Chapter 11 Object-Oriented Design Exception and binary I/O can be covered after Chapter 9

Transcription

1 Chapter 7 Objects and Classes Chapter 6 Arrays Chapter 7 Objects and Classes Chapter 8 Strings and Text I/O Chapter 9 Inheritance and Polymorphism GUI can be covered after 10.2, Abstract Classes Chapter 12 GUI Basics 10.2, Abstract Classes Chapter 13 Graphics 10.4, Interfaces Chapter 14 Event-Driven Programming Chapter 11 Object-Oriented Design Exception and binary I/O can be covered after Chapter 9 Chapter 17 Exceptions and dassertions Chapter 18 Binary I/O 1

2 Objectives To understand objects and classes and use classes to model objects ( 7.2). To learn how to declare a class and how to create an object of a class ( 7.3). To understand the roles of constructors and use constructors to create objects ( 7.3). To use UML graphical notations to describe classes and objects ( 7.3). To distinguish between object reference variables and primitive data type variables ( 7.4). To use classes in the Java library ( 7.5). To declare private data fields with appropriate get and set methods to make class easy to maintain ( ). To develop methods with object arguments ( 7.9). To understand the difference between instance and static variables and methods ( 7.10). To determine the scope of variables in the context of a class ( 7.11). To use the keyword this as the reference to the current object that invokes the instance method ( 7.12). To store and process objects in arrays ( 7.13). 2

3 OO Programming Concepts Object-oriented programming (OOP) involves programming using objects. An object represents an entity in the real world that can be distinctly i identified. ifi d 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, i state, and dbehaviors. 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 ed by a set of methods. 3

4 Objects Class Name: Circle Data Fields: radius is Methods: getarea A class template Circle Object 1 Circle Object 2 Circle Object 3 Three objects of the Circle class Data Fields: radius is 10 Data Fields: radius is 25 Data Fields: radius is 125 An object has both a state and behavior. The state defines the object, and the behavior defines what the object does. 4

5 Classes Classes are construction 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. 5

6 Classes class Circle { /** The radius of this circle */ double radius = 1.0; /** Construct a circle object */ Circle() { /** Construct t a circle object */ Circle(double newradius) { radius = newradius; Data field Constructors /** Return the area of this circle */ double getarea() { return radius * radius * ; Method 6

7 UML Class Diagram UML Class Diagram Circle Class name radius: double Data fields Circle() Circle(newRadius: double) Constructors and Mthd Methods getarea(): double circle1: Circle radius: 10.0 circle2: Circle radius: 25.0 circle3: Circle radius: UML notation for objects UML=Unified U Modeling Language 7

8 Constructors Circle() { Constructors are a special kind of methods that are invoked to construct objects. Circle(double newradius) { radius = newradius; Different signature: order, number, type overload constructor 8

9 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 fiiilii initializing objects. 9

10 Creating Objects Using Constructors ClassName objname = new ClassName(); Example: Circle cir1 = new Circle(); Circle cir2 =new Circle(5.0); 10

11 Default Constructor 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 df default constructor, is provided automatically only if no constructors are explicitly declared in the class. ยกต วอย างประกอบ ยกตวอยางประกอบ 11

12 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; objectrefvar = new ClassName(); Example: Circle mycircle; mycircle = new Circle(); 12

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

14 Accessing Objects Referencing the object s data fields: objectrefvar.data e.g., mycircle.radius Invoking the object s method: objectrefvar.methodname(arguments) e.g., mycircle.getarea() Occasionally, an object does not need to be referenced later, you can create an object without explicitly assigning it to a variable System.out.println( Area is + new Circle(5).getArea() ).getarea()); Call Anonymous object 14

15 public class TestCircle { /** Main method */ public static void main(string[] args) { // Create a circle with radius 5.0 Circle mycircle = new Circle(5.0); System.out.println("The area of the circle of radius " + mycircle.radius + " is " + mycircle.getarea()); System.out.println(myCircle); t yc e); // Create a circle with radius 1 Circle yourcircle = new Circle(); 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()); // Define the circle class with two constructors class Circle { double radius; /** Construct a circle with radius 1 */ Circle() { radius = 1.0; /** Construct a circle with a specified radius */ Circle(double newradius) { radius = newradius; /** Return the area of this circle */ double getarea() { return radius * radius * Math.PI; Circle radius: double Circle() Circle(newRadius: double) getarea(): double TestCircle main(args : String[]) TestCircle1 15

