Java Tutorials and codes

Association in Java Programming

2014-02-11T13:33:00.002-08:00

What is Association in java

Relationship between one or more classes
Aggregation and composition is a special type of association

For example, Student Belongs to a specific Faculty and One faculty has many students

Code Example:


class Student {

  String name;

  Student () {name=null; }

  Student (String n) {name=n; }

  void setName(String n) {name=n;}

  String getName() { return name;}

}

class Faculty{

  Student student[];

  int size;

  Faculty (int s){size=0;

           student=new Student[s];}

void belongsTo(Student p){

    student[size]=p;

    size++;

}

void print(){

   for (int i=0;i<size;i++)

      System.out.println(student[i].getName());

}

}

class Test{

public static void main (String a[]){

    Faculty FIT= new Faculty(5);

    Faculty FMS= new Faculty(5);

    

    Student s1=new Student("Hamza");

    FMS.belongsTo(s1);

    

    Student s2=new Student("Kamil");

    FIT.belongsTo(s2);

    

    Student s3=new Student("Shoaib");

    FMS.belongsTo(s3);

    

    System.out.println("FIT students");

    FIT.print();

    System.out.println("FMS students");

    FMS.print();

    }

}

Practice 2

Student register for one or more courses
One student can register max 5 courses
Each course may have a maximum of 50 registered students


class Student {

String name;

Student () {name=null; }

Student (String n) {name=n; }

void setName(String n) {name=n;}

String getName() { return name;}

}

class Course {

String title;

Course  () {title=null; }

Course  (String n) {title=n; }

void setTitle(String n) {title=n;}

String getTitle() { return title;}

}



class Registration{

Student s;

Course  c;

void register(Student s,Course c ) {

            this.s=s;

            this.c=c;

}

void print(){

 System.out.print(s.getName() + " Register ");

 System.out.println(c.getTitle());

}

}



class Test{

public static void main (String a[]){

Registration r[]=new Registration[10];

Student s1= new Student("Jhon");

Student s2= new Student("Alex");

Student s3= new Student("Jawad");

Course  c1 = new Course ("DSA");

Course  c2 = new Course ("Algo");

r[0] = new Registration();

r[0].register(s1,c1);

r[1] = new Registration();

r[1].register(s2,c1);

r[2] = new Registration();

r[2].register(s3,c2);

r[3] = new Registration();

r[3].register(s1,c2);

for (int i=0;i<4;i++)

   r[i].print();



}

}

Generalization in java with examples

2013-11-20T05:33:00.000-08:00

Generalization basic points

Type/Subtype Relationship
"IS A " Relationship
One class has many subclasses
General class can have many Special classes as subclasses
General names can be used to Address special names
Top to bottom hierarchy goes towards specialization
Bottom to top Hierarchy goes towards generalization

e.g Soap
Bath Soap and Washing Soap is a subclass of Soap

e.g COldDrink
Pepsi Team

-Cold Drink is a general class
-Pepsi and team are special classes

e.g Person

Teacher Student
e.g Car
Sedan Hatchback
Honda Suzuki Suzuki Toyota
civic Liana Alto vitz
....................................................\
Some Examples from Nature

e.g. Animal
Mammals Reptiles

Human Cow Snake Croc

Student Teacher Cobra Python

e.g Fruits
Mango Orange Banana Strawberry

Some Examples from Kitchen

e.g Cutlery
fork Spoon Knife

Some Examples from Library

e.g
Book
Social Science Natural Science

.................................................
Some Examples from Computer Lab

e.g Computer
Desktop Laptop
Compaq HP

e.g Printer
Laser Inkjet DotMatrix

....................................................

