BITS Pilani, Pilani Campus BITS Pilani Pilani Campus Dr. Yashvardhan Sharma CSIS Dept., BITS-Pilani IS F213 Object Oriented Programming

BITS Pilani, Pilani Campus

BITS PilaniPilani Campus

Dr. Yashvardhan SharmaCSIS Dept., BITS-Pilani

IS F213 Object Oriented Programming


Objects as Parameters To Methods

1. Objects are always passed by reference w.r.t its instance field values i.e state of the object. ( State of object Can be changed by using the same reference variable )

2. If you are changing the values of instance fields by using same reference variable then changes will be reflected in the calling program. [ That’s what we mean by callByReference ]

3. Objects are always passed by value w.r.t. its reference. The Called Method can not change the passed parameter reference to some another object . The called method if tries to change the reference of passed parameter then that change remains local only and will not be reflected to calling method.


class A{int a,b;A(int a,int b){this.a =a; this.b=b;}void show(){System.out.println("a="+a);System.out.println("b="+b);}

Example1 (Objects as parameters to methods)

void changeContents(A other){other.a = 20;other.b = 100;}void changeReference(A other){other = new A(-10,-20);other.a = 20;other.b = 100;}} // End of class A


class objref{public static void main(String args[]){A a1=new A(40,50);a1.show();a1.changeReference(a1); a1.show();}}

Objects as Parameters Methods

E:\New Folder\Java>java objrefa=40b=50a=40b=50

a1 40 50a b


class objref{public static void main(String args[]){A a1=new A(40,50);a1.show();a1.changeContents(a1);a1.show();}}

Objects as Parameters Methods

E:\New Folder\Java>java objrefa=40b=50a=20b=100

a1 40 50a b

other

changeContents(A other)Method does not change the reference for other


class greeter{String name;greeter(String name){this.name = name;}String sayHello(){return "Hello"+name+"!";}void copyNameTo(greeter other){other.name = this.name;} void copyGreeterTo(greeter other){other = new greeter(“Ram");}}

TEXT BOOK EXAMPLE

Changing other name value with this name value [ State Change] By Reference w.r.t State

Changing other reference to another greeter [ Reference Change] By value w.r.t to reference change


class greetertest{public static void main(String args[]){greeter worldgreeter = new greeter("World");greeter davegreeter = new greeter("Dave");System.out.println(worldgreeter.name);System.out.println(davegreeter.name);worldgreeter.copyNameTo(davegreeter);System.out.println(worldgreeter.name);System.out.println(davegreeter.name);greeter g1 = new greeter("KAMAL");greeter g2 = new greeter("SHARMA");System.out.println(g1.name);System.out.println(g2.name);g1.copyGreeterTo(g2);System.out.println(g1.name);System.out.println(g2.name);}}


class greeter{String name;greeter(String name) { this.name = name; }void setName(String name){this.name = name;}void copyNameTo(greeter other){other.name = this.name;}void copyLengthTo(int n){n = name.length();}

void copyGreeterTo(greeter other){other = new greeter("Pankaj Vyas");}void swapNames(greeter other){String temp = other.name;other.name = this.name;this.name = temp;}void swapReference(greeter other){greeter temp = other;other = this;this = temp;} }

TEXT BOOK PROBLEM 1.13, 1.14

this is final reference. Cannot be changed by methodswapReference

Cont…..


class greetertest{public static void main(String args[]){greeter g1 = new greeter("OOP");g1.setName("BITS");

greeter g2 = new greeter("KAMAL");g1.copyNameTo(g2);

}}

D:\Java1>javac greetertest.javagreetertest.java:34: cannot assign a value to final variable thisthis = temp;^1 error

