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Chapter 5 Ch 1 – Introduction to Computers and Java Defining Classes and Methods 1.

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Presentation on theme: "Chapter 5 Ch 1 – Introduction to Computers and Java Defining Classes and Methods 1."— Presentation transcript:

1 Chapter 5 Ch 1 – Introduction to Computers and Java Defining Classes and Methods 1

2 Chapter Class and Method Definitions 5.2 Information Hiding and Encapsulation 5.3 Objects and References 5.4 Graphics Supplement 2

3 5.1 Class and Method Definitions 3 Class Name Instance Variables Methods

4 Java is Object Oriented 4 It can model any real world object

5 A class is a blueprint of what an object will look like 5

6 The object is just an instance of the class 6

7 Object Oriented Programming deals with the creation of objects 7 and their relationships and interactions

8 Start by defining the class 8 Car - bodyPaintColor: Color - numberOfTires: int + getBodyPaintColor(): Color + setBodyPaintColor(Color color): void + getNumberOfTires(): int + setNumberOfTires(int tireCount): void Use a UML class diagram instance variables instance methods

9 Code the class definition 9 public class Car { private Color bodyPaintColor; private int numberOfTires; public Color getPaintColor() { return bodyPaintColor; } // end getPaintColor() public void setPaintColor(Color color) { bodyPaintColor = color; } // end setPaintColor() public int getNumberOfTires() { return numberOfTires; } // end getNumberOfTires() public void setNumberOfTires(int tireCount) { numberOfTires = tireCount; } // end setNumberOfTires() } // end Car instance variables instance methods

10 An Object consists of data bodyPaintColorColor.Green numberOfTires4 greenCar object's memory footprint

11 and operations that store and manage the data 11 bodyPaintColorColor.Green numberOfTires4 greenCar bodyPaintColorColor.Red numberOfTires4 redCar Class Car methods methods are shared by all car objects

12 Each class should be in a separate file 12 public class Car { // code omitted } // end Car Car.java public class Driver { // code omitted } // end Driver Driver.java

13 new Creates an instance of a class 13 Car myCar = new Car();

14 Recap  An object is an instance of class  Use new to create an object  Objects have data (instance variables)  Objects offer functionality (methods) 14

15 There are two types of methods 15 Methods that do not return a value (void) System.out.println("println does not return"); and methods that do return a value int num = keyboard.nextInt();

16 Let's see how methods work 16 Car myCar = new Car(); First create the object bodyPaintColornull numberOfTires0 myCar Default values

17 You can then call a method to set an instance variable 17 myCar.setNumberOfTires(4); bodyPaintColornull numberOfTires4 myCar Receiving Object

18 or a get method to retrieve an instance variable 18 int tireCount = myCar.getNumberOfTires() bodyPaintColornull numberOfTires4 myCar 4

19 this Demystified 19 public int getNumberOfTires() { return this.numberOfTires; } // end getNumberOfTires() public void setNumberOfTires(int tireCount) { this.numberOfTires = tireCount; } // end setNumberOfTires() Since each method is shared by all the objects, we need to be able to identify the receiving object. this refers to the receiving object, implicitly.

20 void Method Definition 20 public void setNumberOfTires(int tireCount) { numberOfTires = tireCount; } // end setNumberOfTires() Method is accessible by defining class and any other class Parameter list can be empty or list parameters needed Instance Variable

21 return Method Definition 21 public int getNumberOfTires() { return numberOfTires; } // end getNumberOfTires() return type of intParameter list can be empty or list parameters needed Instance Variable

22 Recap  Methods expose a class's functionality  Call a method on a receiving object  this identifies the receiving object inside the method's definition  Each class is stored in its own.java file 22

23 Local variables are defined within a method 23 public double updateSumAmount(double amount) { double newSumAmount += amount; return newSumAmount; } // end updateSumAmount() local variable

24 Methods can define same name local variables 24 public void method1() { double someDouble = 0; // Code omitted } // end method1() public void method2() { double someDouble = 0; // Code omitted } // end method2() local to method1 local to method2

25 5.2 Information Hiding and Encapsulation 25

26 A method should hide how it is implemented 26 I know what the method does, just not how!

27 Know "what" a method does, not "how" it does it 27

28 Methods can be public 28 These define the class's interface

29 or private 29 These are part of the implementation

30 Instance Variables are private 30 They define the implementation

31 Accessor methods control access to instance variables 31 Getters retrieve instance variables Setters set instance variables

32 Recap  Local variables are defined within a method  Know "what" a method does, not "how" it does it  Public methods define the class's interface  Private instance variables/methods are part of the implementation 32

33 Class Deconstructed 33 Fraction - numerator: int - denominator: int - reduce(): void + getNumerator(): int + setNumerator(int n): void + getDenominator(): int + setDenominator(int d): void + setNumeratorAndDenominator(int n, int d): void + add(Fraction f): Fraction + subtract(Fraction f): Fraction + multiply(Fraction f): Fraction + divide(Fraction f): Fraction + show(): void

