Java 2 chapter 10 - basic oop in java

1. Lecture d by: CHEA VICHET 077 657 007 1

2. 1. Why OOP? Traditional Programming Language (C, Pascal) Object-Oriented Programming Language 2. Class Instance Class Compartment Creating a Class Creating an Instance 3. Method of Class Creating a Method Constructor Information Hiding Encapsulation Overloading Method 4. Helpful Reminder 5. Exercise and Homework 2

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46. Java is an Object Oriented Language. As a language that has the Object Oriented feature Java supports the following fundamental concepts: Polymorphism Inheritance Encapsulation Abstraction Classes Objects Instance Method Message Parsing 6

47. Notable drawbacks for creating reusable software components: Programs are made up of functions. Functions are often not reusable. It is very difficult to copy a function from one program and reuse in another program because the function is likely to reference the headers, global variables and other functions. Not suitable of high-level abstraction for solving real life problems. 7

48. The basic unit of OOP is a class, which encapsulates both the static attributes and dynamic behaviors within a box, and specifies the public interface for using these boxes. 8 OOP languages permit higher level of abstraction for solving real-life problems. The OOP languages (such as Java, C++, C#) let you think in the problem space, and use software objects to represent and abstract entities of the problem space to solve the problem.

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62. ចន 10 Object-Oriented technology has many benefits: • EEEEaaaasssseeee iiiinnnn ssssooooffffttttwwwwaaaarrrreeee ddddeeeessssiiiiggggnnnn as you could think in the problem space rather than the machine's bits and bytes. You are dealing with high-level concepts and abstractions. Ease in design leads to more productive software development. • EEEEaaaasssseeee iiiinnnn ssssooooffffttttwwwwaaaarrrreeee mmmmaaaaiiiinnnntttteeeennnnaaaannnncccceeee: object-oriented software are easier to understand, therefore easier to test, debug, and maintain. • RRRReeeeuuuussssaaaabbbblllleeee ssssooooffffttttwwwwaaaarrrreeee: you don't need to keep re-inventing the wheels and re-write the same functions for different situations. The fastest and safest way of developing a new application is to reuse existing codes - fully tested and proven codes.

63. A class is a blueprint[បង], template[ពម ], or prototype[ គ

64. ] that defines and describes the static attributes and dynamic behaviors common to all objects of the same kind. An instance is an instantiation of a class. All the instances of a class have similar properties, as described in the class definition. For example, you can define a class called Student and create three instances of the class Student for Peter, Paul and Pauline. The term object usually refers to instance. 11

65. A class can be visualized as a three-compartment box: Name Variables Methods 12

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67. The following figure shows two instances of the class Student, identified as paul and perter. 14

68. [AccessControlModifier] class ClassName { // class body contains definition of variables and methods ... } 15 Class Naming Convention: A class name shall be a noun, or a noun phrase made up of several words. All the words shall be initial-capitalized (camel-case). Use a singular noun for class name. Choose a meaningful and self-descriptive classname. ▪ For examples: SoccerPlayer, HttpProxyServer, FileInputStream, PrintStream and SocketFactory.

69. [AccessControlModifier] type variableName [= initialValue]; 16 Example: private double radius; public int length = 1, width = 1;

70. Variable Naming Convention: A variable name shall be a noun, or a noun phrase made up of several words. The first word is in lowercase and the rest of the words are initial-capitalized, ▪ e.g., theFontSize, roomNumber, xMax, yMin and xTopLeft. ▪ Take note that variable name begins with an lowercase, while class name begins with an uppercase. 17

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72. To create an instance of a class, you have to: Declare an instance identifier (instance name) of a particular class. Construct the instance (i.e., allocate storage for the instance and initialize the instance) using the new operator. 19

73. The variables and methods belonging to a class are formally called member variables and member methods. To reference a member variable or method, you must: Identify the instance you are interested in, and Reference the method or variable via the dot operator. 20 In general, suppose there is a class called AClass with a member variable called aVariable and a member method called aMethod(). An instance called anInstance is constructed for AClass. You use anInstance.aVariable and anInstance.aMethod().

