Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All rights reserved.

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1 Chapter 8 Objects and Classes Lecture notes for computer programming 1 Faculty of Engineering and Information Technology Prepared by: Iyad Albayouk 1

2 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 Objectives To describe objects and classes, and use classes to model objects ( 8.2). To use UML graphical notation to describe classes and objects ( 8.2). To demonstrate how to define classes and create 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 the Java library classes Date, Random, and JFrame( 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 primitivetype arguments and object-type arguments ( 8.10). To store and process objects in arrays ( 8.11). 3

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. 4

5 Objects Class Name: Circle A class template Data Fields: radius is Methods: getarea Circle Object 1 Circle Object 2 Circle Object 3 Data Fields: radius is 10 Data Fields: radius is 25 Data Fields: radius is 125 Three objects of the Circle class An object has both a state and behavior. The state defines the object, and the behavior defines what the object does. 5

6 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. Additionally, a class provides a special type of methods, known as constructors, which are invoked to construct objects from the class. 6

7 Classes class Circle { /** The radius of this circle */ double radius = 1.0; /** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newradius) { radius = newradius; } Data field Constructors } /** Return the area of this circle */ double getarea() { return radius * radius * ; } Method 7

8 UML Class Diagram UML Class Diagram Circle Class name radius: double Data fields Circle() Circle(newRadius: double) Constructors and methods getarea(): double circle1: Circle radius = 1.0 circle2: Circle radius = 25 circle3: Circle radius = 125 UML notation for objects 8

9 Two examples for Defining Classes and Creating Objects Objective: Demonstrate creating objects, accessing data, and using methods. 9

10 Example1(TestSimpleCircle.java): 1 public class TestSimpleCircle { 2 /** Main method */ 3 public static void main(string[] args) { 4 // Create a circle with radius 1 5 SimpleCircle circle1 = new SimpleCircle(); 6 System.out.println("The area of the circle of radius + circle1.radius + " is " + circle1.getarea()); 7 8 // Create a circle with radius 25 9 SimpleCircle circle2 = new SimpleCircle(25); 10 System.out.println("The area of the circle of radius " + circle2.radius + " is " + circle2.getarea()); // Create a circle with radius SimpleCircle circle3 = new SimpleCircle(125); 14 System.out.println("The area of the circle of radius " + circle3.radius + " is " + circle3.getarea()); // Modify circle radius 17 circle2.radius = 100; // or circle2.setradius(100) 18 System.out.println("The area of the circle of radius + circle2.radius + " is " + circle2.getarea()); 19 } 20 } 10

11 Example1(TestSimpleCircle.java), cont: 21 // Define the circle class with two constructors 22 class SimpleCircle { 23 double radius; /** Construct a circle with radius 1 */ 26 SimpleCircle() { radius = 1; } /** Construct a circle with a specified radius */ 29 SimpleCircle(double newradius) { radius = newradius; } /** Return the area of this circle */ 32 double getarea() { return radius * radius * Math.PI; } /** Return the perimeter of this circle */ 35 double getperimeter() { return 2 * radius * Math.PI; } /** Set a new radius for this circle */ 38 void setradius(double newradius) { radius = newradius; } 39 } 11

12 Example2(TV.java): 1 public class TV { 2 int channel = 1; // Default channel is 1 3 int volumelevel = 1; // Default volume level is 1 4 boolean on = false; // By default TV is off 5 public TV() { } 6 public void turnon() { on = true; } 7 public void turnoff() { on = false; } 8 9 public void setchannel(int newchannel) { 10 if (on && newchannel >= 1 && newchannel <= 120) 11 channel = newchannel; } public void setvolume(int newvolumelevel) { 14 if (on && newvolumelevel >= 1 && newvolumelevel <= 7) 15 volumelevel = newvolumelevel; } public void channelup() { if (on && channel < 120) channel++; } public void channeldown() { if (on && channel > 1) channel--; } public void volumeup() { if (on && volumelevel < 7) volumelevel++; } public void volumedown() { if (on && volumelevel > 1) volumelevel--; } 24 } 12

