1. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 1 Fall 2013 Chapter 8 Objects and Classes

2. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Motivations After learning the preceding chapters, you are capable of solving many programming problems using selections, loops, methods, and arrays. However, these Java features are not sufficient for developing graphical user interfaces and large scale software systems. Suppose you want to develop a graphical user interface as shown below. How do you program it? 2

3. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Objectives  To describe objects and classes, and use classes to model objects (§8.2).  To use UML graphical notations to describe classes and objects (§8.2).  To demonstrate defining classes and creating objects (§8.3).  To create objects using constructors (§8.4).  To access objects via object reference variables (§8.5).  To define a reference variable using a reference type (§8.5.1).  To access an object’s data and methods using the object member access operator (.) (§8.5.2).  To define data fields of reference types and assign default values for an object’s data fields (§8.5.3).  To distinguish between object reference variables and primitive data type variables (§8.5.4).  To use classes Date, Random, and JFrame in the Java library (§8.6).  To distinguish between instance and static variables and methods (§8.7).  To define private data fields with appropriate get and set methods (§8.8).  To encapsulate data fields to make classes easy to maintain (§8.9).  To develop methods with object arguments and differentiate between primitive-type arguments and object-type arguments (§8.10).  To store and process objects in arrays (§8.11). 3

4. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 OO Programming Concepts 4 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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Classes & Objects 5 An object has both a state and behavior. The state defines the object, and the behavior defines what the object does.

6. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Classes 6 Classes are constructs that define objects of the same type. A Java class uses variables to define data fields and methods to define behaviors. Additionally, a class provides special types of methods, known as constructors, which are invoked to construct objects from the class.

7. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Classes 7

8. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Class Definition public class ClassName { // instance variables for holding object attributes // instance variables // constructor: build the object // default constructor: instantiates class and initializes public ClassName() { // Define the default constructor, i.e., no parameters } // other constructors public ClassName(parameter list) { // Define constructor with parameters list } // mutators: m anage requests for changing attributes // accessors: s upply the values of the attributes // facilitators: uses attributes to calculate responses } 8

9. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 UML Class Diagram 9

10. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Example: Defining Classes and Creating Objects Objective: Demonstrate creating objects, accessing data, and using methods. 10 TestCircle1

11. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Constructors Circle() { } Circle(double newRadius) { radius = newRadius; } 11 Constructors are a special kind of methods that are invoked to construct objects.

12. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Example: Defining Classes and Creating Objects Objective: Demonstrate creating objects, accessing data, and using methods. 12 TestTV TV

13. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 LECTURE 2 13

14. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Instance Variables Tied to specific instance of a calss, that is, the object created – Circle1 has one instant variable, radius – Each time a new Circle1 is created, that radius can only be referenced using the object created. myCircle.radius will return the radius of myCircle. myCircle cannot reference the radius of yourCircle. If instance variables are not initialized, they are assigned default values – int is 0, double is 0.0, boolean is false, char is \u0000, String is null 14

15. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Constructors, cont. 15 A constructor 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 operator when an object is created. Constructors play the role of initializing objects.

16. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Creating Objects Using Constructors new ClassName(); Example: new Circle(); new Circle(5.0); 16

17. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Default Constructor 17 A class may be declared without constructors. In this case, a no-arg constructor with an empty body is implicitly declared in the class. This constructor, called a default constructor, is provided automatically only if no constructors are explicitly declared in the class.

18. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Declaring Object Reference Variables To reference an object, assign the object to a reference variable. To declare a reference variable, use the syntax: ClassName objectRefVar; Example: Circle myCircle; 18

19. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Declaring/Creating Objects in a Single Step ClassName objectRefVar = new ClassName(); Example: Circle myCircle = new Circle(); 19 Create an object Assign object reference

20. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Accessing Objects Referencing the object’s data: objectRefVar.data e.g., myCircle.radius Invoking the object’s method: objectRefVar.methodName(arguments) e.g., myCircle.getArea() 20

21. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 A Simple Circle Class Objective: Demonstrate creating objects, accessing data, and using methods. 21

22. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Circle1.java 22 public class Circle1 { /** The radius of the circle */ double radius; /** Construct a circle with radius 1 */ Circle1() { radius = 1.0; } /** Construct a circle with a specified radius */ Circle1(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double getArea() { return radius * radius * Math.PI; }

23. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TestCircle1.java 23 public class TestCircle1 { /** Main method */ public static void main(String[] args) { // Create a circle with radius 5.0 Circle1 myCircle = new Circle1(5.0); System.out.println("The area of the circle of radius " + myCircle.radius + "is " + myCircle.getArea()); // Create a circle with radius 1 Circle1 yourCircle = new Circle1(); System.out.println("The area of the circle of radius " + yourCircle.radius + " is " + yourCircle.getArea()); // Modify circle radius yourCircle.radius = 100; System.out.println("The area of the circle of radius " + yourCircle.radius + " is " + yourCircle.getArea()); }

24. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code 24 Circle myCircle = new Circle(5.0); SCircle yourCircle = new Circle(); yourCircle.radius = 100; Declare myCircle no value myCircle animation

25. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code, cont. 25 Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; no value myCircle Create a circle animation

26. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code, cont. 26 Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle Assign object reference to myCircle animation

27. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code, cont. 27 Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle no value yourCircle Declare yourCircle animation

28. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code, cont. 28 Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle no value yourCircle Create a new Circle object animation

29. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code, cont. 29 Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle reference value yourCircle Assign object reference to yourCircle animation

30. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code, cont. 30 Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle reference value yourCircle Change radius in yourCircle animation

31. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Caution Recall that you use Math.methodName(arguments) (e.g., Math.pow(3, 2.5)) to invoke a method in the Math class. Can you invoke getArea() using Circle1.getArea()? The answer is 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()). More explanations will be given in the section on “Static Variables, Constants, and Methods.” 31

32. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 32 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' }

33. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 The null Value If a data field of a reference type does not reference any object, the data field holds a special literal value, null. 33

34. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 34 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); }

35. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Example 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); } 35 Compilation error: variables not initialized Java assigns no default value to a local variable inside a method.

36. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Differences between Variables of Primitive Data Types and Object Types 36

37. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Copying Variables of Primitive Data Types and Object Types 37

38. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 LECTURE 3 38

39. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 39

40. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 40

41. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Java APIs Java provides a rich collection of Application Programming Interfaces (API) – Set of classes and methods available for any Java program to use. – Java™ Platform, Standard Edition 7 API Specification (http://docs.oracle.com/javase/7/docs/api/)http://docs.oracle.com/javase/7/docs/api/ – Java API Examples (http://www.java2s.com/Code/JavaAPI/CatalogJ avaAPI.htm) 41

42. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 42

43. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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 2003. 43

44. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 44

45. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 45 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: "); for (int i = 0; i < 10; i++) System.out.print(random2.nextInt(1000) + " "); From random1: 734 660 210 581 128 202 549 564 459 961 From random2: 734 660 210 581 128 202 549 564 459 961

46. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 46 TestFrame

47. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TestFrame.java 47 import javax.swing.JFrame; public class TestFrame { public static void main(String[] args) { JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setLocation(200, 100); frame1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setLocation(410, 100); frame2.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame2.setVisible(true); }

48. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code 48 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); Declare, create, and assign in one statement reference frame1 : JFrame title: width: height: visible: animation

49. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code 49 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); reference frame1 : JFrame title: "Window 1" width: height: visible: Set title property animation

50. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code 50 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); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: Set size property animation

51. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code 51 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); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true Set visible property animation

52. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code 52 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); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true Declare, create, and assign in one statement reference frame2 : JFrame title: width: height: visible: animation

53. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code 53 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); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true reference frame2 : JFrame title: "Window 2" width: height: visible: Set title property animation

54. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code 54 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); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true reference frame2 : JFrame title: "Window 2" width: 200 height: 150 visible: Set size property animation

55. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Trace Code 55 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); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true reference frame2 : JFrame title: "Window 2" width: 200 height: 150 visible: true Set visible property animation

56. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 56 GUIComponents

57. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 GUIComponents.java 57 import javax.swing.*; public class GUIComponents { public static void main(String[] args) { // Create a button with text OK JButton jbtOK = new JButton("OK"); // Create a label with text "Enter your name: " JLabel jlblName = new JLabel("Enter your name: "); // Create a text field with text "Type Name Here" JTextField jtfName = new JTextField("Type Name Here"); // Create a check box with text bold JCheckBox jchkBold = new JCheckBox("Bold"); // Create a radio button with text red JRadioButton jrbRed = new JRadioButton("Red"); // Create a combo box with choices red, green, and blue JComboBox jcboColor = new JComboBox(new String[]{"Red", "Green", "Blue"});

58. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 GUIComponents.java 58 // Create a panel to group components JPanel panel = new JPanel(); panel.add(jbtOK); // Add the button to the panel panel.add(jlblName); // Add the label to the panel panel.add(jtfName); // Add the text field to the panel panel.add(jchkBold); // Add the check box to the panel panel.add(jrbRed); // Add the radio button to the panel panel.add(jcboColor); // Add the combo box to the panel JFrame frame = new JFrame();

59. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 GUIComponents.java 59 // Create a frame frame.add(panel); // Add the panel to the frame frame.setTitle("Show GUI Components"); frame.setSize(450, 100); frame.setLocation(200, 100); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); }

60. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Instance Variables and Methods 60 Instance variables belong to a specific instance. Instance methods are invoked by an instance of the class.

61. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Static Variables, Constants, and Methods 61 Static variables are shared by all the instances of the class. Static methods are not tied to a specific object. Static constants are final variables shared by all the instances of the class.

62. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Static Variables, Constants, and Methods, cont. 62 To declare static variables, constants, and methods, use the static modifier.

63. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Static Variables, Constants, and Methods, cont. 63

64. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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. 64 TestCircle2 Circle2

65. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Circle2.java 65 public class Circle2 { /** The radius of the circle */ double radius; /** The number of the objects created */ static int numberOfObjects = 0; /** Construct a circle with radius 1 */ Circle2() { radius = 1.0; numberOfObjects++; } /** Construct a circle with a specified radius */ Circle2(double newRadius) { radius = newRadius; numberOfObjects++; }

66. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Circle2.java 66 /** Return numberOfObjects */ static int getNumberOfObjects() { return numberOfObjects; } /** Return the area of this circle */ double getArea() { return radius * radius * Math.PI; }

67. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TestCircle2.java 67 public class TestCircle2 { /** Main method */ public static void main(String[] args) { // Create c1 Circle2 c1 = new Circle2(); // Display c1 BEFORE c2 is created System.out.println("Before creating c2"); System.out.println("c1 is : radius (" + c1.radius + ") and number of Circle objects (" + c1.numberOfObjects + ")"); // Create c2 Circle2 c2 = new Circle2(5); // Change the radius in c1 c1.radius = 9;

68. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TestCircle2.java 68 // Display c1 and c2 AFTER c2 was created System.out.println("\nAfter creating c2 and modifying " + "c1's radius to 9"); System.out.println("c1 is : radius (" + c1.radius + ") and number of Circle objects (" + c1.numberOfObjects + ")"); System.out.println("c2 is : radius (" + c2.radius + ") and number of Circle objects (" + c2.numberOfObjects + ")"); }

69. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Visibility Modifiers and Accessor/Mutator Methods By default, the class, variable, or method can be accessed by any class in the same package. 69  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. The get and set methods are used to read and modify private properties.

70. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Lecture 4 70

71. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 71 The private modifier restricts access to within a class, the default modifier restricts access to within a package, and the public modifier enables unrestricted access.

72. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 NOTE 72 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).

73. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Why Data Fields Should Be private? To protect data. To make class easy to maintain. 73

74. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Example of Data Field Encapsulation 74 Circle3 TestCircle3

75. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Circle3.java 75 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++; } /** Construct a circle with a specified radius */ public Circle3(double newRadius) { radius = newRadius; numberOfObjects++; }

76. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Circle3.java 76 /** Return radius */ public double getRadius() { return radius; } /** Set a new radius */ public void setRadius(double newRadius) { 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; }

77. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TestCircle3.java 77 public class TestCircle3 { /** Main method */ public static void main(String[] args) { // Create a Circle with radius 5.0 Circle3 myCircle = new Circle3(5.0); System.out.println("The area of the circle of radius " + myCircle.getRadius() + " is " + myCircle.getArea()); // Increase myCircle's radius by 10% myCircle.setRadius(myCircle.getRadius() * 1.1); System.out.println("The area of the circle of radius " + myCircle.getRadius() + " is " + myCircle.getArea()); }

78. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 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) 78 TestPassObject

79. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TestPassObject 79 public class TestPassObject { /** Main method */ public static void main(String[] args) { // Create a Circle object with radius 1 Circle3 myCircle = new Circle3(1); // Print areas for radius 1, 2, 3, 4, and 5. int n = 5; printAreas(myCircle, n); // See myCircle.radius and times System.out.println("\n" + "Radius is " + myCircle.getRadius()); System.out.println("n is " + n); }

80. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TestPassObject 80 /** Print a table of areas for radius */ public static void printAreas(Circle3 c, int times) { System.out.println("Radius \t\tArea"); while (times >= 1) { System.out.println(c.getRadius() + "\t\t" + c.getArea()); c.setRadius(c.getRadius() + 1); times--; }

81. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Passing Objects to Methods, cont. 81

82. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Array of Objects Circle[] circleArray = new Circle[10]; An array of objects is actually an array of reference variables. So invoking circleArray[1].getArea() involves two levels of referencing as shown in the next figure. circleArray references to the entire array. circleArray[1] references to a Circle object. 82

83. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Array of Objects, cont. Circle[] circleArray = new Circle[10]; 83

84. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Array of Objects, cont. Summarizing the areas of the circles 84 TotalArea

85. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TotalArea.java 85 public class TotalArea { /** Main method */ public static void main(String[] args) { // Declare circleArray Circle3[] circleArray; // Create circleArray circleArray = createCircleArray(); // Print circleArray and total areas of the circles printCircleArray(circleArray); } /** Create an array of Circle objects */ public static Circle3[] createCircleArray() { Circle3[] circleArray = new Circle3[5]; for (int i = 0; i < circleArray.length; i++) { circleArray[i] = new Circle3(Math.random() * 100); } // Return Circle array return circleArray; }

86. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TotalArea.java 86 /** Print an array of circles and their total area */ public static void printCircleArray(Circle3[] circleArray) { System.out.println("Radius\t\t\t\t" + "Area"); for (int i = 0; i < circleArray.length; i++) { System.out.print(circleArray[i].getRadius() + "\t\t" + circleArray[i].getArea() + '\n'); } System.out.println("-----------------------------------------"); // Compute and display the result System.out.println("The total areas of circles is \t" + sum(circleArray)); }

87. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 TotalArea.java 87 /** Add circle areas */ public static double sum(Circle3[] circleArray) { // Initialize sum double sum = 0; // Add areas to sum for (int i = 0; i < circleArray.length; i++) sum += circleArray[i].getArea(); return sum; }

88. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Homework (statement of problem 8.11 * (Algebra: linear equations) Design a class named LinearEquation for a 2 X 2 system of linear equations: ax + by = e cx + dy = f ed - bf af - ec x = ------- y = ------- ad – bc ad - bc The class contains: * Private data fields a, b, c, d, e, and f. * A constructor with the arguments for a, b, c, d, e, and f. * Six get methods for a, b, c, d, e, and f. * A method named isSolvable() that returns true if is not 0. * Methods getX()and getY() that return the solution for the equation. Draw the UML diagram for the class. Implement the class. Write a test program that prompts the user to enter a, b, c, d, e, and f and displays the result. If ad - bc is 0, report that “The equation has no solution.” See Exercise 3.3 for sample runs. 88

89. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Homework (partial UML) 89 LinearEquation -a: double.. +LinearEquation (newa: double. …) +getA():double …..

90. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Homework (partial solution) input java.util.Scanner; public class TestLinear { public static void main (…) { //Make Scanner //Input a, b, c, d, e, f LinearEquation equation = new LinearEquation (…); double valA = equation.getA(); //Check to see if the equation is solvable, if so, getX and getY. If not, display error message //Display X and Y. changeA(5.0); 90

91. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Homework (partial solution) public class LinearEquation { private double a; //other 5 variables //create constructor public double getA() { return ….; } // get for each variable public boolean isSolvable() { return ….; } public double getX () { return ….; } // need getY public void changeA(double newA){ a = newA; } 91

92. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 Creating Objects 92 Note (Page 272) Usually you create an object and assign it to a variable. Later you can use the variable to reference the object. Occasionally an object does not need to be referenced later. In this case, you can create an object without explicitly assigning it to a variable, as shown below: new Circle(); or System.out.println("Area is " + new Circle(5).getArea()); The former statement creates a Circle object. The latter creates a Circle object and invokes its getArea method to return its area. An object created in this way is known as an anonymous object.