Simple class and object examples in java

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Information about Simple class and object examples in java
Education

Published on February 12, 2014

Author: gyananihrithik

Source: slideshare.net

Description

Simple class and object examples in java
classes
objects
fields
properties
variables
instance variables
local variables
class variables
methods
members of class
data members
UML representation of class
instance methods
static methods
create object
instantiate object
object qualifier
reference to object
allocate memory to object
class naming conventions
fields naming conventions

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, Regula rRateCalculator.

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 class 1.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

Fields Naming Conventions 1.Field names should follow the camel naming convention.

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.

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 class Person { String name; int age; } Without methods

class keyword Example: Person Class class Person { String name; int age; } Without methods

class keyword Example: Person Class class Person { String name; int age; } Name of class Without methods

class keyword Example: Person Class class Person { String name; int age; } Name of class Without methods Start of class

class keyword Example: Person Class class Person { String name; int age; } Name of class Without methods Start of class End of class

class keyword Example: Person Class class Person { String name; int age; } End of class Data members of class with default access(instance variable) Name of class Without methods Start of class

How to declare object?

How to declare object? Person p1;

How to declare object? Person p1; //declare reference to object

How to declare object? Person p1; //declare reference to object //Syntax: <classname> <objectname>

How to declare object? Person p1; //declare reference to object //Syntax: <classname> <objectname> • It is simply a variable that can refer to an object.

How to declare object? Person p1; //declare reference to object //Syntax: <classname> <objectname> • It is simply a variable that can refer to an object. Person p1; null p1

Allocate memory

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(); //allocate a Person object //Syntax: <objectname> = new <classname>(); • 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(); //allocate a Person object //Syntax: <objectname> = new <classname>(); • The new operator dynamically allocates (that is, allocates at run time) memory for an object and returns a reference to it. • This reference is, the address in memory of the object allocated by new.

Allocate memory p1 = new Person(); //allocate a Person object //Syntax: <objectname> = new <classname>(); • The new operator dynamically allocates (that is, allocates at run time) memory for an object and returns a reference to it. • This reference is, the address in memory of the object allocated by new. name p1 = new Person(); p1 age Person object

Combination of these statements

Combination of these statements

Combination of these statements

Combination of these statements

Combination of these statements Person p1 = new Person();

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); } }

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

public class for Person class Instance variables are initialized with object name qualifier 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); Syntax to set value in System.out.println(obj1.name); instance variable:} } <objectname>.<variablename> = <value>;

public class for Person class Instance variables are initialized with object name qualifier 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); Syntax to set value in System.out.println(obj1.name); instance variable:} } Syntax to get value <objectname>.<variablename> = <value>; from instance variable:<variable> = <objectname>.<instance_variable_name>

Complete Program 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); } } class Person { String name; int age; }

Output

Example 1: Dog Class class Dog { String breed; int age; String color; Instance variables void barking() {} void hungry() {} void sleeping() {} } In this example, barking(), hungry() and sleeping() are instance methods.

Example 2: Stock class Class Stock { public commodity; public price; public void buy (int no_of commodity) {} public boolean sale () {} }

Example 2: Stock class Class Stock { public commodity; public price; Instance variables public void buy (int no_of commodity) {} public boolean sale () {} }

Example 2: Stock class Class Stock { public commodity; public price; Instance variables public void buy (int no_of commodity) {} public boolean sale () {} } In this example, buy(), and sale() are instance methods.

Example 2: Stock class Class Stock { public commodity; public price; Instance variables public void buy (int no_of commodity) {} public boolean sale () {} } Collectively, the methods and variables defined within a class are called members of the class. In this example, buy(), and sale() are instance methods.

Example 3: Person Class 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); } }

Example 3: Person Class 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); } } With methods

Example 3: Person Class class Person { private String name; private int age; Private instance variables cannot be accessed outside the class 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

Example 3: Person Class class Person { private String name; private int age; Private instance variables cannot be accessed outside the class 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 getData() and display() instance methods are public and can be accessed outside the class.

Example 3: Person Class class Person { private String name; private int age; Private instance variables cannot be accessed outside the class 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 getData() and display() instance methods are public and can be accessed outside 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 Example 3: Person Class class Person { private String name; private int age; Private instance variables cannot be accessed outside the class 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 getData() and display() instance methods are public and can be accessed outside the class. Methods inside class can access private data of class. In this case getData() and display() methods are accessing private data.

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.

public class code for Person class public class abc { public static void main(String args[]) { Person p1 = new Person(); p1.getData(); p1.display(); } } Qualifier : Object name NOTE: getData() and Display() method cannot be called without object qualifier.

public class code for Person class public class abc { public static void main(String args[]) { Person p1 = new Person(); p1.getData(); p1.display(); } } Dot operator NOTE: getData() and Display() method cannot be called without object qualifier.

public class code for Person class public class abc { public static void main(String args[]) { Person p1 = new Person(); p1.getData(); p1.display(); } } Instance method because it is called using object NOTE: getData() and Display() method cannot be called without object qualifier.

public class code for Person class public class abc { public static void main(String args[]) { Person p1 = new Person(); p1.getData(); p1.display(); } } Qualifier : Object name Instance method because it is called using object Dot operator NOTE: getData() and Display() method cannot be called without object qualifier.

Complete program import java.util.Scanner; class Person { private String name; private int age; public class abc { public static void main(String args[]) { Person p1 = new Person(); } 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();

Complete program import java.util.Scanner; class Person { private String name; private int age; public class abc { public static void main(String args[]) { Person p1 = new Person(); } 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

Complete program import java.util.Scanner; class Person { private String name; private int age; public class abc { public static void main(String args[]) { Person p1 = new Person(); p1.getData(); p1.display(); } 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); } } Creating and instantiating Person class object p1. A program can contain multiple classes but only one public class(same name as file name) and contains main method

Complete program import java.util.Scanner; class Person { private String name; private int age; public class abc { public static void main(String args[]) { Person p1 = new Person(); p1.getData(); p1.display(); } 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); } } Creating and instantiating Person class object p1. A program can contain multiple classes but only one public class(same name as file name) and contains main method Calling instance methods of Person Class using p1 object.

Output

Members of class

Members of class Members of class

Members of class Members of class Data members

Members of class Members of class Data members Methods

Members of class Members of class Data members Instance data members Methods

Members of class Members of class Data members Instance data members Static data members/ Class Variables Methods

Members of class Members of class Data members Instance data members Static data members Methods Instance methods

Members of class Members of class Data members Instance data members Static data members Methods Instance methods Static methods/ Class methods

Class diagram in UML

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

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.

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.

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.

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