slide1 l.
Download
Skip this Video
Download Presentation
More OOP

Loading in 2 Seconds...

play fullscreen
1 / 37

More OOP - PowerPoint PPT Presentation


  • 133 Views
  • Uploaded on

More OOP. Extending other’s classes. extend Java platform classes, e.g. class Applet. public class MyApplet extends Applet { public void init() { } public void paint(Graphics g) { } }. overriding the method defininitons. Overriding. class Rectangle {

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'More OOP' - lavada


Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide2

Extending other’s classes

extend Java platform classes, e.g. class Applet

public class MyApplet extends Applet {

public void init() {

}

public void paint(Graphics g) {

}

}

overriding the

method defininitons

slide3

Overriding

class Rectangle {

private int x,y,width,height;

public Rectangle(int width,int height) { ? }

public double area() { ? }

public void draw(Graphics g) { ? }

...

}

class RedRectangle extends Rectangle {

public void draw(Graphics g) {

g.setColor(Color.red);

g.drawRectangle(getX(),getY(),getWidth(),getHeight());

}

}

slide4

Overriding

BounceRectangleAroundTheScreen(Rectangle a);

Rectangle b=new RedRectangle(10,10);

BounceRectangleAroundTheScreen(b);

RedRectangle is a Rectangle (it behaves slightly differently)

everytime draw method is

called for object of type

RedRectangle it is drawn

in RED

A variable of type

reference to Rectangle

can point to object of

type RedRectangle

slide5

Overriding

class Rectangle {

private int x,y,width,height;

public Rectangle(int width,int height) { ? }

public double area() { ? }

public void draw(Graphics g) { ? }

...

}

class RedRectangle extends Rectangle {

public void draw(Graphics g) {

g.setColor(Color.red);

g.drawRectangle(x,y,w,h);

}

}

protected

slide6

Overriding - rules

(non-private instance methods)

methods declared with final cannot be overriden

overriden method has to have the same return type

which method will be used depends only on

the object’s type (casting does not change the

object’s type, just the way we look at it)

you cannot override private methods – you do not

inherit them

slide7

Extending classes – constructors

since RedRectangle is Rectangle the constructor

for Rectangle should be called (something

important might be initialized in the constructor)

class RedRectangle {

public void draw(Graphics g) {

g.setColor(Color.red);

g.drawRectangle(getX(),getY(),getWidth(),getHeight());

}

public RedRectangle(int width,int height) {

super(width,height);

?

}

}

slide8

Constructors - rules

you should call super in the first line of a constructor

if you don’t then

if parent class has constructor with no parameters

=> it is called (before the body of the constructor)

if parent class has no constructor with no parameters

=> ERROR

remember – default constructor has no parameters, but

if it is defined only if there is no other constructor.

slide9

class Number {

protected int x;

public Number(int y) { x=y; }

public void increment() { x++; }

public void increment2() { increment(); increment(); }

public int getNumber() { return x; }

}

class BadNumber extends Number {

public BadNumber(int y) { super(y); }

public void increment() { x--; }

}

Number a=new Number(10);

BadNumber b=new BadNumber(10);

Number c=new BadNumber(10);

a.increment(); System.out.println(“”+a.getNumber());

a.increment2(); System.out.println(“”+a.getNumber());

b.increment(); System.out.println(“”+b.getNumber());

b.increment2(); System.out.println(“”+b.getNumber());

c.increment(); System.out.println(“”+c.getNumber());

c.increment2(); System.out.println(“”+c.getNumber());

Exercise

What is the

output?

slide10

class Number {

protected int x;

public Number(int y) { x=y; increment(); }

public void increment() { x++; }

public void increment2() { increment(); increment(); }

public int getNumber() { return x; }

}

class BadNumber extends Number {

public BadNumber(int y) { super(y); increment(); }

public void increment() { x--; }

}

Number a=new Number(10);

BadNumber b=new BadNumber(10);

Number c=new BadNumber(10);

a.increment(); System.out.println(“”+a.getNumber());

a.increment2(); System.out.println(“”+a.getNumber());

b.increment(); System.out.println(“”+b.getNumber());

b.increment2(); System.out.println(“”+b.getNumber());

c.increment(); System.out.println(“”+c.getNumber());

c.increment2(); System.out.println(“”+c.getNumber());

Exercise

What is the

output?

slide11

class Number {

protected int x;

public Number(int y) { x=y; increment(); }

private void increment() { x++; }

public void increment2() { increment(); increment(); }

public int getNumber() { return x; }

}

class BadNumber extends Number {

public BadNumber(int y) { super(y); increment(); }

private void increment() { x--; }

}

Number a=new Number(10);