13 Example2(TestTV.java),cont : 1 public class TestTV { 2 public static void main(string[] args) { 3 TV tv1 = new TV(); 4 tv1.turnon(); 5 tv1.setchannel(30); 6 tv1.setvolume(3); 7 8 TV tv2 = new TV(); 9 tv2.turnon(); 10 tv2.channelup(); 11 tv2.channelup(); 12 tv2.volumeup(); System.out.println("tv1's channel is " + tv1.channel 15 + " and volume level is " + tv1.volumelevel); 16 System.out.println("tv2's channel is " + tv2.channel 17 + " and volume level is " + tv2.volumelevel); 18 } 19 } 13

14 Constructors Constructors are a special kind of methods that are invoked to construct objects. Circle() { } Circle(double newradius) { radius = newradius; } 14

15 Constructors, cont. 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. 15

16 Creating Objects Using Constructors new ClassName(); Example: new Circle(); new Circle(5.0); 16

17 Default Constructor A class may be defined 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 defined in the class. 17

18 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 Declaring/Creating Objects in a Single Step ClassName objectrefvar = new ClassName(); Example: Assign object reference Create an object Circle mycircle = new Circle(); 19

20 Accessing Object s Members 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 (1) Circle mycircle = new Circle(5.0); (2) Circle yourcircle = new Circle(); (3) yourcircle.radius = 100; Trace Code mycircle reference value yourcircle reference value yourcircle reference value : Circle : Circle : Circle radius: 5.0 radius: 1.0 radius: (1) (2) (3) 21

22 Recall that you use Caution Math.methodName(arguments) (arguments)(e.g.,(e.g., Math.pow(3, 2.5)) to invoke a method in the Mathclass. 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 statickeyword. However, getarea()is non-static. It must be invoked from an object using objectrefvar.methodname(arguments) (arguments)(e.g.,(e.g., mycircle.getarea() ()).). More explanations will be given in the section on Static Variables, Constants, and Methods. 22

23 Reference Data Fields The data fields can be of reference types. For example, the following Studentclass contains a data field nameof 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' 23

24 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. 24

25 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(); System.out.println("name? " + student.name); System.out.println("age? " + student.age); System.out.println(" ("issciencemajor? " + student.issciencemajor); System.out.println("gender? " + student.gender); 25

26 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 26

27 Differences between Variables of Primitive Data Types and Object Types Primitive type int i = 1 i 1 Created using new Circle() Object type Circle c c reference c: Circle radius = 1 27

28 Copying Variables of Primitive Data Types and Object Types Primitive type assignment i = j Before: After: i 1 i 2 j 2 j 2 c1 Before: Object type assignment c1 = c2 After: c1 c2 c2 c1: Circle radius = 5 C2: Circle radius = 9 c1: Circle radius = 5 C2: Circle radius = 9 28

29 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. 29

30 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. 30

31 The Date Class Java provides a system-independent encapsulation of date and time in the java.util.dateclass. You can use the Dateclass to create an instance for the current date and time and use its tostringmethod to return the date and time as a string. The + sign indicates public modifer java.util.date +Date() +Date(elapseTime: long) +tostring(): String +gettime(): long +settime(elapsetime: long): void Constructs a Date object for the current time. Constructs a Date object for a given time in milliseconds elapsed since January 1, 1970, GMT. Returns a string representing the date and time. Returns the number of milliseconds since January 1, 1970, GMT. Sets a new elapse time in the object. 31

32 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 likesun Mar 09 13:50:19 EST

33 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.randomclass. java.util.random +Random() +Random(seed: long) +nextint(): int +nextint(n: int): int +nextlong(): long +nextdouble(): double +nextfloat(): float +nextboolean(): boolean Constructs a Random object with the current time as its seed. Constructs a Random object with a specified seed. Returns a random int value. Returns a random int value between 0 and n (exclusive). Returns a random long value. Returns a random double value between 0.0 and 1.0 (exclusive). Returns a random float value between 0.0F and 1.0F (exclusive). Returns a random boolean value. 33

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

35 Displaying GUI Components When you develop programs to create graphical user interfaces, you will use Java classes such as JFrame, JButton, JRadioButton, JComboBox, and JListto create frames, buttons, radio buttons, combo boxes, lists, and so on. The next slide has an example that creates two windows using the JFrame class. 35