Some Examples from Car Parking

e.g vehicle

Car bike Van Truck

...............................................
Discriminator
- Special types based on discriminator

E.g Fruit (color)

Red Yellow Green

e.g Person (religion)

Muslims Chris Hindu Jew

Person (Designation)

law Student teacher Mula Dako thief Doctor poond

e.g Student (Cgpa)

above3 below3

e.g Student (grade)

AGrade Bgrade

e.g Person(economically)

lowerMiddle Middle UpperMiddle Upper

Types Of Generalization

Inheritance
Realization

Aggregation in java with example codes

2013-08-28T03:47:00.000-07:00

What is aggregation?

"Have a" or "May have" relationship

Aggregation is special type of association

Life time of Component object is independent from its container object

E.g A school has one student




class Student {

String name;

Student () {name=null; }

Student (String n) {name=n; }

void setName(String n) {name=n;}

String getName() { return name;}

}

class School{

Student  st;

School (  ) { st=null;}

School (Student s){st=s;}

void studentName(){

  System.out.println(st.getName());

}

}

class Post{

public static void main (String a[]){

Student st=new Student("Imran");

School s= new School(st);

s.studentName();

}

}

Output
Hello

Problem : A Car must has Engine and body.
A Car may or may not has Ac

Adding Ac as an Aggregate object in car


class Engine{

void start() {

System.out.println("Engine Start");

}

}

class Body{

void type() {

System.out.println("Saloon Type");

}

}

class Ac{

void cool() {

System.out.println("Super Cool");

}

}



class Car{

Engine e;

Body b;

Ac ac;

Car () { e=new Engine();

         b= new Body();

         ac=null;        

}

Car (Ac p) { e=new Engine();

             b= new Body();

             ac=p;        

}

void specification(){

e.start();

b.type();

if (ac!=null)

    ac.cool();

}

}




class Post{

  public static void main(String a[]){

  Ac ac= new Ac();

  Car c1= new Car(ac); // car with AC

  c1.specification();

  Car c2= new Car(); // car without AC

  c2.specification();

  }

  }

Output:

Engine Start
Saloon Type
Super Cool
Engine Start

Saloon Type

Same with Data Members

class Engine{
int hrp;
Engine () { }
Engine(int hp) { hrp=hp;}
void start() {
System.out.println("Engine Power "+hrp);
}
}
class Body{
int size;
Body()  {}
Body(int s){size=s;}
void type() {
System.out.println("Saloon Size "+size);
}
}
class Ac{
int price;
Ac(int p){price=p;}
void cool() {
System.out.println("Super Cool with price =" + price);
}
}
class Car{
Engine e;
Body b;
Ac  ac;
int price;
Car () { e=new Engine();
         b= new Body();
         ac=null;
}
Car (int p,int hp,int s,Ac a) { 
e=new Engine(hp);
b= new Body(s); 
price=p; 
ac=a;
}
void specification(){
e.start();
b.type();
System.out.println("Car Price "+price);
if (ac!=null)
    ac.cool();
}
}

class Post{
public static void main(String a[]){
Ac ac=new Ac(1000);
Car c= new Car(1000,800,5,ac);
c.specification();

}
}

Output:

Engine Power 800
Saloon Size 5
Car Price 1000

Super Cool with price =1000

Composition in Java with code examples

2013-05-18T09:43:00.000-07:00

Composition

"Have a " Relationship OR "Must have" relationship between two or more classes.

Container class contains component's class object

For example a Car is composed of engine and body. Where Car is a container class and both Engine and Body are components class.

Life Time Issue in Composition

Life time of component objects depends on container class
If Container object destroyed, component objects will be also destroyed

Car Example

There is a composition relationship between car's body and engine.
Car must have Engine and body
If Car destroyed, both Engine and Body will be destroyed

class Engine{
Engine () { System.out.println("Engine created");}
void start() {
System.out.println("Engine Start");
}
}
class Body{
Body () {System.out.println("Body created");}
void type() {
System.out.println("Saloon Type");
}
}
class Car{
Engine e;
Body b;
Car () { e=new Engine();
b= new Body();
System.out.println("Car created");
}
void specification(){
e.start();
b.type();
}
}
class Post{
public static void main(String a[]){
Car c= new Car();
c.specification();
}
}