BadNumber b=new BadNumber(10);

Number c=new BadNumber(10);

a.increment2(); System.out.println(“”+a.getNumber());

b.increment2(); System.out.println(“”+b.getNumber());

c.increment2(); System.out.println(“”+c.getNumber());

Exercise

What is the

output?

slide12

Exercise

What is the

output?

class Number {

protected int x;

public Number(int y) { x=y; }

public Number() { x=13; }

public int getNumber() { return x; }

}

class BadNumber extends Number {

public BadNumber(int y) { System.out.println(“Yupeee!”); }

}

Number a=new Number(10);

BadNumber b=new BadNumber(10);

System.out.println(“”+a.getNumber());

System.out.println(“”+b.getNumber());

slide13

Exercise

What is the

output?

class Number {

private int x;

public Number(int y) { x=y; }

public Number() { x=13; }

public int getNumber() { return x; }

}

class BadNumber extends Number {

public BadNumber(int y) { System.out.println(“Yupeee!”); x=y; }

}

Number a=new Number(10);

BadNumber b=new BadNumber(10);

System.out.println(“”+a.getNumber());

System.out.println(“”+b.getNumber());

slide14

Invoking an overriden method

class Rectangle {

private int x,y,width,height;

public Rectangle(int width,int height) { ? }

public double area() { ? }

public void draw(Graphics g) { ? }

...

}

class RectangleWithPrintedArea {

public void draw(Graphics g) {

super.draw(g);

g.drawString(“”+area(),get(X),get(Y)+20);

}

}

slide15

class Number {

protected int x;

public Number(int y) { x=y; }

public void increment() { x++; }

public void increment2() { increment(); increment(); }

public int getNumber() { return x; }

}

class BadNumber extends Number {

public BadNumber(int y) { super(y); }

public void increment() { super.increment(); x--; }

}

Number a=new Number(10);

BadNumber b=new BadNumber(10);

Number c=new BadNumber(10);

a.increment(); System.out.println(“”+a.getNumber());

a.increment2(); System.out.println(“”+a.getNumber());

b.increment(); System.out.println(“”+b.getNumber());

b.increment2(); System.out.println(“”+b.getNumber());

Exercise

What is the

output?

slide16

class Number {

protected int x;

public Number(int y) { x=y; }

private void increment() { x++; }

public void increment2() { increment(); increment(); }

public int getNumber() { return x; }

}

class BadNumber extends Number {

public BadNumber(int y) { super(y); }

private void increment() { super.increment(); x--; }

}

Number a=new Number(10);

BadNumber b=new BadNumber(10);

Number c=new BadNumber(10);

a.increment2(); System.out.println(“”+a.getNumber());

b.increment2(); System.out.println(“”+b.getNumber());

Exercise

What is the

output?

slide17

Inheritance – using others code

class Rectangle {

protected int width,height;

public Rectangle(int width,int height) { ? }

public double diameterOfCircumCircle() { ? }

}

class RectangleOnScreen extends Rectangle {

int x,y;

public RectangleOnScreen(int x,int y,int w,int h) {

super(w,h);

this.x=x; this.y=y;

}

public drawCircumCircle(Graphics g) { ? }

}

slide18

Inheritance – using others code

public drawCircumCircle(Graphics g) {

g.drawCircle(x+width/2,y+height/2, diameterOfCircumCircle());

}

We can easily change/use other’s code!

Private variables cannot be used in the derived

class!

slide19

Polymorphism

We want to have list

of various shapes which

are placed on the screen.

We want to move them,

draw them, compute area.

Shape

Polygon

Square

Circle

slide20

abstract class Shape {

private int x,y; // coordinates

Shape(int initialX,int initialY) {

x=initialX; y=initialY;

}

public void moveUp(int pixels) {

y-=pixels;

}

....

abstract double area();

abstract void draw(Graphics g);

}

Shape is “too

abstract”, we

cannot compute

area or draw

slide21

Shape

class Circle extends Shape {

public static final double PI=3.1415;

private int diameter;

Circle(int x,int y,int diam) {

super(x,y);

diameter=diam;

}

public double area() {

return PI*diameter*diameter/4.0;

}

public void draw(Graphics g) { ... }

}

Circle

slide22

Shape

class Square extends Shape {

public static final double PI=3.1415;

private int side;

Square(int x,int y,int side) {

super(x,y);

this.side=side;

}

public double area() {

return side*side;

}

public void draw(Graphics g) { ... }

}

Square

calling

constructor

of

the parent

class

slide23

Circle is a Shape!

