FUNDAMENTAL PROGRAMMING STRUCTURE IN JAVA – … · Package Statement A package is a collection of...

FUNDAMENTAL PROGRAMMINGSTRUCTURE IN JAVA – CLASSES IN JAVA

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1P.Poovizhi/CS8392/Fundamental programming structure in java – classes in java

Basic Structure of a Simple Java program

7/23/2019P.Poovizhi/CS8392/Fundamental programming structure in java – classes in java

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Documentation Section

It includes the comments that improve thereadability of the program.

A comment is a non-executable statement thathelps to read and understand a programespecially when your programs get more complex.

It is simply a message that exists only for theprogrammer and is ignored by the compiler.


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Java programming language supports three types of comments

Single line (or end-of line) comment Multiline Comment Documentation comments


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Single line (or end-of line) comment

It starts with a double slash symbol (//) andterminates at the end of the current line.

The compiler ignores everything from// to theend of the line.

Example:// Calculate sum of two numbers


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Multiline Comment

Java programmer can use C/C++ comment stylethat begins with delimiter /* and ends with */.

All the text written between the delimiter isignored by the compiler.

This style of comments can be used on part of aline, a whole line or more commonly to definemulti-line comment.

Example/*calculate sum of two numbers and it is a

multiline comment */7/23/2019

P.Poovizhi/CS8392/Fundamental programming structure in java – classes in java

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Documentation comments

This comment style is new in Java. Such comments beginwith delimiter /** and end with */.

The compiler also ignores this type of comments just like itignores comments that use / * and */.

The main purpose of this type of comment is toautomatically generate program documentation.

The java doc tool reads these comments and uses them toprepare your program's documentation in HTML format.

Example/** The text enclosed here will be part of program

documentation */7/23/2019


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Package Statement

A package is a collection of classes, interfaces and sub-packages.

A sub package contains collection of classes,interfaces and sub-sub packages etc.

java.lang.*; package is imported by default andthis package is known as default package.

It must appear as the first statement in the sourcecode file before any class or interface declaration.

This statement is optional.


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Import statements

Java contains many predefined classes that are stored into packages. In order to refer these standard predefined classes in your program,

you need to use fully qualified name (i.e. Packagename.className). But this is a very tedious task as one need to retype the package path

name along with the classname. So a better alternative is to use an import statement. An import statement is used for referring classes that are

declared in other packages.

The import statement is written after a package statement but beforeany class definition.

You can import a specific class or all the classes of the package.


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Example for Import statements

To import Date class of java.util package using importstatement then write.

import java.util.Date; This statement allows the programmer to use the simple

classname Date rather than fully qualified classnamejava.util.Date in the code.Unlike package statement, youcan specify more than one import statement in yourprogram.

Example Import java.util.Date; /* imports only the Dateclass in java.util package */ import java.applet.*; //imports all the classes in java applet


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Interface Section

An interface is similar to a class but contains onlyconstants and method declarations.

Interfaces cannot be instantiated. They can only be implemented by classes or

extended by other interfaces. It is an optional section and is used when we

wish to implement multiple inheritance feature inthe program.


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Class Definition

Java program may contain multiple class definition.Classes are primary feature of Java program.

Exampleclass Addition{

void add(String args[]){int a=2, b=3, c;c=a+b;System.out.println(c);}

}7/23/2019


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Main Method Class Section

The Class section describes the information about user-defined classespresent in the program.

A class is a collection of fields (data variables) and methods thatoperate on the fields.

Every program in Java consists of at least one class, the one thatcontains the main method.

The main () method which is from where the execution of programactually starts and follow the statements in the order specified.

The main method can create objects, evaluate expressions, and invokeother methods and much more.

On reaching the end of main, the program terminates and controlpasses back to the operating system.

The class section is mandatory


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Example

class HelloJava{

public static void main(String args[]){

System.out.println("Hello Java");}

}


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Example


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Structure of Java Programs

class class-name {

public static void main(String args[]) {

statement1;

statement2;

…

…

}

}7/23/2019


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Structure of Java Programs

class keyword is used to declare a class in java. public keyword is an access modifier which

represents visibility, it means it is visible to all. static is a keyword, if we declare any method as static void is the return type of the method, it means it

doesn't return any value. main represents the starting point of the program. String[] args is used for command line argument.


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Simple Program of Java

class Simple{

public static void main(String args[]){System.out.println("Hello Java");}

}


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Simple Program of Java

class Example2 {public static void main(String args[]) {int num; num = 100; System.out.println("This is num: " + num);num = num * 2;System.out.print("The value of num * 2 is ");System.out.println(num);}}


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Classes in Java

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Define Class

A class is a group of objects which have commonproperties.

It is a template or blueprint from which objectsare created.


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Components of class declarations

Modifiers : A class can be public or has default access.

Class name: The name should begin with a initial letter.

Superclass(if any): The name of the class’s parent (superclass), if any,

preceded by the keyword extends. A class can only extend (subclass)

one parent.

Interfaces(if any): A comma-separated list of interfaces implemented

by the class, if any, preceded by the keyword implements. A class can

implement more than one interface.

Body: The class body surrounded by braces, { }.


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Class in Java can contain


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General Form of a Class

class classname {

type instance-variable1;

type instance-variable2;

// ... type instance-variableN;

type methodname1(parameter-list) {

// body of method }

type methodname2(parameter-list) {

// body of method }

// ...type methodnameN(parameter-list) {

// body of method

} }


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Instance Variable

A variable which is created inside the class butoutside the method is known as an instancevariable.

Instance variable doesn't get memory at compiletime.

It gets memory at runtime when an object orinstance is created.