Continue Problem……


class BOX{private double length,width,height;BOX(double l,double b,double h){length = l; width = b; height =h;}// Mutator Methodspublic void setLength(double l) { this.length = l;}public void setWidth(double b) { this.width = b; }public void setHeight(double h) { this.height = h;}// Accessor Methodspublic double getLength() { return this.length;}public double getWidth() { return this.width; }public double getHeight() { return this.height;}void showDimension(){System.out.println("Length:"+this.length);System.out.println("Width:"+this.width);System.out.println("Height:"+this.height);}


double area() { return 2 * (length * width + width*height + height*length);}double volume() { return length*width*height ;}

void swapBoxes(BOX b1, BOX b2){BOX temp = b1;b1 = b2;b2 = temp;}}// End of BOX


class BOXtest{public static void main(String args[]){BOX b1 = new BOX(10,40,60);BOX b2 = new BOX(20,30,80);BOX b3 = new BOX(100,200,300);

b1.showDimension();b2.showDimension();

b3.swapBoxes(b1,b2);

b1.showDimension();b2.showDimension();}}//End of Main

D:\java\bin>java BOXtestLength:10.0Width:40.0Height:60.0

Length:20.0Width:30.0Height:80.0




class BOXtest{public static void main(String args[]){BOX b1 = new BOX(10,40,60);BOX b2 = new BOX(20,30,80);BOX b3 = new BOX(100,200,300);

b1.showDimension();b2.showDimension();

// b3.swapBoxes(b1,b2);

BOX temp = b1;b1 = b2;b2 = temp;

b1.showDimension();b2.showDimension();}}//End of Main

D:\java\bin>java BOXtestLength:10.0Width:40.0Height:60.0





Reading Inputs in Java

1. Use BufferedReader (Use io package [import java.io.*;])

throws IOException

BufferedReader br = new BufferedReader(new InputStreamReader(System.in)));

String str = br.readLine(); int n = Integer.parseInt(str); float f = Float.parseFloat(str); double a = Double.parseDouble(str);


UML Representation for Class Point

Point

x: doubley: double

+getX() : double+getY() : double+setX(x: double) : void+setY(y: double) : void+equals(other : Point) : boolean+computeDistance(other : Point) : double+show() : void

class Point{double x; // x – coordinatedouble y; // y –coordinate

public double getX() { return x; }public double getY() { return y; } public void setX(double x) { this.x = x; }public void setY(double y) { this.y = y; }public boolean equals(Point other){return this.x == other. x && this.y == other.y ;} public double computeDistance(Point other){double a = (this.y – other.y) * (this.y – other.y);double b = this.x – other.x) * (this.x – other.x);return Math.sqrt(a+b); }

public void show(){S.O.P(“ x= “+x);S.O.P(“ y= “+y);}} End of Point Class

Attributes

Operations


Class PointTest

class PointTest{public static void main(String args[ ]){Point P1 = new Point();P1.show();Point P2 = P1;P2.show();System.out.println(P1.equals(P2));System.out.println(P1.computeDistance(P2));}// End of main() Method}// End of PointTest


OBJECT CONTAINMENT

• When one class contains instance of other class as its instance field then it is known as object containment.

• Object Containment defines one of the important concepts of UML known as composition or aggregation or a-part-of or has a relationship

Player Team Aggregation

Engine Car Composition


class Line{private Point x;private Point y; private double length;

Line(Point x,Point y){this.x = x;this.y = y;length = x.computeDistance(y);}

Line(double x1, double y1, double x2, double y2){x = new Point(x1,y1);y = new Point(x2,y2);length = x.computeDistance(y);}

Example Object Containment

Line class

Point Lineinstance variables of class Line

Constructor where Line can be created from two points

computeDistance() method of point class

Constructor where Line can be created from 4 double values


void show(){System.out.println("x="+x);System.out.println("y="+y);System.out.println("Length="+length);}// Introduce a method for computing slope} // End of Line class

