Construction of classes with classes€¦ · Segway Bicycle Car Private Car Truck Vehicle (November...

Construction of classes with classes

Classes can be built on existing classes through attributes of object type.

Example:

I A class PairOfDice can be constructed using two attributes of the

type Dice.

I A class CardDeck can be build with the help of an array of an object

array containing 52 objects of the type Card

I A class ListMap can be constructed with the help of ListNode objects

I A class TrafficSystem can be constructed using objects from the

classes Lane, Light, Vehicle . . .

One use to say that this is a composition relation or a has relation.

(November 4, 2010 � Class hierarchies 1 )

Class hierarchies

A new class can also be built as a specialization of an existing class.

Example:

I The class StackException theat is used in the Stack class is written

as a sub class to Exception

I If we have a class Vehicle, we can build new classes like Segway,

Bicycle and Car. A Car can in turn be a base class for the classes

Private car and Truck

Segway Bicycle Car

Private Car Truck

Vehicle


In these case you say that you have a class hierarchy

I The class Vehicle is called a base class

I The classes Segway, Bicycle and Car are derived classes or sub classes

to the class Vehicle

I A subclass (e. g. the class Car) can be used as a base class for other

classes (e. g. Private car and Truck)

One use to say the the phrase

is a ... that ...

can be used to identify when it is proper to use a class hierarchy.


Inheritance

An object of a sub class

I has all the attributes and methods of the base class and

I can add more by itself

One say that the sub class inherits all the characteristics of the base class.

Example: The class Car can have the attribute weight. A Private car can

add nPassenger while a Truck can add maxLoad.

A subclass can also override methods and attributes in the baseclass.


Uni�ed Modeling Language (UML)

The class hierarchies are often express in class diagrams in UML.

print() : void

print() : void

print() : void

maxLoad : double

print() : void

nPassenger : int

print() : void

Segway

Vehicle

Bicycle Car

TruckPrivate Car

weight : double


The Vehicle hierarchy in code

public class Vehicle {

public void print() {

System.out.print("Vehicle");

}

}

public class Segway extends Vehicle { }

public class Bicycle extends Vehicle {


System.out.print("Bicycle");

}

}

public class Car extends Vehicle {

double weight;

public Car( double v ) {

weight = v;

}


System.out.print("Car, weight " + weight);

}

}(November 4, 2010 � Class hierarchies 6 )

UsageThe code

...

public static void pr(String s) {

System.out.print(s);

}

public static void main(String [] args) {

Vehicle f = new Vehicle();

Segway s = new Segway();

Bicycle c = new Bicycle();

Car b = new Car(1.2);

pr("f: "); f.print(); pr("\n");

pr("s: "); s.print(); pr("\n");

pr("c: "); c.print(); pr("\n");

pr("b: "); b.print(); pr("\n");

...

give the printout

f: Vehicle

s: Vehicle

c: Bicycle

b: Car, weight 1.2


More subclasses

public class PrivateCar extends Car {

int nPassenger = 5;

public PrivateCar(double v) {

super(v);

}

public PrivateCar(double v, int ant) {

super(v);

nPassenger = ant;

}


System.out.print("Private");

super.print();

System.out.print(", number of passengers " + nPassenger);

}

}


Continuation of main

...

PrivateCar pb1 = new PrivateCar(1.5);

PrivateCar pb2 = new PrivateCar(0.9,4);

pr("pb1: "); pb1.print(); pr("\n");

pr("Weight b : " + b.weight + "\n");

pr("Weight pb1: " + pb1.weight + "\n");

f = b;

pr("f: "); f.print(); pr("\n");

f = pb1;

pr("f: "); f.print(); pr("\n");

pb1 = (PrivateCar) f;

pr("pb1: "); pb1.print(); pr("\n");

gives the printout

pb1: PrivateCar, weight 1.5 and number of passengers 5

Vikt b: 1.2

Vikt pb1: 1.5

f: Car, weight 1.2

f: PrivateCar, weight 0.9 and number of passengers 4

pb1: PrivateCar, weight 0.9 and number of passengers 4


Example of illegal operations

pb1 = f;

pb1 = (PrivateCar) c;

pr("Weight: " + f.weight);

pb1 = (PrivateCar) f; // if f don't refer a PrivateCar

The last can be made �safe� through

if ( f instanceof PrivateCar )

pb1 = (PrivateCar) f;

else

...

