simple class and object examples in java

Simple class and object examples in Java

Presented By Harish Gyanani

What is class?

A class is a blue print from which individual objects are created.

Class naming convention

• By convention, class names capitalize the initial of each word.

• For example: Employee, Boss, DateUtility, PostOffice, RegularRateCalculator.

What are members of Class?

• Field: – field is nothing but the property of the class or object which

we are going to create .– Example if we are creating a class called computer then they

have property like model, memSize, hdSize, osType etc.

• Method:– method is nothing but the operation that an object can

perform it define the behavior of object how an object can interact with outside world.

– Example startMethod (), shutdownMethod ().

Fields in class1.Fields are variables.2.They can be primitives or references to objects.For example, the Employee class has two fields, age and salary.

public class Employee{ int age; int salary}

Fields Naming Conventions

1.Field names should follow the camel naming convention.

2.The initial of each word in the field, except for the first word, is written with a capital letter.

3.For example: age, maxAge, address, validAddress, numberOfRows.

Instance variables

Variables within a class but outside any method.

Instance Methods

Methods defined in a class which is only accessible through the Object of the class are called Instance methods.

Example: Person Class

class Person{ String name; int age;}

Example: Person Class

Without methods

Example: Person Classclass keyword

Without methods

Example: Person ClassName of class

class keyword

Without methods

class keyword

Start of class

Without methods

class keyword

Start of class

End of class

Without methods

class keyword

Start of classData members of class

with default access(instance variable)End of

Without methods

How to declare object?

Person p1;

//declare reference to object

Person p1;

//declare reference to object//Syntax: <classname> <objectname>

Person p1;

• It is simply a variable that can refer to an object.

Person p1;

• It is simply a variable that can refer to an object.

nullp1

Person p1;

Allocate memory

p1 = new Person();

Allocate memory

p1 = new Person();

//allocate a Person object

Allocate memory

p1 = new Person();

//allocate a Person object//Syntax: <objectname> = new <classname>();

Allocate memory

p1 = new Person();

• The new operator dynamically allocates (that is, allocates at run time) memory for an object and returns a reference to it.

Allocate memory

p1 = new Person();

• This reference is, the address in memory of the object allocated by new.

Allocate memory

p1 = new Person();

• This reference is, the address in memory of the object allocated by new.

p1 = new Person(); p1

Person object

Combination of these statements

Person p1 = new Person();

//Syntax: <classname> <objectname> = new <classname>();

public class for Person class public class NewClass1{ public static void main(String args[]) { Person obj1 = new Person(); obj1.name=“ramesh"; obj1.age=22;

int a=obj1.age; System.out.println(a); System.out.println(obj1.name); }}

Instance variables are initialized with

object name qualifier

Syntax to set value in instance variable:-

<objectname>.<variablename> = <value>;

public class for Person class

Syntax to get value from instance

variable:-<variable> = <objectname>.<instance_variable_name>

public class NewClass1{ public static void main(String args[]) { Person obj1 = new Person(); obj1.name=“ramesh"; obj1.age=22;

Syntax to set value in instance variable:-

<objectname>.<variablename> = <value>;

Complete Programpublic class NewClass1{ public static void main(String args[]) {

Person obj1 = new Person();obj1.name=“ramesh";obj1.age=22;int a=obj1.age;System.out.println(a);System.out.println(obj1.name);

Output

In this example, barking(), hungry() and sleeping() are instance

methods.

Instance variables

Example 1: Dog Classclass Dog{

String breed;int age;String color;

void barking() {}void hungry() {}void sleeping() {}

Example 2: Stock class

Class Stock {

public commodity;public price;

public void buy (int no_of commodity) {}public boolean sale () {}

Instance variables

Class Stock {

In this example, buy(), and sale() are instance

methods.

Instance variables

Class Stock {

In this example, buy(), and sale() are instance

methods.

Instance variables

Class Stock {

Collectively, the methods and variables defined within a class arecalled members of the class.

Example 3: Person Classclass Person{ private String name; private int age; public void getData() { Scanner sc = new Scanner(System.in); System.out.println("Enter name and age"); name=sc.nextLine(); age=sc.nextInt(); } public void display() { System.out.println("Name ="+name); System.out.println("Age ="+age); }}

With methods

Private instance variables cannot be accessed outside the class

With methods

getData() and display() instance

methods are public and can be accessed outside

the class.

With methods

the class.

Methods inside class can access private data of class. In this case getData() and display() methods

are accessing private data.

With methods

the class.

Methods inside class can access private data of class. In this case getData() and display() methods

are accessing private data.

They are defined inside

class not inside method

With methods

public class code for Person class

public class abc { public static void main(String args[]) { Person p1 = new Person();

p1.getData();

p1.display(); }}

NOTE: getData() and Display()

method cannot be called without

object qualifier.

Qualifier: Object name

p1.getData();

p1.display(); }}

object qualifier.

Dot operator

p1.getData();

p1.display(); }}

object qualifier.

Instance method because it is called

using object

p1.getData();

p1.display(); }}

object qualifier.

Instance method because it is called

using object

Dot operator

Qualifier: Object name

p1.getData();

p1.display(); }}

object qualifier.

Complete programimport java.util.Scanner;

class Person{ private String name; private int age; public void getData() { Scanner sc = new Scanner(System.in); System.out.println("Enter name and age"); name=sc.nextLine(); age=sc.nextInt(); } public void display() { System.out.println("Name ="+name); System.out.println("Age ="+age); }}

p1.getData(); p1.display(); }}

A program can contain multiple classes but only one public

class(same name as file name) and contains main method

Creating and instantiating Person class object p1.

Calling instance methods of

Person Class using p1 object.

Creating and instantiating Person class object p1.

Output

Members of class

Data members

Members of class

Data members Methods

Members of class

Data members

Instance data members

Methods

Members of class

Data members

Static data members/

Class Variables

Methods

Members of class

Data members

Static data members

Methods

Instance methods

Members of class

Data members

Static data members

Methods

Instance methods

Static methods/

Class methods

Class diagram in UML

UML class is represented by the diagram shown below. The diagram is divided into four parts:-

•The top section is used to name the class.

•The second one is used to show the attributes of the class.

•The third section is used to describe the operations performed by the class.

Variable Types

A class can contain any of the following variable types.• Local variables: Variables defined inside methods,

constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.

• Instance variables: Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.

• Class variables: Class variables are variables declared with in a class, outside any method, with the static keyword.

simple class and object examples in java

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