Output
Engine created
Body created
Car created
Engine Start
Saloon Type

Same problem with Data Members

Example : Create car(price) class as container object with engine(hp) and body(size) as component objects

class Engine{
int hrp;
Engine () { }
Engine(int hp) { hrp=hp;}
int getHrp() { return hrp;}
void setHrp(int h){hrp=h;}
void start() {
System.out.println("Engine Power "+hrp);
}
}
class Body{
int size;
Body() {}
Body(int s){size=s;}
void setSize(int s){size=s;}
int getSize(){ return size;}
void type() {
System.out.println("Saloon Size "+size);
}
}
class Car{
Engine e;
Body b;
int price;
Car () { e=new Engine();
b= new Body();
}
Car (int p,int hp,int s) {
e=new Engine(hp);
b= new Body(s);
price=p;
}
void specification(){
e.start();
b.type();
System.out.println("Car Price "+price);
}
}
class Post{
public static void main(String a[]){
Car c= new Car(40000,800,5);
c.specification();
}
}

Output:
Engine Power 800
Saloon Size 5
Car Price 40000

Example : Create 4 cup of tea and serve into tray.

CupofTea consist of milk, sugar, Teabag and water.

Container Class :
CupofTea

Component Class :
Milk (qty)
sugar(spoone)
Teabag(make)
Water(qty)
Tray :
Array of cupoftea

class Milk {
private int qty;
public Milk () {}
public Milk (int q) {qty=q;}
public void setQty(int q) {qty=q;}
public int getQty(){ return qty;}
}
class Sugar{
private int spoone;
public Sugar () {}
public Sugar (int q) {spoone=q;}
public void setSpoone(int q) {spoone=q;}
public int getSpoone(){ return spoone;}
}
class Teabag{
private String make;
public Teabag () {}
public Teabag (String q) {make=q;}
public void setMake(String q) {make=q;}
public String getMake(){ return make;}
}
class Water{
private int qty;
public Water () {}
public Water (int q) {qty=q;}
public void setQty(int q) {qty=q;}
public int getQty(){ return qty;}
}
class CupOfTea{
private Milk m;
private Sugar s;
private Teabag t;
private Water w;
public CupOfTea() {
m=new Milk();
s=new Sugar();
t=new Teabag();
w= new Water();
}
public CupOfTea(int q,int s1, String m1, int w1){
m=new Milk(q);
s=new Sugar(s1);
t=new Teabag(m1);
w= new Water(w1);
}
public void setMilk(int q){m.setQty(q);}
public void setSugar(int t){s.setSpoone(t);}
public void setTeabag(String q){t.setMake(q);}
public void setWater(int q){w.setQty(q);}
public int getMilk(){return m.getQty();}
public int getSugar(){return s.getSpoone();}
public String getTeabag(){return t.getMake();}
public int getWater(){return w.getQty();}
}
class Post {
public static void main(String o[]){
CupOfTea tray[] = new CupOfTea[4];
CupOfTea c1=new CupOfTea(5,1,"Lipton",3);
CupOfTea c2=new CupOfTea(5,10,"Lipton",3);
CupOfTea c3=new CupOfTea(5,0,"Olpers",3);
CupOfTea c4=new CupOfTea(5,15,"TarakChai",3);
tray[0]=c1;
tray[1]=c2;
tray[2]=c3;
tray[3]=c4;
for (int i=0;i<tray.length;i++){
System.out.println("Cup no = " + i);
System.out.println("Milk="+tray[i].getMilk());
System.out.println("sugar="+tray[i].getSugar());
System.out.println("Teabag="+tray[i].getTeabag());
System.out.println("water="+tray[i].getWater());
System.out.println("...........................");
}
}
}

Output:

Cup no = 0

Milk=5

sugar=1

Teabag=Lipton

water=3

...........................