16 animation Trace Code Declare mycircle Circle mycircle = new Circle(5.0); Circle yourcircle = new Circle(); yourcircle.radius = 100; mycircle no value 16

17 animation Trace Code, cont. Circle mycircle = new Circle(5.0); Circle yourcircle = new Circle(); yourcircle.radius = 100; mycircle :Circle no value radius: 5.0 Create a circle 17

18 animation Trace Code, cont. Circle mycircle = new Circle(5.0); Circle yourcircle = new Circle(); mycircle reference value yourcircle.radius = 100; Assign object reference to mycircle radius: 5.0 :Circle 18

19 animation Trace Code, cont. Circle mycircle = new Circle(5.0); Circle yourcircle = new Circle(); yourcircle.radius = 100; mycircle reference value : Circle radius: 5.0 yourcircle no value Declare yourcircle 19

20 animation Trace Code, cont. Circle mycircle = new Circle(5.0); Circle yourcircle = new Circle(); yourcircle.radius = 100; mycircle reference value : Circle radius: 5.0 yourcircle no value Create a new radius: 0.0 Circle object : Circle 20

21 animation Trace Code, cont. Circle mycircle = new Circle(5.0); Circle yourcircle = new Circle(); yourcircle.radius = 100; mycircle reference value : Circle radius: 5.0 yourcircle reference value Assign object reference to yourcircle : Circle radius:

22 animation Trace Code, cont. Circle mycircle = new Circle(5.0); Circle yourcircle = new Circle(); yourcircle.radius = 100; mycircle reference value : Circle radius: 5.0 yourcircle reference value : Circle Change radius in radius: yourcircle 22

23 1 public class Circle { 2 /** Main method */ 3 public static void main(string[] args) { C 4 // Create a circle with radius Circle mycircle = new Circle(5.0); 6 System.out.println("The area of the circle of radius " 7 + mycircle.radius + " is " + mycircle.getarea()); System.out.println(myCircle); t yc e); // Create a circle with radius 1 Circle yourcircle = new Circle(); 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()); Circle() { // Define the circle class with two constructors double radius; /** Construct a circle with radius 1 */ radius = 1.0; /** Construct a circle with a specified radius */ Circle(double newradius) { radius = newradius; /** Return the area of this circle */ double getarea() { return radius * radius * Math.PI; Circle radius: double main(args : String[]) Circle() Circle(newRadius: double) getarea(): double -สร างเมธอด main ไว ในคลาสเพ อทดสอบการท างานของคลาส -ส วน fields วางไว ส วนใดก ได แต ให อย ใน scope ของคลาส 23

24 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 Section 7.7, Static Variables, Constants, and Methods. 24

25 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 literal value, null. Student name: String age : int issciencemajor : boolean gender : char 25

26 Default Value for a Data Field The default value of a data field is null for a reference type 0.0 for a float, double type 0 for a numeric type, (byte, short, int, long) false for a boolean type, and '\u0000' for a char type. Student name: String age : int issciencemajor : boolean gender : char 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); //null System.out.println("age? " + student.age); //0 System.out.println("isScienceMajor? " + student.issciencemajor); //false System.out.println("gender? " + student.gender);//space character System.out.println(student.name.length());//NullPointerException Student std = null; std.age = 20; //NullPointerException is a common runtime error 26

27 Example Java assigns no default value to a local variable inside id 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 27

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

29 Copying Variables of Primitive Data Types and Object Types Before: Primitive type assignment i = j After: Circle radius: double Circle() Circle(newRadius: double) getarea(): double i 1 i 2 j 2 j 2 int i=1; int j=2; i=j; c1 Before: Circle c1 = new Circle(5); Circle c2 = new Circle(9); c1=c2; Object type assignment c1 = c2 After: c1 c2 c2 c1: Circle c2: Circle c1: Circle c2: Circle radius = 5.0 radiu s = 9.0 radius = 5.0 radius =

30 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. The JVM detects garbage and automatically reclaims the space it occupies. This process is called garbage collection 30

31 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. Circle c1 = new Circle(5); Circle c2 = new Circle(9); c1=null; c1=c2; 31

32 Using Classes from the Java Library Example 7.1 declared the Circle1 class and created objects from the class. Often you will use the classes in the Java library to develop programs. You learned to obtain the current time using System.currentTimeMillis() in Example 2.5, Displaying Current Time. You used the division and remainder operators to extract current second, minute, and hour. 32