36 Example(TestFrame.java): 1 import javax.swing.jframe; 2 3 public class TestFrame { 4 public static void main(string[] args) { 5 JFrame frame1 = new JFrame(); 6 frame1.settitle("window 1"); 7 frame1.setsize(200, 150); 8 frame1.setlocation(200, 100); 9 frame1.setdefaultcloseoperation(jframe.exit_on_close); 10 frame1.setvisible(true); JFrame frame2 = new JFrame(); 13 frame2.settitle("window 2"); 14 frame2.setsize(200, 150); 15 frame2.setlocation(410, 100); 16 frame2.setdefaultcloseoperation(jframe.exit_on_close); 17 frame2.setvisible(true); 18 } 19 } 36

37 JFrame frame1 = new JFrame(); frame1.settitle("window 1"); frame1.setsize(.setsize(200, 150); frame1.setlocation(.setlocation(200, 100); frame1.setdefaultcloseoperation( JFrame.EXIT_ON_CLOSE); frame1.setvisible(.setvisible(true); Trace Code frame1 reference : JFrame title: "Window 1" width: 200 height: 150 x:200 y:100 visible: true JFrame frame2 = new JFrame(); frame2.settitle("window 2"); frame2.setsize(.setsize(200, 150); frame2.setlocation(.setlocation(410, 100); frame2.setdefaultcloseoperation( JFrame.EXIT_ON_CLOSE); frame2.setvisible(.setvisible(true); 37 frame2 reference : JFrame title: "Window 2" width: 200 height: 150 x:410 y:100 visible: true

38 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. The next two slides have an example to create buttons, labels, text fields, check boxes, radio buttons, and combo boxes. 38

39 Example(GUIComponents.java): 1 import javax.swing.*; 2 public class GUIComponents { 3 public static void main(string[] args) { 4 // Create a button with text OK 5 JButton jbtok = new JButton("OK"); 6 // Create a button with text Cancel 7 JButton jbtcancel = new JButton("Cancel"); 8 // Create a label with text "Enter your name: " 9 JLabel jlblname = new JLabel("Enter your name: "); 10 // Create a text field with text "Type Name Here" 11 JTextField jtfname = new JTextField("Type Name Here"); 12 // Create a check box with text bold 13 JCheckBox jchkbold = new JCheckBox("Bold"); 14 // Create a check box with text italic 15 JCheckBox jchkitalic = new JCheckBox("Italic"); 16 // Create a radio button with text red 17 JRadioButton jrbred = new JRadioButton("Red"); 18 // Create a radio button with text yellow 19 JRadioButton jrbyellow = new JRadioButton("Yellow"); 20 // Create a combo box with several choices 21 JComboBox jcbocolor = new JComboBox(new String[]{"Freshman", 22 "Sophomore", "Junior", "Senior"}); 23 39

40 Example(GUIComponents.java), cont. : 24 // Create a panel to group components 25 JPanel panel = new JPanel(); 26 panel.add(jbtok); // Add the OK button to the panel 27 panel.add(jbtcancel); // Add the Cancel button to the panel 28 panel.add(jlblname); // Add the label to the panel 29 panel.add(jtfname); // Add the text field to the panel 30 panel.add(jchkbold); // Add the check box to the panel 31 panel.add(jchkitalic); // Add the check box to the panel 32 panel.add(jrbred); // Add the radio button to the panel 33 panel.add(jrbyellow); // Add the radio button to the panel 34 panel.add(jcbocolor); // Add the combo box to the panel JFrame frame = new JFrame(); // Create a frame 37 frame.add(panel); // Add the panel to the frame 38 frame.settitle("show GUI Components"); 39 frame.setsize(450, 100); 40 frame.setlocation(200, 100); 41 frame.setdefaultcloseoperation(jframe.exit_on_close); 42 frame.setvisible(true); 43 } 44 } 40

41 Instance Variables, and Methods Instance variables belong to a specific instance. Instance methods are invoked by an instance of the class. 41

42 Static Variables, Constants, and Methods 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. 42

43 Static Variables, Constants, and Methods, cont. To declare static variables, constants, and methods, use the static modifier. 43

44 Static Variables, Constants, and Methods, cont. instantiate circle1 Memory Circle radius: double numberofobjects: int getnumberofobjects(): int +getarea(): double instantiate radius = 1 numberofobjects = 2 circle2 1 radius 2 numberofobjects After two Circle objects were created, numberofobjects is 2. UML Notation: +: public variables or methods underline: static variables or methods radius = 5 numberofobjects = 2 5 radius 44