Cup no = 1

Milk=5

sugar=10

Teabag=Lipton

water=3

...........................

Cup no = 2

Milk=5

sugar=0

Teabag=Olpers

water=3

...........................

Cup no = 3

Milk=5

sugar=15

Teabag=TarakChai

water=3

...........................

Life Time of Objects in Java with code Examples

2013-05-15T09:53:00.000-07:00

Lifetime Concept in Java Programming

Time between allocation and de-allocation of memory

Life time depends on block i.e { }

life time starts when declaration statement elaborated and expires at end of block

There is no delete keyword in java

Garbage collector is responsible for all deletes

Java is free from dangling reference and garbage problem

Why java does not support delete keyword?

Java does not support delete keyword because there is a garbage collector in java
This automatically runs and collects dangling or un-referenced objects.That is why there is no delete keyword in java like C++.
Garbage collector can be explicitly like that System.gc();
When a life of object ends garbage collector runs automatically

{
int a=5;// life Time start here
a++;
} //life time expire

Example 2
Let Student is a class
Student s; // creating a reference of Student class
{Student t; // l
t= new Student(); // Life Time Start
s=t;
}//garbage collector expires Student t reference from stack

When life of an object ends it is available for garbage collector to collect.

Example 3

{Student t;
t= new Student(); // Life Time Start
t= new Student();
t= new Student();
t= new Student();

}//life time expires

Note: We can explicitly request garbage collector to invoke using System class gc method but it will again depends of garbage thread

System.gc();

Example 4

{Student t;
t= new Student(); // Life Time Start
System.gc(); // invokeing garbage collector
t= new Student();
t= new Student();
System.gc();
t= new Student();

}//
Destructors in java

No explicit destructors in java as c++
finalize method can be used as action on expiry

protected void finalize() {}
Example
class Car{
String make;
Car (String m){make=m;}
String getMake() { return make;}
void setMake(String m){make=m;}
protected void finalize(){
// action on expiry
}
}

There are no explicit pointers in java like C++

Problem with Pointers

1- Garbage (Memory Leakage)
c++
Student *t;
t= new Student(); // Life Time Start
t= new Student();
t= new Student();
t= new Student();

2- Dangling Reference
c++
Student *c1=new Student();
Student *c2=c1;
delete c2;
c1->getName();

What are Objects In Java Programming

2013-05-10T03:03:00.003-07:00

Java Programming Objects

Object name in java is a reference
References are used to store address
All objects created in heap with new statement
There is no delete statement in java
Deletion of object will be handled by garbage collector
A special value null can be assigned
Always passed by reference

Object Example
Let Student is a class
Student t; //here t is a reference variable

t=null; // Assigning a reference to null

// t=NULL Error here as Java is case sensitive null and NULL are different

t = new Student(); // created in heap
- Alias name can created
for example
Student y=t; // here y and t referring to same object in heap

Parameter Passing

Primitives by value
Objects by reference

Object as parameter parameter passing by reference
Example 1

class Car{
String make;
String getMake() { return make;}
void setMake(String m){make=m;}

}
class ChangeCarMake{
static void changeMake(Car p){
p.setMake("Suzuki");
}
}
class Test {
public static void main(String o[]){
Car c1=new Car();
c1.setMake("Honda");
ChangeCarMake.changeMake(c1); // passing a reference of class Car System.out.println(c1.getMake());

}
}
output
Suzuki

A program which Exchange Marks

class Student {
String name;
int marks;
Student ( ) { }
Student (String n, int m ){name=n; marks=m;}
String getName() {return name;}
int getMarks() {return marks;}
void setName(String n) {name=n;}
void setMarks(int m) { marks=m;}
}

class Test {
static void exchangeMarks(Student a, Student b){
int t=a.getMarks();
a.setMarks(b.getMarks());
b.setMarks(t);

