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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 {

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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 {

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());

}

}


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


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


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


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);

?

}

}


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.


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?


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?


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?


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());


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());


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);

}

}


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?


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?


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) { ? }

}


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!


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


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


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


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


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();


Shape is not a Circle

Circle a=new Shape();

Circle b=new Rectangle();

Shape c=new Shape();


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();


Applets


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


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


Applets – start(), stop(), destroy()


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


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();


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!


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


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


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


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


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


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