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78. A method: receives parameters from the caller, performs the operations defined in the method body, and returns a piece of result (or void) to the caller. Syntax: [AccessControlModifier] returnType methodName ([argumentList]) { // method body or implementation ...... } Example: public double getArea() { double area = radius * radius * Math.PI; return area; } 23

79. A method name shall be a verb, or a verb phrase made up of several words. The first word is in lowercase and the rest of the words are initial-capitalized. For example, getRadius(), getParameterValues(). Note: variable name is a noun method name is a verb Methods take arguments in parentheses (possibly zero argument with empty parentheses), but variables do not. 24

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90. A constructor is a special method that has the same method name as the class name. It is used to initialize the instance constructed. To create a new instance of a class, you need to use a special new operator followed by a call to one of the constructors. Circle c1 = new Circle(); Circle c2 = new Circle(2.0); Circle c3 = new Circle(3.0, red); A constructor is different from an ordinary method in the following aspects: The name of the constructor method is the same as the class name, and begins with an uppercase. Constructor has no return type (or implicitly returns void). Hence, no return statement is allowed inside the constructor's body. Constructor can only be invoked via the new operator. It can only be used once to initialize the instance constructed. You cannot call the constructor afterwards. Constructors are not inherited (to be explained later). 29

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93. Member variables of a class are typically private They are hidden from the outside world (i.e., the other classes) Access to the member variables are provided via the public assessor methods, e.g., getRadius() and setRadius(). This follows the principle of information hiding. Objects communicate with each others using well-defined interfaces. Objects are not allowed to know the implementation details of others. Note: Do not make any variable public, unless you have a good reason. 31

94. Mutator/Setter: Any method that changes a property of an object, normally setXXX() methods. public void setName(String name) public void setAge(int age) Accessor: It is a method that returns a property of an object, normally getXXX() methods. public String getName() public int getAge() 32

95. An access control modifier can be used to control the visibility of a class, or a member variable or a member method within a class. public: The class/variable/method is accessible and available to all the other objects in the system. private: The class/variable/method is accessible and available within this class only. UML Notations: In UML notations, public entity is denoted with a +, while private entity is denoted with a - in the class diagram. More access control modifiers will be discussed later. 33

96. You can use keyword this to refer to this class/instance inside a class definition. One of the main usage of keyword this is to resolve ambiguity [Eពម,នចF##]. 34

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98. 36 Circle -radius: double -color: String create+Circle() create+Circle(radius: double) create+Circle(radius: double, color: String) +getRadius(): double +setRadius(radius: double) +getColor(): String +setColor(color: String) +getArea(): double

99. Every well-designed Java class should have a public method called toString() that returns a string description of the object. You can invoke the toString() method explicitly by calling anInstanceName.toString() or implicitly via println() or String concatenation operator '+'. Running println(anInstance) 37

100. 38 Rectangle -width: int -heigh: int create+Rectangle(width: int, heigh: int) +getWidth(): int +setWidth(width: int) +getHeigh(): int +setHeigh(heigh: int) +getArea(): int Student -name: String -grade: double create+Student(name: String, grade: double) +getName(): String +setName(name: String) +getGrade(): double +setGrade(grade: double) +toString(): String

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116. import java.awt.Point; class Box { int x1 = 0; // these 4 variables int y1 = 0; // are instance variable int x2 = 0; // int y2 = 0; // (default is package) Box buildBox(int x1, int y1, int x2, int y2) { this.x1 = x1; this.y1 = y1; this.x2 = x2; this.y2 = y2; return this; } Box buildBox(Point topLeft, Point bottomRight) { x1 = topLeft.x; y1 = topLeft.y; x2 = bottomRight.x; y2 = bottomRight.y; return this; } Box buildBox(Point topLeft, int w, int h) { x1 = topLeft.x; y1 = topLeft.y; x2 = (x1 + w); y2 = (y1 + h); return this; } This method takes four integer arguments and returns a reference to the resulting Box object. Because the arguments have the same names as the instance variables, the keyword this is used inside the method when referring to the instance variables. You can overload buildBox() by creating a second version of the method with an argument list that takes two Point objects: Another possible way to define the rectangle is to use a top corner, a height, and a width: 1: 2: 3: 4: 5: 6: 7: 8: 9: 10: 11: 12: 13: 14: 15: 16: 17: 18: 19: 20: 21: 22: 23: 24: 25: 26: 27: 28: 29: 30: 31: 3 Methods are overloaded 41

117. void printBox(){ System.out.print(Box: + x1 + , + y1); System.out.println(, + x2 + , + y2 + ); } public static void main(String[] arguments) { Box rect = new Box(); System.out.println(Calling buildBox with coordinates + (25,25) and (50,50):); rect.buildBox(25, 25, 50, 50); rect.printBox(); System.out.println(nCalling buildBox with points + (10,10) and (20,20):); rect.buildBox(new Point(10, 10), new Point(20, 20)); rect.printBox(); System.out.println(nCalling buildBox with 1 point + (10,10), width 50 and height 50:); rect.buildBox(new Point(10, 10), 50, 50); rect.printBox(); } } This method is to display the rectangle’s coordinates, and a main() method tries everything out. 31: 32: 33: 34: 35: 36: 37: 38: 39: 40: 41: 42: 43: 44: 45: 46: 47: 48: 49: 50: 51: 52: 53: 54: 55: 55: 57: 42