}
public static void main(String o[]){
Student s1=new Student("Amir",20);
Student s2=new Student("Imran",90);
exchangeMarks(s1,s2);
System.out.println(s1.getName() + " "+ s1.getMarks());
System.out.println(s2.getName() + " "+ s2.getMarks());
}
}

Naming Conventions and Indentation in Java

2013-04-09T09:19:00.000-07:00

Quality coding Standards

Naming Rules

a) Declare name for a class in Java     class Book
     classs EnglishBook
     class Car
     class Student
     class StudentTranscript

b) Variables/function Name
     int book
     int englisBook
     int enonomyOfAmerica
     int car
     int student
     int studentTranscript

c) Constants (AllUpper)
    final int A=5;
    final int SHOAIB=5;
    final int ECONOMYOFPAKISTAN=34;
.....................................
Indentation in Java Programming
class Test{
}
void f (){
}
while (a>b){

}
.....................................
Data Types in Java
1- int (4 bytes) short (2 byte) long (8)
2- float (4bytes) 2.5f
      float a=2.5f;
3- double (8 bytes) 2.5
4- byte ( 1 byte)
5- char (2 bytes) unicode ( Range 0 to 65536)
     'a' '\n'
     '\uxxxx'   '\u2345'
    '\u' Followed by exactly four hexadecimal digits

6- boolean (true, false)
.....................................
Type Conversion in Java    Two Methods
     a) Coercion
     b) Casting

1- Coercion
Implicit type conversion by compiler
      int a=5;
      float b=a;//promotion
      double g=2.5;
      int y=g;//demotion (data loss)
               // not allowed
     float a=2.5; //demotion
                  //not allowed

2- Casting
Explicit type conversion by programmer

       double b=2.5;
       int x=(int)b;
....................................
Practice 1
class Test{
public static void main(String o[]) {
float a=2.5;

}
}
...................................
Practice 2
class Test{
public static void main(String o[]) {
float a=2.5f;

}
}
.....................................
Practice 3
class Test{
public static void main(String o[]) {
float a=2.5f;
int b=a;

}
}
.........................
Practice 4
class Test{
public static void main(String o[]) {
float a=2.5f;
int b=(int)a;
}
}
....................................
Some Hint from C++
void main() {
int i=1;
int sum=0;
while(i<=3){
float j;
cin>>j;
sum=sum+j;
i++;
}
cout<<sum;
}

Java Basics Quick Review with examples

2013-04-09T09:11:00.000-07:00

Java Programming Language

High Level
Strongly Typed means no automatic type conversions in java like C++
Case Sensitive
Successor of c++
Pure Object Oriented Language

Hello World Problem in java

class Test{
public static void main(String a[]) {
System.out.println("Hello World");
}

}

More than one Classes

class Test1{
void print () {
System.out.println("Test class");
}
}
class Test{
public static void main(String a[]) {

Test1 t=new Test1();
t.print();
}

}

Rule of Thumb: Don't write extra functions in driver class

Extra function and data in driver class

class Test{
int sq(int p){
   return p*p;
}
public static void main(String a[]) {
    Test t = new Test();
System.out.println(t.sq(4));
}

}
..................................
call without object instantiation
..................................

class Test{
static int sq(int p){
   return p*p;
}
public static void main(String a[]) {
System.out.println(sq(4));
}

}
.....................................

Math Class Example
class MyMath{
int sq(int p){
return p*p;
}
int power(int x , int y){
if (y==0)
return 1;
else
return x * power(x,y-1);
}
}
class Test{
public static void main(String a[]) {
MyMath m= new MyMath();
System.out.println(m.sq(4));
System.out.println(m.power(2,3));
}
}
.....................................

calling methods without object instantiation

..................................
class MyMath{
static int sq(int p){
return p*p;
}
static int power(int x, int y){
if (y==0)
return 1;
else
return x * power(x,y-1);
}
}
class Test{
public static void main(String a[]) {
System.out.println(MyMath.sq(4));
System.out.println(MyMath.power(2,3));
}
}