45 Example of Using Instance and Class Variables and Method Objective: Demonstrate the roles of instance and class variables and their uses. The next two slides have an example adds a class variable numberofobjects to track the number of Circle objects created. 45

46 Example(CircleWithStaticMembers.java): 1 public class CircleWithStaticMembers { 2 /** The radius of the circle */ 3 double radius; 4 /** The number of the objects created */ 5 static int numberofobjects = 0; 6 /** Construct a circle with radius 1 */ 7 CircleWithStaticMembers() { 8 radius = 1.0; 9 numberofobjects++; 10 } 11 /** Construct a circle with a specified radius */ 12 CircleWithStaticMembers(double newradius) { 13 radius = newradius; 14 numberofobjects++; 15 } 16 /** Return numberofobjects */ 17 static int getnumberofobjects() { 18 return numberofobjects; 19 } 20 /** Return the area of this circle */ 21 double getarea() { 22 return radius * radius * Math.PI; 23 } 24 } 46

47 Example(TestCircleWithStaticMembers.java): 1 public class TestCircleWithStaticMembers { 2 /** Main method */ 3 public static void main(string[] args) { 4 System.out.println("Before creating objects"); 5 System.out.println("The number of Circle objects is " + CircleWithStaticMembers.numberOfObjects); 6 // Create c1 7 CircleWithStaticMembers c1 = new CircleWithStaticMembers(); 8 // Display c1 BEFORE c2 is created 9 System.out.println("\nAfter creating c1"); 10 System.out.println("c1: radius (" + c1.radius + ") and number of Circle objects (" + 11 c1.numberofobjects + ")"); 12 // Create c2 13 CircleWithStaticMembers c2 = new CircleWithStaticMembers(5); 14 // Modify c1 15 c1.radius = 9; 16 // Display c1 and c2 AFTER c2 was created 17 System.out.println("\nAfter creating c2 and modifying c1"); 18 System.out.println("c1: radius (" + c1.radius + ") and number of Circle objects (" + 19 c1.numberofobjects + ")"); 20 System.out.println("c2: radius (" + c2.radius + ") and number of Circle objects (" + 21 c2.numberofobjects + ")"); 22 } 23 } 47

48 Visibility Modifiers and Accessor/MutatorMethods By default, the class, variable, or method can be accessed by any class in the same package. public 48 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.

49 package p1; package p2; public class C1 { public int x; int y; private int z; } public void m1() { } void m2() { } private void m3() { } public class C2 { void amethod() { C1 o = new C1(); can access o.x; can access o.y; cannot access o.z; } } can invoke o.m1(); can invoke o.m2(); cannot invoke o.m3(); public class C3 { void amethod() { C1 o = new C1(); can access o.x; cannot access o.y; cannot access o.z; } } can invoke o.m1(); cannot invoke o.m2(); cannot invoke o.m3(); package p1; package p2; class C1 {... } public class C2 { can access C1 } public class C3 { cannot access C1; can access C2; } 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. 49

50 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). public class F { public class Test { private boolean x; public static void main(string[] args) { F f = new F (); System.out.println(f.x); System.out.println(f.convert()); } } public static void main(string[] args) { Foo f = new F(); System.out.println(f.x); System.out.println(f.convert(f.x)); } } private int convert(boolean b) { return x? 1 : -1; } (a) This is OK because object f is used inside the F class (b) This is wrong because x and convert are private in F. 50

51 Why Data Fields Should Be private? To protect data. To make class easy to maintain. 51

52 Example of Data Field Encapsulation The - sign indicates private modifier -radius: double Circle -numberofobjects: int The radius of this circle (default: 1.0). The number of circle objects created. +Circle() +Circle(radius: double) +getradius(): double +setradius(radius: double): void +getnumberofobject(): int +getarea(): double Constructs a default circle object. Constructs a circle object with the specified radius. Returns the radius of this circle. Sets a new radius for this circle. Returns the number of circle objects created. Returns the area of this circle. 52

53 Example(CircleWithPrivateDataFields.java): 1 public class CircleWithPrivateDataFields { 2 /** The radius of the circle */ 3 private double radius = 1; 5 /** The number of the objects created */ 6 private static int numberofobjects = 0; 8 /** Construct a circle with radius 1 */ 9 public CircleWithPrivateDataFields( ) { numberofobjects++; } 13 /** Construct a circle with a specified radius */ 14 public CircleWithPrivateDataFields(double newradius) { 15 radius = newradius; 16 numberofobjects++; 17 } 19 /** Return radius */ 20 public double getradius( ) { return radius; } 24 /** Set a new radius */ 25 public void setradius(double newradius) { radius = (newradius >= 0)? newradius : 0; } 29 /** Return numberofobjects */ 30 public static int getnumberofobjects( ) { return numberofobjects; } 34 /** Return the area of this circle */ 35 public double getarea() { return radius * radius * Math.PI; } 38 } 53

