1. Chapter 9 Objects and Classes

2. Motivations
After learning the preceding chapters, you are capable of solving many programming problems using tools such as selections, loops, methods, and arrays. However, these Java features are not sufficient for developing graphical user interfaces (GUI) and large scale software systems. Suppose you want to develop a graphical user interface as shown below. How do you program it?

3. Objectives
To describe objects and classes, and use classes to model objects (§9.2).
To use UML graphical notations to describe classes and objects (§9.2).
To demonstrate defining classes and creating objects (§9.3).
To create objects using constructors (§9.4).
To access objects via object reference variables (§9.5).
To define a reference variable using a reference type (§9.5.1).
To access an object’s data and methods using the object member access operator (.) (§9.5.2).
To define data fields of reference types and assign default values for an object’s data fields (§9.5.3).
To distinguish between object reference variables and primitive data type variables (§9.5.4).
To use classes Date, Random, and JFrame in the Java library (§9.6).
To distinguish between instance and static variables and methods (§9.7).
To define private data fields with appropriate get and set methods (§9.8).
To encapsulate data fields to make classes easy to maintain (§9.9).
To develop methods with object arguments and differentiate between primitive-type arguments and object-type arguments (§9.10).
To store and process objects in arrays (§9.11).

4. OO Programming Concepts
Object-oriented programming (OOP) involves programming using objects.
An object represents an entity in the real world that can be distinctly identified.
For example, a student, a desk, a circle, a button, and even a loan can all be viewed as objects.
An object has a unique identity, state, and behaviors.
The state of an object consists of a set of data fields (also known as properties) with their current values. The behavior of an object is defined by a set of methods.

5. Classes Classes are constructs that define objects of the same type.
A Java class uses variables to define data fields and methods to define behaviors.
The behavior of an object also known as its actions which is define by methods.
Additionally, a class provides a special type of methods, known as constructors, which are invoked to construct objects from the class.

6. More Classes
The access level modifiers will determine whether other classes can use a particular field or invoke a particular method.
There are two level of access control:
public, or package-private //top level
public, private, protected, or package-private // member level
A class may be declared with the modifier public, which is visible to all classes anywhere. However, if a class doe not have modifier (public or private, or else), then, it is only visible within its own package.

7. Objects
An object has both a state and behavior. The state defines the object, and the behavior defines what the object does.
The State of an object, know as properties or attributes is represent as data fields.
A circle object has data fields of radius, which is the property define circle.
A rectangle object has data fields width and height, which are properties and characterize a rectangle.

8. More Objects
A Java class uses variable to define data fields and methods.
A class provides methods of special type, know as constructors which is invoked to create a new object.
A constructor can performs any actions, but they do initial action. Example:
Circle3 myCircle = new Circle3 (5.0);
class object constructor (keyword new that creates new object
myCirlce from Circle 3 class with the argument of 5.0)

9. Constructors
The declaration for a method or a constructor type of the arguments for that method or constructor.
Constructors are a special kind of methods that are invoked to construct objects.
Constructors with no parameters is referred to as a no-arg constructor.
Constructors must have the same name as the class itself.
Constructors do not have a return type—not even void.
Constructors are invoked using the new
new ClassName();
Example:
new Circle();
new Circle(5.0);

10. Create Objects
A class provides the blueprint for objects; that you create an object from a class.
Each statement below taken from CreateObjectDemo program creates an object and assigns it to a variable:
Point originOne = new Point (2, 9); //create object Point
Rectangle recOne = new rectangle (originOne, 10, 20);
//create object Rectangle
Rectangle rectwo = new Rectangle (50, 10); //Create object Rectangle

11. Classes

12. UML Class Diagram
Unified Modeling Language (UML) is an object-oriented analysis and design language from the Object Management Group (OMG).
The constructor is denoted as
ClassName (parameterName: parameterType)
The Method is denoted as
methodName(parameterName: parameterType): retrunType

13. Example: Defining Classes and Creating Objects
1 public class TestCircle1 { 2 /** Main method */ 3 public static void main(String[] args) { // Create a circle with radius Circle1 myCircle = new Circle1(5.0); // create new object System.out.println("The area of the circle of radius " myCircle.radius + " is " + myCircle.getArea()); // Create a circle with radius Circle1 yourCircle = new Circle1(); // create new object System.out.println("The area of the circle of radius " yourCircle.radius + " is " + yourCircle.getArea()); // Modify circle radius yourCircle.radius = 100; //Changes new object's with new value of System.out.println("The area of the circle of radius " yourCircle.radius + " is " + yourCircle.getArea()); 18 } 19 }
21 // Define the circle class with two constructors 22 class Circle1 { 23 double radius; /** Construct a circle with radius 1 */ 26 Circle1() { radius = 1.0; 28 } /** Construct a circle with a specified radius */ 31 Circle1(double newRadius) { radius = newRadius; 33 } 34 /** Return the area of this circle */ 36 double getArea() { return radius * radius * Math.PI; 38 } 39 }
Example: Defining Classes and Creating Objects
Objective: Demonstrate creating objects, accessing data, and using methods.
TestCircle1
Run