34 Application Deconstructed package fractiondemo; public class Fraction { private int numerator; private int denominator; 34 private void reduce() { int u = numerator; int v = denominator; int temp; while (v != 0) { temp = u % v; u = v; v = temp; }// end while numerator /= u; denominator /= u; }// end reduce()

35 Application Deconstructed public int getNumerator() { return numerator; }// end getNumerator() 35 public void setNumerator(int n) { setNumeratorAndDenominator(n, denominator); }// end setNumerator() public int getDenominator() { return denominator; }// end getDenominator() public void setDenominator(int d) { setNumeratorAndDenominator(numerator, d); }// end setDenominator()

36 Application Deconstructed public void setNumeratorAndDenominator(int n, int d) { numerator = n; if (d == 0) { System.err.println("ERROR: Invalid parameter (" + d + ") in setNumeratorAndDenonimator"); System.exit(1); } else { denominator = d; }// end if }// end setNumeratorAndDenominator() 36 public Fraction add(Fraction f) { Fraction sum = new Fraction(); sum.setNumeratorAndDenominator(numerator * f.denominator + denominator * f.numerator, denominator * f.denominator); sum.reduce(); return sum; }// end add()

37 Application Deconstructed public Fraction subtract(Fraction f) { Fraction difference = new Fraction(); difference.setNumeratorAndDenominator( numerator * f.denominator - denominator * f.numerator, denominator * f.denominator); difference.reduce(); return difference; }// end subtract() 37 public Fraction multiply(Fraction f) { Fraction product = new Fraction(); product.setNumeratorAndDenominator( numerator * f.numerator, denominator * f.denominator); product.reduce(); return product; }// end multiply()

38 Application Deconstructed public Fraction divide(Fraction f) { Fraction division = new Fraction(); division.setNumeratorAndDenominator( numerator * f.denominator, denominator * f.numerator); division.reduce(); return division; }// end divide() 38 public void show() { System.out.print("(" + numerator + " / " + denominator + ")"); }// end show() }// end Fraction()

39 Application Deconstructed package fractiondemo; public class FractionDemo { public static void main(String[] args) { Fraction f1 = new Fraction(); Fraction f2 = new Fraction(); Fraction result = new Fraction(); // Set f1 to 1 / 4. f1.setNumeratorAndDenominator(1, 4); // Set f2 to 1 / 2. f2.setNumeratorAndDenominator(1, 2); 39

40 Application Deconstructed 40 // Output their sum, difference, product and division. result = f1.add(f2); f1.show(); System.out.print(" + "); f2.show(); System.out.print(" = "); result.show(); System.out.println(); result = f1.subtract(f2); f1.show(); System.out.print(" - "); f2.show(); System.out.print(" = "); result.show(); System.out.println();

41 Application Deconstructed 41 result = f1.multiply(f2); f1.show(); System.out.print(" * "); f2.show(); System.out.print(" = "); result.show(); System.out.println(); result = f1.divide(f2); f1.show(); System.out.print(" / "); f2.show(); System.out.print(" = "); result.show(); System.out.println(); }// end main() }// end FractionDemo

42 Application Deconstructed 42

43 5.3 Objects and References 43

44 There are two types of variables 44 Value: Stores the actual value 1 Reference: Stores a reference to the actual value 2

45 Value types store values 45 int x = 100; 100 x

46 Reference types store references 46 Fraction f = new Fraction(); f numerator ? denominator ?

47 Lets compare value types 47 int x = 100;x 100 int y = 200;y 200 x == y ? false x = y; x 200 y x == y ?true

48 Now lets compare reference types 48 f1 == f2 ? false f1 == f2 ?true Fraction f1 = new Fraction(); f1.setNumeratorAndDenominator(1,2); f1 200 numerator 1 denominator Fraction f2 = new Fraction(); f2.setNumeratorAndDenominator(1,2); f2 208 numerator 1 denominator f1 = f2; f1 208 f2 208 numerator 1 denominator 2 208

49 The solution to the == problem? 49 Define an equals method

50 Code Deconstructed public boolean equals(Fraction f) { return this.numerator == f.numerator && this.denominator == f.denominator; }// end equals() 50 Two fractions are equal if both their numerator and denominator values are the same.

51 Code Deconstructed Fraction f1 = new Fraction(); f1.setNumeratorAndDenominator(1, 2); Fraction f2 = new Fraction(); f2.setNumeratorAndDenominator(1, 2); 51 if (f1 == f2) System.out.println("Both variables refer to the same object"); else System.out.println("Each variable refers to a different object"); if ( f1.equals(f2) ) System.out.println("Both objects have the same value"); else System.out.println("Each object has a different value");

52 Code Deconstructed 52 Fraction f1 = new Fraction(); f1.setNumeratorAndDenominator(1,2); f1 200 numerator 1 denominator Fraction f2 = new Fraction(); f2.setNumeratorAndDenominator(1,2); f2 208 numerator 1 denominator if (f1.equals(f2) {...}... public boolean equals(Fraction f) {...} f2 208 numerator 1 denominator f Both the argument (f2) and the parameter (f) point to the same object. Notice how the parameter (f) refers to the same object as the argument (f2) and thus object can be changed from within the method.

