x<0){

}

x = -x;dx = -dx;

if (y<0) { ... }if (x>maxx) { ... }if (y>maxy) { ... }


}



public Particle (Color k, ){ col = k; maxx = maxy =}

Space r

r . getSize( ) . width;r . getSize( ) . height;

int maxx, maxy;

Summary of classes

class Simulation extends Applet impl ActList init actionPerformed

class Space extends Canvas Space paint

class Particle Particle setPos, setVelo, doStep, draw

public class Simulation extends Applet impl ActionListener {

}

Methods of class Simulation

Button step, auto; Space r1, r2, r3;

public void init ( ){ stap = new Button (“step”); auto = new Button (“start”); r1 = new Space (100, 196); r2 = new Space (196, 150); r3 = new Space ( 60, 75);

}

this.add(stap); this.add(auto);this.add(r1); this.add(r2); this.add(r3);step.addActionListener(this);auto.addActionListener(this);

public class Simulation extends Applet impl ActionListener {

}

Methods of class Simulation

Button step, auto; Space r1, r2, r3;

public void actionPerformed (ActionEvent e ){ if (e.getSource() == stap) this.doStep( ); else ...}

private void doStep ( ){

}

r1.doStep(); r2.doStep();r3.doStep();r1.repaint (); r2.repaint ();r3.repaint ();

private class Space extends Canvas {

}

Methods of class Space

Particle d1, d2, d3;public Space (int width, int height){ this.setSize (width, height); this.setColor (Color.gray); d1 = new Particle(Color.red, this); d2 = new Particle(Color.red, this); d3 = new Particle(Color.red, this); d1.setPos(30, 40); d1.setVelo(10,10); ...}

private class Space extends Canvas {

}

Method of class Space

Particle d1, d2, d3;

public void doStep ( ){ d1 . doStep (); d2 . doStep (); d3 . doStep ();}public void paint (Graphics gr){ d1 . draw (gr); d2 . draw (gr); d3 . draw (gr);}

Animation

Animationprogram shows changing picturesautomatically

Easily programmed using library class

Thread

“thread”of events

which occur,simultaneously

with other threads

Programming animations

animation . start ( ); this . run ( );

calls run of the object

that was passedto the constructor

Thread animation;animation = new Thread ( this );

and returnsimmediately

(even when run is still running)

Method run

class Simulation extends Applet{

}

implements Runnable

public void run ( ){

}

while (true)this . doStep ( );{

}

Thread . sleep (50);try{ Thread . sleep (50); }catch (Exception e){ }

infinite repetition!

millisecondsmilliseconds

if (moving){ auto . setLabel (“Start”); moving = false;}else{

}

Starting the animation


}

if (e.getSource ( )==auto){

}else this . doStep ( );

animation = new Thread(this);animation . start ( );auto . setLabel (“Stop”); moving = true;

Method run revisited

class Simulation extends Applet{

}

implements Runnable

public void run ( ){

}

while (true)this . doStep ( );{

}

try{ Thread . sleep (50); }catch (Exception e){ }

oneindige herhaling!

while (moving)

finite repetition.

if (beweging){ auto . setLabel (“Start”); beweging = false;}else{

}

Starting the animationrevisited


}

if (e.getSource ( )==auto){

}else this . doStep ( );

animatie = new Thread(this);animatie . start ( );auto . setLabel (“Stop”); moving = true;

(animation != null )animation = null;

The value null

null : reference to nothing

x = null;

x

null is a valid valuefor each object reference type

and use

What is object?declarations of object variables

Wat can you do with the object?method headers

How is it done?method bodies

Class design

Ways to design classes

Top-downstart with the most complicated classusing classes and methods which areyet to be written

Bottom-upstart with the simplest classcreating a library of methodswhich come in useful writingthe other classes

Summary ch. 9

Make objects of a home-made class variables: what is the object methods: what can the object do

Animation

Thread a;a = new Thread(this);a . start ( );

public void run ( ){ while (true) { this . doStep (); Thread . sleep(50); }}

Useful classes (1/4)

Things from reality String

length, equals, substring, concat Color

Color, red, green, blue, black, white Dimension

Dimension, getWidth, getHeight

Useful classes(2/4)