The operator instanceof should be used with caution


Conclusions

If the class S is a subclass to the class B and if s is declared as a reference

to S and b to B we have

I An object of the class S has all attributes and methods that the class B

have

I If S declares an attribute that are declared B, that attribute is

overridden by B's attribute.

I A reference can refer object of the declared class and its subclasses.

I The construction b.x (or b.x()) requires that the attribute (or

method) x is declared in B or any of B's baseclasses.

I When you use an attribute a through dot notation p.a it is the

declaration of p that settles what attributes are used.

I When you call a method m using p.m() it is the type of the element

that p refers that settles what method is selected. (so called dynamic

or late binding).


Methods that only are declared in selected classes

Private Car

nPassenger : int

print() : void

tax() : double

print() : void

Segway

Vehicle

Truck

maxLoad : double

print() : void

tax() : double

Bicycle

print() : void weight : double

print() : void

Car

Cars are taxed, but the calculations are done in di�erent way for private

cars and trucks.


Usage of the tax-method

Car b = new Truck(3., 2.);

...

if (...) {

b = new PrivateCar(1.2, 5);

}

...

double s;

s = b.tax(); // Illegal! No tax in b

s = ((Truck) b).tax(); // Risky!

if ( b instanceof Truck ) // Works but clumsy

s = ((Truck) b).tax(); // and inflexible

else if ( b instanceof PrivateCar )

s = ((PrivateCar) b).tax();

else

s = 0;


BetterDe�ne also a method tax-method in the class Car or, better, in the class

Vehicle:

Private Car

nPassenger : int

print() : void

tax() : double

Vehicle

Bicycle

tax() : double

Truck

maxLoad : double

print() : void

tax() : double

Segway

print() : void weight : double

print() : void

Car

print() : void

public class Vehicle {


System.out.print("Vehicle");

}

public double tax() {

return 0;

}

}

Then the method can be used for all objects of the type Vehicle or its sub

classes.

In general: Don't use instanceof! Use the dynamic binding!


Levels of protection

I private

Only accessible from the classes own methods

I public

Accessible for all

I protected

Accessible from subclasses and classes in the same package

I package

If no level of protection is given. all classes are assign package access

level


Constructors in inheritance

Constructors are not inherited

When an object of a subclass is created, the following steps are taken:

1. Instance variables take their default values

2. A constructor for the base class (super class) is called. You use the

keyword super to specify which constructor in the base class that

should be used. If super is not used, the parameterless constructor in

the base class are used, in this case there has to exist such a

constructor.

3. If there are initializarions they are evaluated and assigned to the

respective attributes.

4. The statements in the constructor in the subclass are executed.

Use super to avoid repeating code!


Standard software in Java

The Java environment is based on inheritance and class hierarchies!

Example:

1. The classes for exceptions: Throwable with the subclasses Error and

Exception with the subclasses . . .

2. Graphical components: Component with subclasses Button, Checkbox,

Container, . . . where e. g. Container has the subclasses Panel,

Window . . .

3. Maps: Map with (among others) the subclasss HashMap

4. The Collection classes: Collection with (among others) the subclass

List that have the subclasses Vector and LinkedList

(Map, Collection are List actually interfaces interface and not classes)


The Class Object

The class Object is the baseclass to all classes.

A reference to Object can thus refer any object.

This can be used to construct general �collection classes� (lists, tables or. . . ):

class ListNode {

Object info;

ListNode next;

ListHead(Object i, ListNode n) {

info = l;

next = n;

}

}

public class List {

ListNode head;

public void insertFirst(Object o) {

head = new ListNode(o, head)

}

...

}

From Java 5 and onward, you usually use �generics� instead.


The classen Object cont

Some methods in the class Object:

I Object clone()

I boolean equals(Object o)

I String toString()

You often have reasons to override these!


An example of using inheritance: geometrical �gures

Write a program that can handle pictures of circles, rectangles and other

geometrical shapes.

The program should be able to

I represent a number of di�erent shapes.

I Draw an image of the represented shapes.

I Compute the accumulated area of all �gures.

I Read rules about the structure of a certain shape.

