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
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
Example: Person Classclass keyword
class Person{ String name; int age;}
Without methods
Example: Person ClassName of class
class keyword
class Person{ String name; int age;}
Without methods
Example: Person ClassName of class
class keyword
Start of class
class Person{ String name; int age;}
Without methods
Example: Person ClassName of class
class keyword
Start of class
End of class
class Person{ String name; int age;}
Without methods
Example: Person ClassName of class
class keyword
Start of classData members of class
with default access(instance variable)End of
class
class Person{ String name; int age;}
Without methods
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.
nullp1
Person 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.
p1 = new Person(); p1
name
age
Person object
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();
Combination of these statements
//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 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;
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>;
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);
}}
class Person{ String name; int age;}
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
Example 2: Stock class
Class Stock {
public commodity;public price;
public void buy (int no_of commodity) {}public boolean sale () {}
}
In this example, buy(), and sale() are instance
methods.
Instance variables
Example 2: Stock class
Class Stock {
public commodity;public price;
public void buy (int no_of commodity) {}public boolean sale () {}
}
In this example, buy(), and sale() are instance
methods.
Instance variables
Example 2: Stock class
Class Stock {
public commodity;public price;
public void buy (int no_of commodity) {}public boolean sale () {}
}
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); }}
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
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
getData() and display() instance
methods are public and can be accessed outside
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
getData() and display() instance
methods are public and can be accessed outside
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); }}
Methods inside class can access private data of class. In this case getData() and display() methods
are accessing private data.
With methods
Private instance variables cannot be accessed outside the class
getData() and display() instance
methods are public and can be accessed outside
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); }}
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.
public class code for Person class
Qualifier: Object name
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
Dot operator
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
Instance method because it is called
using object
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
Instance method because it is called
using object
Dot operator
Qualifier: Object name
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.
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); }}
public class abc { public static void main(String args[]) { Person p1 = new Person();
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 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); }}
public class abc { public static void main(String args[]) { Person p1 = new Person();
p1.getData(); p1.display(); }}
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 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); }}
public class abc { public static void main(String args[]) { Person p1 = new Person();
p1.getData(); p1.display(); }}
Calling instance methods of
Person Class using p1 object.
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 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); }}
public class abc { public static void main(String args[]) { Person p1 = new Person();
p1.getData(); p1.display(); }}
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.