Shape a=new Circle(10,10,5);

a.moveUp(5);

System.out.println(“The area is ”+a.area());

Shape[] a=new Shape[2];

a[0]=new Circle(10,10,5);

a[1]=new Square(20,20,10);

if (a[1].area()>a[0].area())

a[1].draw();

else

a[0].draw();

slide24

Shape is not a Circle

Circle a=new Shape();

Circle b=new Rectangle();

Shape c=new Shape();

slide25

EXERCISE #12:

class Animal {

void wish() { System.out.println(“I don’t know!”); }

}

class Rabbit extends Animal {

void wish() { System.out.println(“I want carrot!”); }

}

class Turtle extends Animal {

void wish() { System.out.println(“I want shrimp!”); }

}

Animal a=new Animal(); a.wish();

Rabbit b=new Rabbit(); b.wish();

Animal c=new Rabbit(); c.wish();

Animal d=new Turtle(); ((Turtle) d).wish();

slide27

Simple applets

public class MyApplet extends Applet {

public void init() {

}

public void paint(Graphics g) {

}

}

executed at the beginning

executed when the applet

is redrawn

slide28

Simple applets

public class MyApplet extends Applet {

int x=0;

public void init() {

setBackground(Color.black);

}

public void paint(Graphics g) {

x++;

g.setColor(Color.white);

g.drawString(“”+x,20,20);

}

}

Import?

java.applet.Applet

java.awt.Graphics

java.awt.Color

MyApplet

slide30

Applets that run

public class MyApplet extends Applet implements Runnable {

public void init() {

}

public void paint(Graphics g) {

}

public void run() {

}

}

executed at the beginning

executed when the applet

is redrawn

we will create a new Thread

which will execute this method

slide31

Interfaces

we will not go into details

public class MyApplet extends Applet implementsRunnable {

public void init() {

}

public void

paint(Graphics g) {

}

public void run() {

}

}

says which methods must be

implemented in MyApplet

expects parameter of type Runable

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

slide32

Applets that run

public class MyApplet extends Applet implements Runnable {

public void init() {

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

}

public void paint(Graphics g) {

}

public void run() {

}

}

we will create a new Thread

out of this Object

(note that this Object is Runnable)

its run method will be this.

We will start it!

slide33

Applets that run

public class MyApplet extends Applet implements Runnable {

int x=0;

public void init() {

setBackground(Color.black);

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

}

public void paint(Graphics g) {

g.setColor(Color.white);

g.drawString(“”+x,20,20);

}

public void run() {

while (true) {

x++;

repaint();

}

}

}

different Thread

request for repainting

(If there are enough

resources – it is repainted)

MyApplet

slide34

Applets that run – wasting resources

public class MyApplet extends Applet implements Runnable {

int repaints=0,requests=0;

public void init() {

setBackground(Color.black);

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

}

public void paint(Graphics g) {

g.setColor(Color.white);

g.drawString(“Repaints: ”+(++repaints),20,20);

g.drawString(“Requests: “+requests,20,40);

}

public void run() {

while (true) {

requests++;

repaint();

}

}

}

request for repainting

(If there are enough

resources – it is repainted)

MyApplet

slide35

Applets that run

public class MyApplet extends Applet implements Runnable {

int repaints=0,requests=0;

public void init() {

setBackground(Color.black);

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

}

public void paint(Graphics g) {

g.setColor(Color.white);

g.drawString(“Repaints: ”+(++repaints),20,20);

g.drawString(“Requests: “+requests,20,40);

}

public void run() {

while (true) {

requests++;

repaint();

}

}

}

try {

Thread.currentThread().sleep(40);

}

catch (InterruptedException e) {};

MyApplet

slide36

Why not run in init?

public class MyApplet extends Applet {

int repaints=0,requests=0;

public void init() {

setBackground(Color.black);

while (true) {

requests++;

repaint();

}

}

public void paint(Graphics g) {

g.setColor(Color.white);

g.drawString(“Repaints: ”+(++repaints),20,20);

g.drawString(“Requests: “+requests,20,40);

}

}

MyApplet

slide37

Why not run in init?

public class MyApplet extends Applet {

int repaints=0,requests=0;

public void init() {

setBackground(Color.black);

while (true) {

requests++;

repaint();

}

}

public void paint(Graphics g) {

g.setColor(Color.white);

g.drawString(“Repaints: ”+(++repaints),20,20);

g.drawString(“Requests: “+requests,20,40);

}

}

the thread which runs init needs

to do some things – e.g. start the

paint thread...

MyApplet