118. void printBox(){ System.out.print(Box: + x1 + , + y1); System.out.println(, + x2 + , + y2 + ); } public static void main(String[] arguments) { Box rect = new Box(); System.out.println(Calling buildBox with coordinates + (25,25) and (50,50):); rect.buildBox(25, 25, 50, 50); rect.printBox(); System.out.println(nCalling buildBox with points + (10,10) and (20,20):); rect.buildBox(new Point(10, 10), new Point(20, 20)); rect.printBox(); System.out.println(nCalling buildBox with 1 point + (10,10), width 50 and height 50:); rect.buildBox(new Point(10, 10), 50, 50); rect.printBox(); } } 31: 32: 33: 34: 35: 36: 37: 38: 39: 40: 41: 42: 43: 44: 45: 46: 47: 48: 49: 50: 51: 52: 53: 54: 55: 55: 57: Create a Box’s instance, rect. This line of code calls to the default constructor. 43

119. void printBox(){ System.out.print(Box: + x1 + , + y1); System.out.println(, + x2 + , + y2 + ); } public static void main(String[] arguments) { Box rect = new Box(); System.out.println(Calling buildBox with coordinates + (25,25) and (50,50):); rect.buildBox(25, 25, 50, 50); rect.printBox(); System.out.println(nCalling buildBox with points + (10,10) and (20,20):); rect.buildBox(new Point(10, 10), new Point(20, 20)); rect.printBox(); System.out.println(nCalling buildBox with 1 point + (10,10), width 50 and height 50:); rect.buildBox(new Point(10, 10), 50, 50); rect.printBox(); } } 31: 32: 33: 34: 35: 36: 37: 38: 39: 40: 41: 42: 43: 44: 45: 46: 47: 48: 49: 50: 51: 52: 53: 54: 55: 55: 57: rect is built by calling the method of the class Box on line 9 44

120. void printBox(){ System.out.print(Box: + x1 + , + y1); System.out.println(, + x2 + , + y2 + ); } public static void main(String[] arguments) { Box rect = new Box(); System.out.println(Calling buildBox with coordinates + (25,25) and (50,50):); rect.buildBox(25, 25, 50, 50); rect.printBox(); System.out.println(nCalling buildBox with points + (10,10) and (20,20):); rect.buildBox(new Point(10, 10), new Point(20, 20)); rect.printBox(); System.out.println(nCalling buildBox with 1 point + (10,10), width 50 and height 50:); rect.buildBox(new Point(10, 10), 50, 50); rect.printBox(); } } 31: 32: 33: 34: 35: 36: 37: 38: 39: 40: 41: 42: 43: 44: 45: 46: 47: 48: 49: 50: 51: 52: 53: 54: 55: 55: 57: print the information of rect by calling the method on line 32 45

121. void printBox(){ System.out.print(Box: + x1 + , + y1); System.out.println(, + x2 + , + y2 + ); } public static void main(String[] arguments) { Box rect = new Box(); System.out.println(Calling buildBox with coordinates + (25,25) and (50,50):); rect.buildBox(25, 25, 50, 50); rect.printBox(); System.out.println(nCalling buildBox with points + (10,10) and (20,20):); rect.buildBox(new Point(10, 10), new Point(20, 20)); rect.printBox(); System.out.println(nCalling buildBox with 1 point + (10,10), width 50 and height 50:); rect.buildBox(new Point(10, 10), 50, 50); rect.printBox(); } } 31: 32: 33: 34: 35: 36: 37: 38: 39: 40: 41: 42: 43: 44: 45: 46: 47: 48: 49: 50: 51: 52: 53: 54: 55: 55: 57: Now, a box is built by a method on line 17 46

122. void printBox(){ System.out.print(Box: + x1 + , + y1); System.out.println(, + x2 + , + y2 + ); } public static void main(String[] arguments) { Box rect = new Box(); System.out.println(Calling buildBox with coordinates + (25,25) and (50,50):); rect.buildBox(25, 25, 50, 50); rect.printBox(); System.out.println(nCalling buildBox with points + (10,10) and (20,20):); rect.buildBox(new Point(10, 10), new Point(20, 20)); rect.printBox(); System.out.println(nCalling buildBox with 1 point + (10,10), width 50 and height 50:); rect.buildBox(new Point(10, 10), 50, 50); rect.printBox(); } } 31: 32: 33: 34: 35: 36: 37: 38: 39: 40: 41: 42: 43: 44: 45: 46: 47: 48: 49: 50: 51: 52: 53: 54: 55: 55: 57: Now, aa bbooxx iiss bbuuiilltt by a method on line 25 47