33 Using Classes from the Java Library :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. 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. java.util.date d = new java.util.date(24*60*60*1000); System.out.println(d.toString());// Fri Jan 02 07:00:00 ICT

34 The Date Class Example For example, the following code java.util.date date = new java.util.date(); System.out.println(date.toString()); //เวลาป จจ บ น displays a string like: Mon Jul 04 14:42:18 ICT 2011 ต อง import java.util.date; //หร อไม 34

35 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. java.util.random +Random() Constructs a Random object with the current time as its seed. +Random(seed: long) Constructs a Random object with a specified seed. +nextint(): int Returns a random int value. +nextint(n: int): int Returns a random int value between 0 and n (exclusive). +nextlong(): long Returns a random long value. +nextdouble(): double Returns a random double value between 0.0 and 1.0 (exclusive). +nextfloat(): float Returns a random float value between 0.0F and 1.0F (exclusive). +nextboolean(): boolean Returns a random boolean value. 35

36 The Random Class Example If two Random objects have the same seed, they will generate e 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: "); for (int i = 0; i < 10; i++) System.out.print(random2.nextInt(1000) + " "); From random1: From random2: Random random3 = new Random(9); System.out.print("\nFrom random3: "); for (int i = 0; i < 10; i++) System.out.print(random3.nextInt(1000) ( ( ) + " "); o/p: From random3:

37 Instance Variables, and Methods Instance variables belong to a specific instance, it is not shared among objects of the same class double radius; Instance methods are invoked by an instance of the class. void getarea(){st; t Instance methods and instance variables belong to instances and can only be used after the instances are created. They are accessed via a reference variable Circle obj = new Circle(); obj.getarea(); obj.radius; 37

38 UML Class Diagram UML Class Diagram Circle Class name radius: double Data fields Circle() Circle(newRadius: double) Constructors and Mthd Methods getarea(): double circle1: Circle radius: 10.0 circle2: Circle radius: 25.0 circle3: Circle radius: UML notation for objects Circle cir1 = new Circle(10); cir1.getarea(); cir1.radius; 38

39 Static Variables, Constants,and and Methods Static variables are shared by all the instances of the class. static int numberofobjects;//class variable Static ti methods are not tied to a specific object. static void getnumberofobjects(){st;//class method Static constants are final variables shared by all the instances of the class. final static int MAX = 100; To declare static variables, constants, and methods, use the static modifier. Static ti methods and static ti variables can be accessed from a reference variable or from their class name. 39

40 Static Variables, Constants, and Methods, cont. instantiate circle1 Memory Circle radius = 1.0 numberofobjects = radius radius: double numberofobjects: int getnumberofobjects(): int +getarea(): double instantiate circle2 2 numberofobjects UML Notation: underline: static variables or methods radius = 5.0 numberofobjects = radius Circle circle1 = new Circle(1); Circle circle2 = new Circle(5); circle1.numberofobjects = 2; System.out.println(circle2.getNumberOfObjects()); ถ าเป นแบบ static ควรใช คลาสอ างด กว าเพ อให ทราบว าค อต วแปรและ เมธอดแบบ static Circle.numberOfObjects; Circle.getNumberOfObjects() Access by using class name 40

41 static variable final static variable obj1 obj2... obj1 objn obj2... objn static int numobj; final static int MAX; final variable final variable final variable... public class A { obj1 obj2 objn final int MIN; final int MIN; public A(int val){ MIN = val; 41

42 Example of Using Instance and Class Variables and dmethod 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. Circel2, TestCircle2 42

43 Static Variables, Constants, and Methods, cont. Static variables and methods can be used from instance or static methods in the class Instance variables and methods can only be used from instance methods, not from static method public class Foo { int i = 5; static int k = 2; public static void main(string[] args) { int j = i; // wrong because i is an instance var m1(); // wrong because m1 is an instance method public void m1(){ public static void m() Can call -static vars, methods i = i + k + m2(i, k); public void m2() // correct since instance and static var and methods // can be used in an instance method public void m2(int m, int n){ return(int)(math.pow(m, n)); Can call -static vars,methods -instance vars,methods 43

44 JDK 1.5:import static variables, methods New JDK 1.5 feature to directly import static variables and methods from a class The imported data and methods can be referenced or called without specifying a class import static packagename.classname.*; * represent all static variables and static methods import static java.lang.math.*; import static java.lang.math.*; public class Test { public static void main(string[] args) { System.out.println(abs( outprintln(abs(-7.8)); System.out.println(max(9,100)); System.out.println(PI); If use: import static java.lang.math.abs; 44

