1. Chapter 7 Objects and Classes

2. OO Programming Concepts
An Object corresponds to entity in the real world.
Each object has a unique
identity
state – properties that can change in value
behaviors – what can the object do?

3. OO Programming Concepts
An object encapsulates (wraps together)
attributes (data)
behaviors (methods)
Goal: Information Hiding

4. OOP Interface - what the object can do, proper usage vs.
Implementation – how the object works

5. Objects
An object has both a state and behavior. The state defines the object, and the behavior defines what the object does.

6. Classes Classes define objects of the same type. Class consists of
Variables – data
Methods – behavior

7. Constructors Methods that are used to instantiate a class
Have the same name as the class
Do not return a value
Note: a method can have the same name as the class and have a return type -
It’s not a constructor!

8. Classes

9. UML Class Diagram

10. Constructors
Constructors are a special kind of methods that are invoked to construct objects.
Circle() { } Circle(double newRadius) { radius = newRadius;

11. Constructors, cont. Constructors · have the same name as class
·       have no return type—not even void
·       invoked using new operator at object creation
Constructors initialize objects.
Constructors can be overloaded.

12. Creating Objects Using Constructors
new ClassName();
Example:
new Circle();
new Circle(5.0);

13. Default Constructor A class may be declared without constructors.
A constructor with no parameters is referred to as a no-arg constructor.
No-arg constructor with empty body is implicitly declared.
This default constructor is provided automatically only if no constructors are explicitly declared.

14. Declaring Object Reference Variables
To reference an object, assign the object to a reference variable.
Syntax:
ClassName objectRefVar;
Example:
Circle myCircle;

15. Declaring/Creating Objects in a Single Step
ClassName objectRefVar = new ClassName();
Example:
Circle myCircle = new Circle();
Assign object reference
Create an object

16. Dot Operator Referencing the object’s data: objectRefVar.data
e.g., myCircle.radius
Invoking the object’s method:
objectRefVar.methodName(arguments)
e.g., myCircle.getArea()

17. A Simple Circle Class
Objective: Demonstrate creating objects, accessing data, and using methods.
TestCircle1
Run

18. Trace Code animation Declare myCircle
Circle myCircle = new Circle(5.0);
SCircle yourCircle = new Circle();
yourCircle.radius = 100;
myCircle
no value

19. Trace Code, cont. animation Circle myCircle = new Circle(5.0);
Circle yourCircle = new Circle();
yourCircle.radius = 100;
myCircle
no value
Create a circle

20. Assign object reference to myCircle
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
Circle yourCircle = new Circle();
yourCircle.radius = 100;
myCircle
reference value
Assign object reference to myCircle

21. Trace Code, cont. Declare yourCircle animation
Circle myCircle = new Circle(5.0);
Circle yourCircle = new Circle();
yourCircle.radius = 100;
myCircle
reference value
yourCircle
no value
Declare yourCircle

22. Create a new Circle object
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
Circle yourCircle = new Circle();
yourCircle.radius = 100;
myCircle
reference value
yourCircle
no value
Create a new Circle object

23. Assign object reference to yourCircle
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
Circle yourCircle = new Circle();
yourCircle.radius = 100;
myCircle
reference value
yourCircle
reference value
Assign object reference to yourCircle

24. Change radius in yourCircle
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
Circle yourCircle = new Circle();
yourCircle.radius = 100;
myCircle
reference value
yourCircle
reference value
Change radius in yourCircle

25. Caution We use Math.methodName(arguments)
(e.g., Math.pow(3, 2.5))
to invoke a method in the Math class.
Can you invoke getArea() using Circle.getArea()? NO!
All the methods used before this chapter are static methods, which are defined using the static keyword. However, getArea() is non-static. It must be invoked from an object using
objectRefVar.methodName(arguments)
(e.g., myCircle.getArea()).

26. Reference Data Fields The data fields can be of primitive types OR
reference types - Composition of objects
For example, the following Student class contains a data field name of the String type.
public class Student {
String name; // name has default value null
int age; // age has default value 0
boolean isScienceMajor; // isScienceMajor has default value false
char gender; // c has default value '\u0000' }

27. The null Value
If data field of reference type does not reference any object, it holds a special literal value, null.

28. Default Values
Data fields of an object have default values. Variables defined in a method do not assume default values.

29. Default Value for a Data Field
Default value of data field:
null for reference type
0 for a numeric type
false for a boolean type
'\u0000' for char type.
public class Test {
public static void main(String[] args) {
Student student = new Student();
System.out.println("name? " + student.name);
System.out.println("age? " + student.age);
System.out.println("isScienceMajor? " + student.isScienceMajor);
System.out.println("gender? " + student.gender); }

30. Example
Java assigns no default value to a local variable inside a method.
public class Test { public static void main(String[] args) { int x; // x has no default value String y; // y has no default value System.out.println("x is " + x); System.out.println("y is " + y); }
Compilation error: variables not initialized

31. Differences between Variables of Primitive Data Types and Object Types

32. Copying Variables of Primitive Data Types and Object Types

33. Garbage Collection
An assignment statement such as c1=c2 results in 2 objects pointing to the same object. The object previously referenced by c1 is no longer referenced so it is garbage. Garbage is automatically collected by JVM.