Method for printing the values for x and y and length


E:\New Folder\Java>java LineDemox=Point@256a7cy=Point@720eeb

Length=25.612496949731394

class LineDemo{public static void main(String args[]){Point p1 = new Point();Point p2 = new Point(4,true);Point p4 = new Point(10,8);Point p5 = new Point(-10,-8);Line l2 = new (new Point(),new Point(3,5));Line l1 = new Line(p4,p5);l1.show();}}

Origin pointX =4 , Y =0X =10,Y =8X =-10,Y =-8


There are Four different uses of static keyword in java.1. static instance variables2. static methods3. static classes4. static blocks

Use of static keyword in Java

Note :static field/methods of a class can be accessed/referenced even withoutcreating the objects of that class [ Provided They are not static].

Syntax : <classname> . <fieldname> OR

<classname> . <methodname>


static instance variables/fields

• Static field/instance variables are allocated space in the heap area.

• Static field is just like a global variable for a class that is allocated memory once and all objects of that class share that common copy.

• For a static fields of any class, all the objects of that class share a common copy.

• Declaring an instance field as static makes it class variable that belongs to a whole class not an individual object.

• Any Field/Attribute/instance field of a class can be declared as static.


Example (Static Fields)

class circle{static double PI=3.14156; double radius; double area(){return PI * radius * radius;} double perimeter(){return 2*PI*radius;}} // End of circle class

Static Member

Non-static instance field

circle c1 = new circle();circle c2 = new circle();

c1 c2radius radius

PI = 3.14156

Memory Map


Example (Static Fields)

class A{static int a = 10; double b , c; ……….}

A a1 = new A();

A a2 = new A();

Memory Map

a1 a2b c b c

a=10


Static Methods

• static methods can use only static data• static methods can be called/accessed/referenced even without

creating the objects that class.• Syntax <class name> . < method name(parameter list)>• static method can not call non static methods.• Examples: Math.sqrt (all math functions)• Static method can declare local variables but from outside it can

access only static data(fields/methods)


class num{int a,b,c;static int d = 10;static double e = 20.56;num(int a,int b,int c){ this.a = a; this.b =b; this.c =c; }static int sum(int a1 , int b1){//System.out.println(“a=”+a+”b=”+b+”c=”+c);System.out.println(“d=”+d+”e=”+e);return 40;}

Static Method Example

a,b,c are non static fields and can not be accessed from a static method

non static instance fields

static instance fields

static method


static double sum(double a , double b){System.out.println(“d=”+d+”e=”+e);return 40.56;}static void pr(){System.out.println(“This is method pr”);}void print(){System.out.println(“This is method print”);pr(); // call to static method from a non static method ---- VaildSystem.out.println(“a=”+a+”b=”+b+”c=”+c);System.out.println(“d=”+d+”e=”+e);}}


class BOX{private double l,b,h; // Instance FieldsBOX(double a,double b,double c) { l=a;this.b=b;h=c;} // Constructorboolean isEqual(BOX other){if (this.l == other.l && this.b == other.b && this.h == other.h)return true;elsereturn false;}static boolean isEqual(BOX b1, BOX b2){if (b1.l == b2.l && b1.b == b2.b && b1.h == b2.h)return true;elsereturn false;}} // End of BOX class


class statictest{public static void main(String args[]){BOX b1 = new BOX(10,6,8);BOX b2 = new BOX(10,6,8);BOX b3 = new BOX(1,16,18);BOX b4 = new BOX(2,6,8);

System.out.println(b1.isEqual(b2));System.out.println(BOX.isEqual(b1,b2));System.out.println(b3.isEqual(b1,b2));System.out.println(b4.isEqual(b2));System.out.println(b4.isEqual(b4,b2));}}

D:\Java1>java statictesttruetruetruefalsefalse


1. final keyword in java can be used for following (i) class declaration (ii) variable declartion (iii) method decalaration

Use of final keyword in java

4. final keyword used with method definition means method can not be overridden by subclasses ( Makes sense

only when inheritance is used)

3. final keyword used with variable declaration makes it constant whose value cannot be changed. Final variables should be initialized to some values at the time of declaration.