45 instance one or a static one? How do you decide whether a variable or method should be an instance one or a static one? A variable or method that is dependent on a specific instance of fthe class should ldbe an instance variable or method. A variable or method that is not dependent on a specific instance of the class should be a static variable or method. radius, //specific use Math class : pow(), abs() // common use Student -studentid -tuitionfees getstudentid() gettuitionfees() instance or static? 45

46 Visibility Modifiers If public or private is not used, then by default the classes, methods and data fields They are accessible by any class in the same package, known packageaccess double radius; 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. private class Test { //compile error, outer class 46

47 package p1; public class C1 { public class C2 { public int x; void amethod() { int y; C1 o = new C1(); private int z; can access o.x; can access o.y; public void m1() { void m2() { private void m3() { cannot access o.z; o can invoke o.m1(); can invoke o.m2(); cannot invoke o.m3(); ต วแปร ตวแปร z เล นได ท ไหนบ าง? เลนไดทไหนบาง? package p2; import p1.c1; 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 { import p1.c2; 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 t access. 47

48 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 Foo { private boolean x; public static void main(string[] args) { Foo foo = new Foo(); System.out.println(foo.x); System.out.println(foo.convert()); ()) public class Test { public static void main(string[] i args) { Foo foo = new Foo(); System.out.println(foo.x); System.out.println(foo.convert(foo.x)); private int convert(boolean b) { return x? 1 : -1; (a) This is OK because object foo is used inside the Foo class (b) This is wrong because x and convert are private in Foo. 48

49 NOTE Visibility modifiers are used for the members of the class, not local variables inside the method Cause compile error private constructor If you want to prohibit the user from creating an instance of a class private Math() { public class Test{ public static void main(string[] args) { static int m = 5; public int n = 10; private int p = 15; final int x = 100; // ok 49

50 Why Data Fields Should Be private? To protect data. To make class easy to maintain. Accessor/Mutator Methods The get and set methods are used to read and modify private properties. Accessor : public returntype getpropertyname() { Mutator : public void setpropertyname() { Return type boolean : public void ispropetyname() { 50

51 Example of Data Field Encapsulation The - sign indicates private modifier Circle -radius: double The radius of this circle (default: 1.0). -numberofobjects: int 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. Circle3, TestCircle3 51

52 Immutable Objects and Classes Its class is called an immutable class. If you delete the set methods in the Circle class in the preceding example, the class would be immutable because radius is private and cannot be changed without a set method. If the contents of an object cannot be changed once the object is created, the object is called an immutable object and A class with all private data fields and without mutators t is not necessarily immutable. For example, the following class Student has all private data fields and no mutators (พวกเมธอด set, but it is mutable.) 52

53 Example public class Student t { private int id; private BirthDate birthdate; public class BirthDate { private int year; private int month; private int day; public Student(int ssn, int year, int month, int day) { id = ssn; birthdate = new BirthDate(year, month, day); public int getid() { return id; public BirthDate getbirthdate() { return birthdate; public BirthDate(int newyear, int newmonth, int newday) { year = newyear; month = newmonth; day = newday; public void setyear(int newyear) { year = newyear; public class Test { public static void main(string[] args) { Student student = new Student( , 1970, 5, 3); BirthDate date = student.getbirthdate(); date.setyear(2010); // Now the student birth year is changed! For a class to be immutable, it must mark all data fields private and provide no mutator methods 53 and no accessor methods that would return a reference to a mutable data field object.

54 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) TestPassObject 54

55 1 public class TestPassObject { 2 /** Main method */ 3 public static void main(string[] args) { 4 // Create a Circle object with radius 1 5 Circle mycircle = new Circle(1); 6 7 // Print areas for radius 1, 2, 3, 4, and 5. 8 intn=5; n printareas(mycircle, n); // See mycircle.radius and times 12 System.out.println("\n" t tl " + "Radius is " mycircle.getradius()); System.out.println("n is " + n); 17 /** Print a table of areas for radius */ 18 public static void printareas(circle c, int times) { 19 System.out.println("Radius \t\tarea"); 20 while (times >= 1) { System.out.println(c.getRadius() + "\t\t" + c.getarea()); c.setradius(c.getradius() + 1); times--; Radius Area

56 Passing Objects to Methods, cont. Stack Space required for the printareas method int times: 5 Circle c: reference Space required for the main method it int n: 5 mycircle: reference Pass by value (here the value is 5) Pass by reference for the object Heap A circle object 56