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Method in Java

A method is like a function which is used toexpose the behavior of an object.

Advantage of MethodCode ReusabilityCode Optimization


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new keyword in Java

Used to allocate memory at runtime. All objects get memory in Heap memory area


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Object and Class Example

class Student{//defining fieldsint id;//field or data member or instance variableString name;//creating main method inside the Student classpublic static void main(String args[]){//Creating an object or instanceStudent s1=new Student();//creating an object of Student//Printing values of the objectSystem.out.println(s1.id);//accessing member through reference variableSystem.out.println(s1.name);

}}


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Main within class

Output0

null

Object and Class Example

class Student{int id;String name;}//Creating another class TestStudent1 which contains the main methodclass TestStudent1{public static void main(String args[]){Student s1=new Student();System.out.println(s1.id);System.out.println(s1.name);}}


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Main outside class

3 Ways to initialize object

By reference variable By method By constructor


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Initialization through reference

Initializing an object means storing data into the objectclass Student{int id;String name;}class TestStudent2{public static void main(String args[]){Student s1=new Student();s1.id=101;s1.name="Sonoo";System.out.println(s1.id+" "+s1.name);//printing members with a white space

}}


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Output

101 Sonoo

Create multiple objects

class Student{int id;String name;}class TestStudent3{public static void main(String ar

gs[]){//Creating objectsStudent s1=new Student();Student s2=new Student();


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//Initializing objectss1.id=101;s1.name="Sonoo";s2.id=102;s2.name="Amit";

//Printing dataSystem.out.println(s1.id+" "+s1.n

ame);System.out.println(s2.id+" "+s2.n

ame);}} Output

101 Sonoo 102 Amit

Initialization through method

class Student{int rollno;String name;void insertRecord(int r, String n){rollno=r;name=n;}void displayInformation(){System.o

ut.println(rollno+" "+name);}}


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class TestStudent4{public static void main(String args[]){Student s1=new Student();

Student s2=new Student();

s1.insertRecord(111,"Karan");s2.insertRecord(222,"Arya

n");s1.displayInformation();s2.displayInformation();}}

Initialization through a constructor

class Employee{int id;String name;float salary;void insert(int i, String n, float s) {

id=i;name=n;salary=s;

}void display(){System.out.println(id

+" "+name+" "+salary);}}


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public class TestEmployee {public static void main(String[] args){

Employee e1=new Employee();Employee e2=new Employee();Employee e3=new Employee();e1.insert(101,"ajeet",45000);e2.insert(102,"irfan",25000);e3.insert(103,"nakul",55000);e1.display();e2.display();e3.display();

}}

101 ajeet 45000.0 102 irfan 25000.0 103 nakul 55000.0

Rectangle

class Rectangle{int length;int width;void insert(int l, int w){length=l;width=w;}void calculateArea(){System.out.pr

intln(length*width);}}


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class TestRectangle1{public static void main(String args[]){Rectangle r1=new Rectangle();Rectangle r2=new Rectangle();r1.insert(11,5);r2.insert(3,15);r1.calculateArea();r2.calculateArea();

}}

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Anonymous object

Anonymous simply means nameless. An object which has no reference is known as an

anonymous object. It can be used at the time of object creation only. Example:

new Calculation();


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Calling method through a reference Calling method through an anonymous object

Calculation c=new Calculation();c.fact(5);

new Calculation().fact(5);

Example for Anonymous object

class Calculation{void fact(int n){int fact=1;for(int i=1;i<=n;i++){fact=fact*i;

}System.out.println("factorial is "+fact);}public static void main(String args[]){new Calculation().fact(5);//calling method with anonymous object}}


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Factorial is 120

Creating multiple objects by one type only

Rectangle r1=new Rectangle(), r2=new Rectangle();


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Creating multiple objects by one type only - Example

class Rectangle{int length;int width;void insert(int l,int w){length=l;width=w;}void calculateArea(){System.out.pri

ntln(length*width);}}


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class TestRectangle2{public static void main(String args[]){Rectangle r1=new Rectangle(),r2=n

ew Rectangle();//creating two objectsr1.insert(11,5);r2.insert(3,15);r1.calculateArea();r2.calculateArea();

}}

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Assignment

Write a java program to demonstrate the working of Bank System.

where we deposit and withdraw amount from our account. Create an Account class which has deposit() and withdraw

() methods method to check the balance of the account method to display the values of an object Create a test class to deposit and withdraw amount


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Solution - Real Time Example

class Account{int acc_no;String name;float amount;//Method to initialize objectvoid insert(int a,String n,float amt){acc_no=a;name=n;amount=amt;}


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Contd..,

//deposit methodvoid deposit(float amt){amount=amount+amt;System.out.println(amt+" deposited");}//withdraw methodvoid withdraw(float amt){if(amount<amt){System.out.println("Insufficient Balance");}else{amount=amount-amt;System.out.println(amt+" withdrawn");}}


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Contd..,

//method to check the balance of the account

void checkBalance(){System.out.println("Balance is: "+amount);}

//method to display the values of an object

void display(){System.out.println(acc_no+" "+name+" "+amount);}

}//Creating a test class to deposit

and withdraw amount


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class TestAccount{public static void main(String[] args){Account a1=new Account();a1.insert(832345,"Ankit",1000);a1.display();a1.checkBalance();a1.deposit(40000);a1.checkBalance();a1.withdraw(15000);a1.checkBalance();}}

Reference

https://www.javatpoint.com/java-tutorial

http://nptel.ac.in/courses/106106147/

http://www.nptelvideos.com/java/java_video_lect

ures_tutorials.php

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