Computer related things BufferedImage

setRGB, getRGB, getGraphics AudioClip

play, stop, loop File

getName, getDirectory, exists, delete

Useful classes(3/4)

Objects that can do something for you Graphics

drawLine, fillRect, setColor StringTokenizer

nextToken, hasMoreTokens

Useful classes(4/4)

Interaction componentento use

ScrollBar Button TextField TextArea Label CheckBox

to extend Applet Canvas Frame

problems withversion management

Extending previous work

First try: “cut&paste”class Two{ int x, y;

int old ( ) { return x+y; }}

class Twee{ int x, y;

int old ( ) { return x+y; }

}

int z;

int extra ( ) { return x+y+z; }

Three

+1;

assymmetrical

clumsy


Second try: “delegation”class Two{ int x, y;


class Twee{ Twee t;

int old ( ) { return t.old(); }

}

int z;

int extra ( ) { return t.x+t.y+z; }

Three


Third try: “subclasses”class Two{ int x, y;


class Twee extends Two{

}

int z;


Three

Subclasses

class Space extends Canvas

Space is a subclass of Canvas Canvas is the superclass of Space

Each Space-object is also a Canvas-object A Space-object is a special case of a

Canvas-object

Inheritance

Objects of a subclassinherit variables and methodsof the superclass

meter

min max

Thermo

inhertited

declaredby yourself

Superclasses in standard packages

Applet extends Panel Panel extends Container

superclass

super-superclass

Inherited methods

Applet inherits add from class Container

and manymany more...

Class hierarchies

Van

Car MotorBike

MotorVehicle Bicylce

Vehicle Plane

SteamBoat

MotorBoat SailBoat

Boat

Transporter

design:medium precedes

motorization

design:motorbike is moremotor than bike

Class hierarchies

Vehicle Boat

Transporter

class Vehicle extends Transporterclass Boat extends Transporter

“is a”

Class hierarchy in AWT

Applet

FileDialog

Frame Dialog

Panel Window TextArea TextField

Button Container Canvas Label TextComponent Scrollbar

Component

Button is a Component Applet is also a Component

(more precicely, a Container, and even a Panel)

Not every hierarchy isa class hierarchy

d1Particle

d2Particle

d3Particle

d1Particle

d2Particle

d3Particle

d1Particle

d2Particle

d3Particle

stepButton

autoButton

r1Space

r2Space

r3Space

browser's currentSimulatie

“has a”

Classes vs. Interfaces

Class:Group of methods and declarations

Interface:Group of method headers

“wish list”to be implemented

by another class

“Interface” in 3 readings

Hardware:connectors are the“face” of the computer

Software:buttons are the“face” of the software

Programming:methods are the“face” of the program

class Hallo implements ActionListener {

“GUI”

Interface hierarchy in AWT

Action-Listener

Adjustment-Listener

Component-Listener

Mouse-Listener

Event-Listener


ActionEvent AdjustmentEvent

KeyEvent MouseEvent

InputEvent FocusEvent

ComponentEvent

AWTEvent

EventObject

Everything in one hierarchy

String Component EventObject Image

Object

Methods in class Object : toString redefinable in a subclass clone makes exact copy

TextArea and String

class SymCount extends Applet

{

void init ( ){ input = new TextArea (5, 40); output = new TextField(40); count = new Button(“Count”); this.add(input); this.add(output); this.add(count); count.addActionListener(this);}

TextArea input; TextField output; Button count;


Useful classes (1/4)

Things from reality String

length, equals, substring, concat Color

Color, red, green, blue, black, white Dimension

Dimension, getWidth, getHeight

Useful classes(2/4)

Computer related things BufferedImage

setRGB, getRGB, getGraphics AudioClip

play, stop, loop File

getName, getDirectory, exists, delete

Useful classes(3/4)

Objects that can do something for you Graphics

drawLine, fillRect, setColor StringTokenizer

nextToken, hasMoreTokens

Useful classes(4/4)

Interaction componentento use

ScrollBar Button TextField TextArea Label CheckBox

to extend Applet Canvas Frame

problems withversion management


First try: “cut&paste”class Two{ int x, y;


class Twee{ int x, y;

int old ( ) { return x+y; }

}

int z;


Three

+1;