Simple class and object examples in java

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

1. Simple class and object examples in Java Presented By Harish Gyanani

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

3. Class naming convention By convention, class names capitalize the initial of each word. For example: Employee, Boss, DateUtility , PostOffice , RegularRateCalculator .

4. 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 ().

5. Fields in class Fields are variables. 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 }

6. Fields Naming Conventions

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

8. Fields Naming Conventions Field names should follow the camel naming convention. The initial of each word in the field, except for the first word, is written with a capital letter.

9. Fields Naming Conventions Field names should follow the camel naming convention. The initial of each word in the field, except for the first word, is written with a capital letter. For example: age, maxAge, address, validAddress, numberOfRows.

10. Instance variables Variables within a class but outside any method .

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

12. Example: Person Class class Person { String name; int age; }

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

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

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

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

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

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

19. How to declare object?

20. How to declare object? Person p1;

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

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

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

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

25. Allocate memory

26. Allocate memory p1 = new Person();

27. Allocate memory p1 = new Person(); //allocate a Person object

28. Allocate memory p1 = new Person(); //allocate a Person object //Syntax: <objectname> = new <classname>();

29. 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.

30. 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 .

31. 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

32. Combination of these statements

33. Combination of these statements

34. Combination of these statements

35. Combination of these statements

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

37. Person p1 = new Person(); Combination of these statements //Syntax: < classname > < objectname > = new < classname >();

38. 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); } }

39. 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

40. 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>;

41. 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>;

42. 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; }

43. O utput

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

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

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

47.  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 () {} }

48.  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 are called members of the class.

49. 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); } }

50. 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

51. Private instance variables cannot be accessed outside the class 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

52. 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 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

53. 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 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); } } Methods inside class can access private data of class. In this case getData () and display() methods are accessing private data. With methods

54. 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 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); } } 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

55. 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.

56. 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.

57. 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.

58. 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.

59. 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.

60. Complete program import 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(); } }

61. 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 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(); } }

62. 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 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(); } }

63. 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 program import 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(); } }

64. Output

65. Members of class

66. Members of class Members of class

67. Members of class Members of class Data members

68. Members of class Members of class Data members Methods

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

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

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

72. Members of class Members of class Data members Instance data members Static data members Methods Instance methods Static methods/ C lass methods

73. Class diagram in UML

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

75. 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 .

76. 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 .

77. 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.

78. 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 .