123. A final primitive variable cannot be re-assigned a new value. A final object cannot be re-assigned a new address. A final class cannot be sub-classed (or extended). A final method cannot be overridden. 48

124. public static Double add(Double a, Double b) { return a + b; } . . . int a = 1; int b = 2; double c = 3; double d = 4; System.out.println(add(a, b)); // invalid System.out.println(add(c, d)); // OK 49

125. FFFFoooorrrr rrrreeeeuuuussssaaaabbbblllleeee ccccooooddddeeee :::: If you need to do the same thing, or almost the same thing, many times, write a method to do it, then call the method each time you have to do that task. បB:ន

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169. 4. Class Math is part of java.lang package, which is implicitly imported by the compiler, so it is not necessary to import class Math to use its methods. 5. Methods that modify the values of private variables should verify that that intended new values are proper. If they are not, the set methods should place the private variables into an appropriate consistent state. 6. The online Java API documentation is easy to search and provides many details about each class. As you learn one class, you should get in the habit of looking a the class in the online documentation for additional information. 4. Class MMMMaaaatttthhhh Zផកមយ¥ន jjjjaaaavvvvaaaa....llllaaaannnngggg package U8 LM;ន import

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185. A method that has many parameters my be performed too many tasks. Consider dividing the method into smaller methods that perform separate tasks. As a guideline, try to fit the method header on one line if possible. public void doThisAndDoThat(int a, int b, String str, boolean isOK, char c, float f, double d, int[] someArray, boolean success) Declaring a method outside the body of a class declaration or inside the body of another method is a syntax error. public class Monkey { public void cry() { public void eat() { . . . } error! Method can not be in a method } } public void jump() { . . . } error! Method cannot be outside class Omitting return-value type in a method declaration is a syntax error. public void methodA(int a) { . . . } error! no return type 54

186. Placing a semicolon after the right parenthesis enclosing the parameter list of a method declaration is a syntax error. public void methodA(int a) ; error here { . . . } Redeclaring a method parameter as a local variable in the method’s body is a compilation error. public void methodA(int a) { int a; // error here } 55

187. Forgetting to return a value from a method that should return a value is a compilation error. If a return-value type other than void is specified, the method must contain a return statement that returns a value consistent with the method’s return-value type. Returning a value from a method whose return type has been declared void is a compilation error. public int addition(int a, int b) { int total = a + b; return total; error! if you forget this statement } public int addition(int a, int b) { float total = a + b; return total; error! } public void doSomething(int a, int b) { int total = a + b; return total; error! } 56

188. Add one construction to the following class: class Car { private String color; private int width; private int height; private String brandName; private String countryName; // add one constructor } // In NetBeans: Alt + Insert 57

189. Write the Author class. Also write a test program called TestAuthor to test the constructor and public methods. 58 Author -name: String -email: String -gender: char create+Author(name: String, email: String, gender: char) +getGender(): char +setGender(gender: char) +getName(): String +setName(name: String) +getEmail(): String +setEmail(email: String) +toString(): String Try changing the email of an author, e.g Author anAuthor = new Author(Tan Ah Teck, [email protected], 'm'); System.out.println(anAuthor); // call toString() anAuthor.setEmail([email protected]) System.out.println(anAuthor);

190. Book Author -name: String -author: Author -price: double -qtyInStock: int = 0 create+Book(name: String, author: Author, price: double) create+Book(name: String, author: Author, price: double, qtyInStock: int) +getQtyInStock(): int +setQtyInStock(qtyInStock: int) +getName(): String +setName(name: String) +getAuthor(): Author +setAuthor(author: Author) +getPrice(): double +setPrice(price: double) +toString(): String Write the class Book (which uses the Author class written earlier). Also write a test program called 59 TestBook to test the constructor and public methods in the class Book. Take Note that you have to construct an instance of Author before you can construct an instance of Book. E.g., Author anAuthor = new Author(......); Book aBook = new Book(Java for dummy, anAuthor, 19.95, 1000); // Use an anonymous instance of Author Book anotherBook = new Book(more Java for dummy, new Author(......), 29.95, 888);