34. Garbage Collection, cont
TIP: If an object is no longer needed, explicitly assign null to a variable that references the object. The JVM automatically collects the space of an object not referenced by any variable.

35. The Date Class
Java provides a system-independent encapsulation of date and time in the java.util.Date class. You can use the Date class to create an instance for the current date and time and use its toString method to return the date and time as a string.

36. The Date Class Example For example, the following code
java.util.Date date = new java.util.Date();
System.out.println(date.toString());
displays a string like Sun Mar 09 13:50:19 EST

37. The Random Class
You have used Math.random() to obtain a random double value between 0.0 and 1.0 (excluding 1.0). A more useful random number generator is provided in the java.util.Random class.

38. The Random Class Example
If two Random objects have the same seed, they will generate identical sequences of numbers. For example, the following code creates two Random objects with the same seed 3.
Random random1 = new Random(3);
System.out.print("From random1: ");
for (int i = 0; i < 10; i++)
System.out.print(random1.nextInt(1000) + " ");
Random random2 = new Random(3);
System.out.print("\nFrom random2: ");
System.out.print(random2.nextInt(1000) + " ");
From random1:
From random2:

39. Instance Variables, and Methods
Instance variables belong to a specific instance. Instance methods are invoked by an instance of the class.
The dot operator

40. Static Variables, Constants, and Methods
Static variables are shared by all the instances of the class, class-wide information. Static methods are not tied to a specific object.
Static constants are final variables shared by all the instances of the class.

41. Static Variables, Constants, and Methods, cont.
To declare static variables, constants, and methods, use the static modifier.
The members of the math class are defined as static.
That’s why we never create an object of type Math,
yet we can use its methods.

42. Static Variables, Constants, and Methods, cont.

43. Example of Using Instance and Class Variables and Method
Objective: Demonstrate the roles of instance and class variables and their uses. This example adds a class variable numberOfObjects to track the number of Circle objects created.
Circle2
TestCircle2
Run

44. Final instance variables
A final variable is not modifiable.
A constant can be initialized
In its declaration
By constructors
Syntax error: not initializing constant variable at declaration or in every constructor.

45. Access to a Class
Access level modifiers determine whether other classes can use a particular field OR or invoke a particular method.

46. Access Control Top Level Member Level Public
Package-private (no explicit modifier)
Member Level
Private
Protected – in any subclass or within package

47. Access Levels Modifier Class Package Subclass World Public Y ProtectedN
No modifier
Private

48. Visibility Modifiers and Accessor/Mutator Methods
By default, the class, variable, or method can be accessed by any class in the same package.
public
The class, data, or method is visible to any class in any package.
private
The data or methods can be accessed only by the declaring class.

50. Top Level Access Modifiers
Member Level Access Modifiers

51. NOTE
An object cannot access its private members, as shown in (b). It is OK, however, if the object is declared in its own class, as shown in (a).

52. Summary Modifier Access Private within class within package Public
unrestricted access

53. Why Data Fields Should Be private?
To protect data. To make class easy to maintain.

54. Private Variables How can private data be used by other objects?
Accessor (get) methods
To read private data
Mutator (set) methods
To modify private data

55. Example of Data Field Encapsulation
Circle3
TestCircle3
Run

56. Passing Objects to Methods
Passing by value for primitive type value (the value is passed to the parameter)
Passing by value for reference type value (the value is the reference to the object, essentially pass by reference)
TestPassObject
Run

57. Passing Objects to Methods, cont.

58. This An object can access a reference to itself: keyword this
Method can declare a local variable with same name as a field.
Local variable shadows field
Use this.field_name to access field
A constructor can invoke another constructor of the same class.
this (parameter_list);

59. Reference the Hidden Data Fields

60. Calling Overloaded Constructor

61. Array of Objects Array of objects: array of reference variables.
Circle[] circleArray = new Circle[10];
Array of objects: array of reference variables.
Invoking circleArray[1].getArea() involves two levels of referencing.
circleArray references to the entire array. circleArray[1] references to a Circle object.

62. Array of Objects, cont.
Circle[] circleArray = new Circle[10];

63. Summarizing the areas of the circles
Array of Objects, cont.
Summarizing the areas of the circles
TotalArea
Run

64. Packages Manage complexity of applications Software reuse – import
Unique class names
Import classes that are in a different package.

65. Packages Definition: Compilation (with directory setup) Usage:
package package_name; //first line of code
The class file is stored in package_name
Compilation (with directory setup)
Javac –d directory_above_package source_file
Usage:
import package_name;