14. public class TV { int channel = 1; // Default channel is 1 int volumeLevel = 1; // Default volume level is 1 boolean on = false; // By default TV is off public TV() { } public void turnOn() { //Turn on TV on = true; } public void turnOff() { //Turn off TV on = false; } public void setChannel(int newChannel) { // set a new channel if (on && newChannel >= 1 && newChannel <= 120) channel = newChannel; } public void setVolume(int newVolumeLevel) { // set a new volume if (on && newVolumeLevel >= 1 && newVolumeLevel <= 7) volumeLevel = newVolumeLevel; } public void channelUp() { //increase channel if (on && channel < 120) channel++; } public void channelDown() { //decrease channel if (on && channel > 1) channel--; } public void volumeUp() { //increase volume if (on && volumeLevel < 7) volumeLevel++; } public void volumeDown() { //decrease volume if (on && volumeLevel > 1) volumeLevel--; } }
Example: Defining Classes and Creating Objects, Accessing Data and Using Methods

15. public class TestTV { // Main method public static void main(String[] args) { TV tv1 = new TV(); // create a TV tv1.turnOn(); // turn on tv1.setChannel(30); //set a new channel tv1.setVolume(3); //set a new volume TV tv2 = new TV(); //create a TV - object tv2 - TV class tv2.turnOn(); //turn on tv2.channelUp(); // trun on tv2.channelUp(); //increase channel tv2.volumeUp(); // increase volume System.out.println("tv1's channel is " + tv1.channel //display state " and volume level is " + tv1.volumeLevel); System.out.println("tv2's channel is " + tv2.channel " and volume level is " + tv2.volumeLevel); } }

16. Encapsulation
Encapsulation is one of the four fundamental OOP concepts. The other three are inheritance (is the capability of a class to use the properties and methods of another class), polymorphism (more than one form), and abstraction (simplifying complex reality by modeling classes ).
Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface.
Encapsulation provides a technique of making the fields in a class private and providing access to the fields via public methods.
If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding.
The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code.

17. Benefits of Encapsulation:
The fields of a class can be made read-only or write-only.
A class can have total control over what is stored in its fields.
The users of a class do not know (like a blackbox) how the class stores its data. A class can change the data type of a field, and users of the class do not need to change any of their code.
Link – example of Encapsulation program

18. Example of Data Field Encapsulation
Video – Data Field Encapsulation
Circle3
TestCircle3
Run
main method

19. public class Circle3 { /. The radius of the circle
public class Circle3 { /** The radius of the circle */ private double radius = 1; /** The number of the objects created */ private static int numberOfObjects = 0; /** Construct a circle with radius 1 */ public Circle3() { numberOfObjects++; } public Circle3(double newRadius) {/** Construct a circle with a specified radius */ radius = newRadius; numberOfObjects++; } public double getRadius() {/** Return radius */ return radius; } public void setRadius(double newRadius) { /** Set a new radius */ radius = (newRadius >= 0) ? newRadius : 0; } /** Return numberOfObjects */ public static int getNumberOfObjects() { return numberOfObjects; } /** Return the area of this circle */ public double getArea() { return radius * radius * Math.PI; } }

20. 1 public class TestCircle3 { 3 /. Main method
1 public class TestCircle3 { 3 /** Main method */ 4 public static void main(String[] args) { // Create a Circle with radius Circle3 myCircle = new Circle3(5.0); System.out.println("The area of the circle of radius " myCircle.getRadius() + " is " + myCircle.getArea()); Circle3 yourCircle = new Circle3(6.0); System.out.println("The area of the yourcircle of radius " yourCircle.getRadius() + " is " + yourCircle.getArea()); Circle3 hisCircle = new Circle3(); System.out.println("The area of the hiscircle of radius " hisCircle.getRadius() + " is " + hisCircle.getArea()); } 22 }

21. Declaring/Creating Objects in a Single Step
ClassName objectRefVar = new ClassName();
Example:
Circle myCircle = new Circle();
//the variable myCirlce holds a reference to a Circle object.
Assign object reference
Create an object

22. Trace Code // Circle & SCircle - class
animation
Trace Code
Circle myCircle = new Circle(5.0);
SCircle yourCircle = new Circle();
yourCircle.radius = 100;
Declare myCircle
myCircle
no value
// Circle & SCircle - class
// myCircle & yourCircle– object
// new – keyword that creates object from the class Circle
// new Circle(5.0) - constructor
// (5.0)– argument(s) or instance variable value of 5.0.
//() - is null parameter, goes to default main method parameter value.