191. You are required to: 1. Write the code for the class MyPoint. Also write a test program (called TestMyPoint) to test all the methods defined in the class. // Overloading method distance() // this version takes two ints as arguments public double distance(int x, int y) { int xDiff = this.x – x; int yDiff = ...... return Math.sqrt(xDiff*xDiff + yDiff*yDiff); } // this version takes a // MyPoint instance as argument public double distance(MyPoint another) { int xDiff = this.x – another.x; ....... } //Test program MyPoint p1 = new MyPoint(3, 0); MyPoint p2 = new MyPoint(0, 4); ...... //Testing the overloaded method distance() System.out.println(p1.distance(p2)); // which version? System.out.println(p1.distance(5, 6)); // which version? ..... 60 MyPoint -x: int = 0 -y: int = 0 create+MyPoint() create+MyPoint(x: int, y: int) +getY(): int +setY(y: int) +getX(): int +setX(x: int) +toString(): String +distance(x: int, y: int): double +distance(another: MyPoint): double

192. 61 2. Write a program that allocates 10 points in an array of MyPoint, and initializes to (1, 1), (2, 2), ... (10, 10). Hints: You need to allocate the array, as well as each of the ten MyPoint instances. // Declare and allocate an array of MyPoint MyPoint[] points = new MyPoint[10]; for (......) { points[i] = new MyPoint(...); // Allocate each of MyPoint instances System.out.println(points[i]); }

193. Write the MyCircle class. Also write a test program (called TestMyCircle) to test all the methods defined in the class. 62

194. Write the MyTriangle class. Also write a test program (called TestMyTriangle) to test all the methods defined in the class. 63

195. 64

196. 65

197. ច7eប

198. ងv class មយ

199. ‰r` Invoice U8កªមg«Jន8ក Hardware

200. បBpប ie8ក ទន:ញ¬ a) Invoice គeនព នចនន ® U7ចZ

201. 8ខក7Uទន:ញ (part number: String) ie បe€:យ (part description: String) ចនន¯កl8ក (quantity: int) ន:ង ¥8¯កl (unit price: double) 9 b) Class

202. ន`គeន constructor មយBpប initialize instance variable ˆង ® ង

203. 8 c) ប

204. ងv set ន:ង get method Bpប instance variables ន:មយ] d) ប

205. ងv method មយ

206. ‰r` getInvoiceAmount Bpបគ Šf ¥8BeបJកJង Invoice eច

207. gយ return ¥8Z double e) ប

208. ងv method មយ

209. ‰r` printInvoice

210. Uម“bប‘™ញព ន8:

211. @

212. 8 Invoice 9 បB:ន

213. ប quantity មន:មនZចននMជ:±ន LMកŠ

214. [យkន ¥8 0 9 បB:ន

215. ប ¥8pយ មន:មនZចននMជ:±ន LMកŠ

216. [យkន ¥8 0.0 9 f) ប

217. ងv កមrMធ:bមយ

218. ‰r` InvoiceTest Bpប— ក8“ង

219. ប Invoice 9 66

220. Invoice -partNumber: String -partDescription: String -quantity: int -unitPrice: double create+Invoice(partNumber: String, partDescription: String, quantity: int, unitPrice: double) +setPartNumber(partNumber: String) +setPartDescription(partDescription: String) +setQuantity(quantity: int) +setUnitPrice(unitPrice: double) +getPartNumber(): String +getPartDescription(): String +getQuantity(): int +getUnitPrice(): double +getInvoiceAmount(): double +printInvoice() 67

221. Employee -firstName: String -lastName: String -monthlySalary: float create+Employee(firstName: String, lastName: String, monthlySalary: float) +setFirstName(firstName: String): void +setLastName(lastName: String): void +setMonthlySalary(monthlySalary: float): void +getFirstName(): String +getLastName(): String +getMonthlySalary(): float +getYearlySalary(): float ច7eប

222. ង v class មយ

223. ‰r` Employee U8នព នចនន ³ Z instance variable U7ចZ first name, last name ន:ង monthly salary 9 ប

224. ង v constructor មយBpប initialize instance variable ˆង ³ ប

225. ង v set ន:ង get method Bpប instance variable ន:មយ] 9 បB:ន

226. ប monthly salary មន:មនZចននMជ:±ន LM

227. [យk

228. B rn 9 ប

229. ង vកមMrធ:bមយ

230. ‰r` EmployeeTest Bpប

231. ប Employee

232. 3យប

233. ង v Employee object ចនន 2 eច

234. gយប‘™ញ yearly salary eបB object ន: មយ] 9 ចJងប56ប

235. ង;ក ខចនន 10% ន:ងប‘™ញ yearly salary —eZថbមrងK

236. ទ^ 9 68

Java 2 chapter 10 - basic oop in java

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