54 Example(TestCircleWithPrivateDataFields.java): 1 public class TestCircleWithPrivateDataFields { 2 /** Main method */ 3 public static void main(string[] args) { 4 // Create a Circle with radius CircleWithPrivateDataFields mycircle = new CircleWithPrivateDataFields(5.0); 6 System.out.println("The area of the circle of radius " + mycircle.getradius() + 7 " is " + mycircle.getarea()); 8 // Increase mycircle's radius by 10% 9 mycircle.setradius(mycircle.getradius() * 1.1); 10 System.out.println("The area of the circle of radius " + mycircle.getradius() + 11" is " + mycircle.getarea()); 12 } 13 } 54

55 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) 55

56 Example(TestPassObject.java): 1 public class TestPassObject { 2 /** Main method */ 3 public static void main(string[] args) { 4 // Create a Circle object with radius 1 5 CircleWithPrivateDataFields mycircle = new CircleWithPrivateDataFields(1); 7 8 // Print areas for radius 1, 2, 3, 4, and 5. 9 int n = 5; 10 printareas(mycircle, n); // See mycircle.radius and times 13 System.out.println("\n" + "Radius is " + mycircle.getradius()); 14 System.out.println("n is " + n); 15 } /** Print a table of areas for radius */ 18 public static void printareas( CircleWithPrivateDataFields c, int times) { 20 System.out.println("Radius \t\tarea"); 21 while (times >= 1) { 22 System.out.println(c.getRadius() + "\t\t" + c.getarea()); 23 c.setradius(c.getradius() + 1); 24 times--; 25 } 26 } } 56

57 Passing Objects to Methods, cont. Stack Space required for the printareas method int times: 5 Circle c: reference Space required for the main method int n: 5 mycircle: reference Pass by value (here the value is 5) Pass by value (here the value is the reference for the object) Heap A circle object 57

58 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. 58

59 Array of Objects, cont. Circle[] circlearray = new Circle[10]; circlearray reference circlearray[0] Circle object 0 circlearray[1] Circle object 1 circlearray[9] Circle object 9 59

60 Array of Objects, cont. Example of the areas of the circles 60

61 Example(TotalArea.java): 1 public class TotalArea { 2 /** Main method */ 3 public static void main(string[] args) { 4 // Declare circlearray 5 CircleWithPrivateDataFields[] circlearray; 6 7 // Create circlearray 8 circlearray = createcirclearray(); 9 10 // Print circlearray and total areas of the circles 11 printcirclearray(circlearray); 12 } /** Create an array of Circle objects */ 15 public static CircleWithPrivateDataFields[] createcirclearray() { 16 CircleWithPrivateDataFields[] circlearray = 17 new CircleWithPrivateDataFields[5]; for (int i = 0; i < circlearray.length; i++) { 20 circlearray[i] = 21 new CircleWithPrivateDataFields(Math.random() * 100); 22 } // Return Circle array 25 return circlearray; 26 } 27 61

62 Example(TotalArea.java), cont. : 28 /** Print an array of circles and their total area */ 29 public static void printcirclearray( 30 CircleWithPrivateDataFields[] circlearray) { 31 System.out.printf("%-30s%-15s\n", "Radius", "Area"); 32 for (int i = 0; i < circlearray.length; i++) { 33 System.out.printf("%-30f%-15f\n", circlearray[i].getradius(), 34 circlearray[i].getarea()); 35 } System.out.println(" "); // Compute and display the result 40 System.out.printf("%-30s%-15f\n", "The total areas of circles is", 41 sum(circlearray)); 42 } /** Add circle areas */ 45 public static double sum( 46 CircleWithPrivateDataFields[] circlearray) { 47 // Initialize sum 48 double sum = 0; // Add areas to sum 51 for (int i = 0; i < circlearray.length; i++) 52 sum += circlearray[i].getarea(); return sum; 55 } } 62