2. final keyword for class means class cannot be inherited. Final classes can not have subclasses.


final class ABC{…………………………………..}class a extends ABC{……} // Wrong / Invalid

final int x = 40;final double x = 3.4;final double PI = 3.14

class ABC{…………………final void show()..}class a extends ABC{void show()……} // Wrong / Invalid

final classfinal instance

variable final methods

If it is declared as private final void show() then a can have methodvoid show()


final Keyword Example1class ABC{//final int a;final int a = 10;int b;double c;//final static int d;static int d;ABC(int x,double c,int d){ b=x; this.c = c; this.d =d ;}void show(){System.out.println("a="+a);System.out.println("b="+b);System.out.println("c="+c);System.out.println("d="+d);}}

Final field/variable should be initialized to some default value


class ABCDemo{public static void main(String args[]){ABC a1 = new ABC(10,8,5);ABC a2 = new ABC(-10,18,15);ABC a3 = new ABC(67,80.56,50);ABC a4 = new ABC(76,-6.45,95);a1.show(); }}


class A{final int a = 10;}class ABC{ public static void main(String args[]){A a1 = new A();final int a=10;System.out.println("a="+a);}}

final Keyword Example2

Instance field is declared as final

Local Variable is declared as final


final Object Reference

• You can declare any object reference as final• Declaring a Object reference as final does not mean

that Object’s state i.e attribute values can not be changed. We can change the values of attributes provided it is allowed.

• Declaring a Object reference as final makes the reference itself as final and it can not point to any other object.


final Object reference Example

class circle{double radius;circle(double radius){this.radius = radius;}double getRadius(){return radius;}void setRadius(double radius){this.radius = radius;}}// End of circle

final circle c1 = new circle(20);

c1.radius = 30;

circle c2 = new circle(30);

c1 = c2 WRONG

OK

c1 final object reference it can not point to another Object


Use of toString() Method

1. toString() method is defined by Object class(Supermost super class for all java classes

2. toString() defines the String form of any object.

3. Syntax :

public String toString() { …… }

4. Any class can override this method to provide String form of any object.

5. If it is not overridden then this method will be called from Object class.


class box{int l,b,h;box(int a,int b,int c){l=a;this.b=b;this.h=c;}void show(){System.out.println("length="+l);System.out.println(“Width="+b);System.out.println(“height="+h);}} // End of class box


class boxtest{public static void main(String args[]){box b1 = new box(10,20,30);box b2 = new box(4,5,6);System.out.println(10);System.out.println(10.25+10);System.out.println(b1);System.out.println(b2);}} // End of boxtest

?


/* Out PutE:\New Folder\Java>java boxtest1020.25box@256a7cbox@720eeb*/

Output GeneratedBy callingtoString() from Object


class box{int l,b,h;box(int a,int b,int c){l=a;this.b=b;this.h=c;}void show(){System.out.println("length="+l);System.out.println("Width="+b);System.out.println("height="+h);}public String toString(){return "Hello How are You";}} // End of box class


class boxtest1{public static void main(String args[]){box b1 = new box(10,20,30);box b2 = new box(4,5,6);System.out.println(10);System.out.println(10.25+10);System.out.println(b1);System.out.println(b2);}}

?


/* Out PutE:\New Folder\Java>java boxtest11020.25Hello How are YouHello How are You*/ Out Put generated By

toString() defined by box class


Replace toString() method in box class as follows

public String toString(){return “Length: ”+l+” “+”Width: ”+b+” ”+”Height: ”+h;}

OUTPUT1020.25Length:10 Width:20 Height:30Length:4 Width:5 Height:6

Documents

BITS Pilani, Pilani Campus BITS Pilani Pilani Campus Dr. Yashvardhan Sharma CSIS Dept., BITS-Pilani IS F213 Object Oriented Programming