57 Scope of Variables ab The scope of instance and static variables is the entire class. They can be declared d anywhere inside id a class. public class Circle{ public double getarea(){ return radius*radius*math.pi; private double radius = 1; The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be initialized explicitly before it can be used. 57

58 Local variable same name as class s variable The local variable takes precedence and the class s variable with the same name is hidden 1 public class Foo { 2 int x = 19; 3 int y = 20; 4 public void p(){ 5 int x = 1; 6 System.out.println( x= + x); 7 System.out.println( y= + y); x= 1 public static void main(string[] args) { Foo obj = new Foo(); obj.p(); y= 20 58

59 The this Keyword this.instance_method(); or this.instance_data filed Use this to refer to the object that invokes the instance method. Use this to refer to an instance data field. this(); this(args); Use this to invoke an overloaded constructor of the same class. 59

60 Serving as Proxy to the Calling Object class Foo { int i = 5; static double k = 0; void seti(int i) { this.i = i; static void setk(double k) { Foo.k = k; //Suppose that f1 and f2 are two objects of Foo. Foo f1 = new Foo(); Foo f2 = new Foo(); //Invoking f1.seti(10) is to execute f1.i = 10;// where this is replaced by f1 //Invoking f2.seti(45) is to execute f2.i = 45;// where this is replaced by f2 -ห ามใช -หามใช this ใน static method, compile error this.k = k ; -หล กเล ยงการต งช อซ าก บ fields 60

61 Calling Overloaded Constructor public class Circle { private double radius; public Circle(double radius) { this.radius = radius; public Circle() { this(1.0); this must be explicitly used to reference the data field radius of the object being constructed this is used to invoke another constructor public double getarea() { return this.radius * this.radius * Math.PI; Every instance variable belongs to an instance represented by this, which is normally omitted public class Test{ public static void main(string[] args){ Circle obj = new Circle(); 61

62 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. 62

63 Array of Objects, cont. Circle[] circlearray = new Circle[10]; circlearray[0] = new Circle(4); circlearray[9] = new Circle(11); circlearray reference circlearray[0] Circle object 0 circlearray[1] Circle object 1 Summarizing the areas of the circles circlearray[9] Circle object 9 TotalArea 63

64 Example: The Loan Class Loan -annualinterestrate: double -numberofyears: int -loanamount: double -loandate: Date +Loan() +Loan(annualInterestRate: double, numberofyears: int, loanamount: double) +getannualinterestrate(): double +getnumberofyears(): int +getloanamount(): double +getloandate(): Date +setannualinterestrate( annualinterestrate: double): void +setnumberofyears( numberofyears: int): void +setloanamount( loanamount: double): void +getmonthlypayment(): double +gettotalpayment(): double The annual interest rate of the loan (default: 2.5). The number of years for the loan (default: 1) The loan amount (default: 1000). The date this loan was created. Constructs a default Loan object. Constructs a loan with specified interest rate, years, and loan amount. Returns the annual interest rate of this loan. Returns the number of the years of this loan. Returns the amount of this loan. Returns the date of the creation of this loan. Sets a new annual interest rate to this loan. Sets a new number of years to this loan. Sets a new amount to this loan. Returns the monthly payment of this loan. Returns the total payment of this loan. Loan,TestLoanClass 64

65 Example: The Course Class Course -name: String -students: String[] -numberofstudents: int +Course(name: String) +getname(): String +addstudent(student: String): void +getstudents(): t String[] +getnumberofstudents(): int The name of the course. The students who take the course. The number of students (default: 0). Creates a Course with the specified name. Returns the course name. Adds a new student to the course list. Returns the students for the course. Returns the number of students for the course. Course,TestCourse 65

66 Example: The StackOfIntegers Class StackOfIntegers -elements: int[] -size: int +StackOfIntegers() +StackOfIntegers(capacity: int) +empty(): boolean +peek(): int +push(value: int): int +pop(): int +getsize(): int An array to store integers in the stack. The number of integers in the stack. Constructs an empty stack with a default capacity of 16. Constructs an empty stack with a specified capacity. Returns true if the stack is empty. Returns the integer at the top of the stack without removing it from the stack. Stores an integer into the top of the stack. Removes the integer at the top of the stack and returns it. Returns the number of elements in the stack. TestStackOfIntegers t 66

67 Implementing StackOfIntegers Class elements[capacity 1] elements[size-1]... top capacity elements[1] elements[0]... bottom size StatckOfIntegers 67