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

24. 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

25. 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

26. 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

27. 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

28. 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

29. Reference Data Fields
The data fields can be of reference types. 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' }
If a data field of a reference type does not reference any object, the data field holds a special Java value, null.
Null is a literal just like true and false, Boolean type, null is a literal for a reference type.

30. Default Value for a Data Field
The default value of a data field is null for a reference type, 0 for a numeric type, false for a boolean type, and '\u0000' for a char type. However, Java assigns no default value to a local variable inside a method.
public class Test {
public static void main(String[] args) {
Student student = new Student();
/* default values of data fields name, aga, isScienceMajor, and gender for a Student object.*/
System.out.println("name? " + student.name);
System.out.println("age? " + student.age);
System.out.println("isScienceMajor? " + student.isScienceMajor);
System.out.println("gender? " + student.gender); }

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

32. Copying Variables of Primitive Data Types and Object Types

33. Garbage Collection
As shown in the previous figure, after the assignment statement c1 = c2, c1 points to the same object referenced by c2.
The object previously referenced by c1 is no longer referenced. This object is known as garbage. Garbage is automatically collected by JVM.

34. Garbage Collection, cont
TIP: If you know that an object is no longer needed, you can explicitly assign null to a reference variable for the object. The JVM will automatically collect the space if the object is 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());
// date is an object for the class library of java.util.Date
displays a string like Sun Mar 09 13:50:19 EST 2003.

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. Displaying GUI Components
When you develop programs to create graphical user interfaces, you will use Java classes such as JFrame, JButton, JRadioButton, JComboBox, and JList to create frames, buttons, radio buttons, combo boxes, lists, and so on. Here is an example that creates two windows using the JFrame class.
TestFrame
Run

40. Declare, create, and assign in one statement
animation
Trace Code
Declare, create, and assign in one statement
JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);
frame1
reference
: JFrame
title:
width:
height:
visible:
frame1 & frame2 – object created from Jframe class
setTtile, setSize, setVisible, setTitle – methods that set the properties of the objects.

41. animation
Trace Code
JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);
frame1
reference
Set title property
: JFrame
title: "Window 1"
width:
height:
visible:
frame1.setTtile(“Window1”);
// setTitle method sets a title for Window 1

42. animation
Trace Code
JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);
JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);
frame1
reference
Set size property
: JFrame
title: "Window 1"
width: 200
height: 150
visible:
//setSize method defined the size of 200 & 150 (width, height).

43. animation
Trace Code
JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);
JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);
frame1
reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
Set visible property
//setVisible method displays the window.

44. Declare, create, and assign in one statement
animation
Trace Code
JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);
JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);
frame1
reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
Declare, create, and assign in one statement
frame2
reference
: JFrame
title:
width:
height:
visible:
frame2 – object created from Jframe class with another sets of methods; setTitle, setSize, setVisible

45. animation
Trace Code
JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);
JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);
frame1
reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
frame2
reference
Set title property
: JFrame
title: "Window 2"
width:
height:
visible:

46. animation
Trace Code
JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);
JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);
frame1
reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
frame2
reference
: JFrame
title: "Window 2"
width: 200
height: 150
visible:
Set size property

47. animation
Trace Code
JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);
JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);
frame1
reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
frame2
reference
: JFrame
title: "Window 2"
width: 200
height: 150
visible: true
Set visible property

48. Adding GUI Components to Window
You can add graphical user interface components, such as buttons, labels, text fields, combo boxes, lists, and menus, to the window. The components are defined using classes. Here is an example to create buttons, labels, text fields, check boxes, radio buttons, and combo boxes.
Video on the GUI Components
GUIComponents
Run

49. Static Variables, Constants, and Methods
A static variable is shared by all objects of the class. A static method cannot access instance member of the class.
The data field radius in the circle class in listing 7.1 I know as an instance variable. An instance variable is tied to a specific instance of the class; it is not shared among objects of the same class.
Video – Static Variables, Constants, and Methods

50. The Fan class Program
program 8 - (The fan class) Design a class named Fan to represent a fan.
VideoNote – End of the chapter