(other operations are of course possible: move, rotate, scale . . . )


Cont geometrical �gures

1. Which classes are needed to represent the �gures?

Circle, Rectangle, . . .

2. What attributes and methods should we have?I constructorsI methods for calculating areasI methods for drawing

3. How should one represent a collection of such �gures?I array?I list?I other structure?

What type should be used for the elements in the structure?



paint():void

RectangleCircle

radius: int

area() : double

height : intarea(): doublepaint():void

paint():void

Shape

width : int

xCoord : int yCoord : int

area() : double



Now we can e. g. do

Shape[] fig = new Shape[100];

fig[0] = new Rectangle(...);

fig[1] = new Circle(...);

...

(but we are to use a more �exible structure than an array)

There is no meningful implementation of area() and paint() in Shape!

Declare these methods and the class Shape as abstract.



import java.awt.*;

public abstract class Shape {

protected int xCoord, yCoord;

public Shape(int x, int y) {

xCoord = x;

yCoord = y;

}

public abstract double area();

public abstract void paint(Graphics g);

}



import java.awt.*;

public class Circle extends Shape {

protected int radius;

public Circle(int x, int y, int r){

super(x,y);

radius = r;

}

public double area(){

return Math.PI*radius*radius;

}

public void paint(Graphics g){

g.setColor(Color.RED);

g.fillOval(xCoord-radius, yCoord-radius, 2*radius, 2*radius);

}

}



import java.awt.*;

public class Rectangle extends Shape {

protected int width, height;

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

super(x,y);

width = w;

height = h;

}


return width*height;

}

public void paint(Graphics g){

g.setColor(Color.BLUE);

g.fillRect(xCoord,yCoord,width,height);

}

}



To collect a number of di�erent �gures we use one of the collection classes

in Java: LinkedList that is a (indirect) subclass to Collection (actually

an interface).

For this task, it is su�cient to

I Crete a LinkedList object:Collection<Shape> shapes = new LinkedList<Shape>();

I Add �gures to the list, e. g.:shapes.add(new Circle(x,y,r));

I Iterate over the elements and access their methods. E. g:for (Shape s:shapes)

s.paint();



import javax.swing.*;

import java.awt.*;

import java.util.*;

public class Drawing extends JPanel {

private Collection<Shape> shapes;

public Drawing(Collection<Shape> s, int w, int h) {

shapes = s;

setBackground(Color.WHITE);

setPreferredSize(new Dimension(w,h));

}


double a = 0;

for (Shape s:shapes) a += s.area();

return a;

}

public void paintComponent(Graphics g){

super.paintComponent(g);

for (Shape s:shapes) s.paint(g);

}

}(November 4, 2010 � Class hierarchies 28 )


import javax.swing.*;

import java.awt.*;

import java.util.*;

public class DrawTest extends JFrame {

public static void main(String[] args){

Collection<Shape> shapes = read(new Scanner(System.in));

Drawing d=new Drawing(shapes,400,400);

System.out.println("Total area: " + (int)d.area());

new DrawTest(d);

}

public DrawTest(Drawing d){

getContentPane().add(d);

pack();

setTitle("DrawTest");

setDefaultCloseOperation(EXIT_ON_CLOSE);

setVisible(true);

}



private static Collection<Shape> read(Scanner sc){

Collection<Shape> shapes = new LinkedList<Shape>();

while (sc.hasNext()){

String s=sc.next();

if (s.equals("circle")){

int x, y, r;

x = sc.nextInt();

y = sc.nextInt();

r = sc.nextInt();

shapes.add(new Circle(x,y,r));

}

else {

int x, y, w, h;

x = sc.nextInt();

y = sc.nextInt();

w = sc.nextInt();

h = sc.nextInt();

shapes.add(new Rectangle(x,y,w,h));

}

}

return shapes;

}

} // end of class DrawTest



Note how the classes are based on inheritance. The inherit from the

graphical classes in Java. Details in graphics are outside the scope of this

course.

If you run the program with

java DrawTest < figure.txt

and the �le figure.txt contain

rectangle 20 50 200 30

circle 350 100 30

rectangle 300 200 50 150

circle 100 300 80


Construction of classes with classes€¦ · Segway Bicycle Car Private Car Truck Vehicle (November...

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