53 Code Deconstructed 53 Fraction f1 = new Fraction(); f1.setNumeratorAndDenominator(1,2); f1 200 numerator 1 denominator resetFraction(f1);... public void resetFraction(Fraction f) f1 200 numerator 1 denominator f Notice here how the object variable (f) has been assigned a new object, and that f1 stills refers to its original object. { f = new Fraction(); f.setNumeratorAndDenominator(1, 1); } f1 200 numerator 1 denominator f 208 numerator 1 denominator 1 208

54 5.4 Graphic Supplement 54

55 Graphics class Revisited 55 The Graphics object defines the client area of the applet window.

56 Applet Deconstructed package smileyfacemethods; import javax.swing.JApplet; import java.awt.Color; import java.awt.Graphics; public class SmileyFaceMethods extends JApplet { // All the geometric constants remain the same //... 56

57 Applet public void paint(Graphics canvas) { // Draw a yellow filled face. drawFace(canvas, X_FACE, Y_FACE, FACE_DIAMETER, FACE_DIAMETER, Color.YELLOW); // Draw outline in black. drawOutline(canvas, X_FACE, Y_FACE, FACE_DIAMETER, FACE_DIAMETER, Color.BLACK); 57

58 Applet Deconstructed // Draw a blue left eye. drawEye(canvas, X_LEFT_EYE, Y_LEFT_EYE, EYE_WIDTH, EYE_HEIGHT, Color.BLUE); // Draw a blue right eye. drawEye(canvas, X_RIGHT_EYE, Y_RIGHT_EYE, EYE_WIDTH, EYE_HEIGHT, Color.BLUE); 58

59 Applet Deconstructed // Draw the black nose. drawNose(canvas, X_NOSE, Y_NOSE, NOSE_DIAMETER, NOSE_DIAMETER, Color.BLACK); // Draw the red mouth. drawMouth(canvas, X_MOUTH, Y_MOUTH, MOUTH_WIDTH, MOUTH_HEIGHT, MOUTH_START_ANGLE, MOUTH_EXTENT_ANGLE, Color.RED); }// end paint() 59

60 Applet Deconstructed private void drawFace(Graphics g, int x, int y, int width, int height, Color color) { g.setColor(color); g.fillOval(x, y, width, height); }// end drawFace() 60 private void drawOutline(Graphics g, int x, int y, int width, int height, Color color) { g.setColor(color); g.drawOval(x, y, width, height); }// end drawFace()

61 Applet Deconstructed private void drawEye(Graphics g, int x, int y, int width, int height, Color color) { g.setColor(color); g.fillOval(x, y, width, height); }// end drawEye() 61 private void drawNose(Graphics g, int x, int y, int width, int height, Color color) { g.setColor(color); g.fillOval(x, y, width, height); }// end drawNose()

62 Applet Deconstructed private void drawMouth(Graphics g, int x, int y, int width, int height, int start, int swipe, Color color) { g.setColor(color); g.drawArc(x, y, width, height, start, swipe); }// end drawNose() }// end SmileyFaceMethods 62

63 Applet Deconstructed 63

64 init() Vs. paint() 64

65 Both have a place in Applets  An applet could have both an init() and paint() or neither (unusual)  Both get called automatically  Paint() repaints controls as needed  Init() initialization before applet starts 65

66 Code Deconstructed 66 import javax.swing.JLabel;... JLabel firstNameLabel = new JLabel("First name: "); JLabel lastNameLabel = new JLabel("Last name: "); A label is a control for displaying static text in an applet or frame. Create and add the label in the init() method

67 Code Deconstructed 67 import java.awt.Container;... Container contentPane = getContentPane(); A Container is a control that can contain other controls The applet's ContentPane is one such container

68 Code Deconstructed 68 import java.awt.FlowLayout;... contentPane.setLayout( new FlowLayout() ); contentPane.add(firstNameLabel); contentPane.add(lastNameLabel); A FlowLayout control helps in laying out the controls in a container Each container can be associated with a FlowLayout.

69 Application Deconstructed package labelsdemo; import javax.swing.JLabel; import javax.swing.JFrame; import java.awt.Container; import java.awt.FlowLayout; public class LabelsDemo { public static void main(String[] args) { JFrame frame = new JFrame(); JLabel salutationLabel = new JLabel("Hello there,"); JLabel nameLabel = new JLabel("Mr. Magoo!"); Container contentPane = frame.getContentPane(); 69

70 Application Deconstructed contentPane.setLayout( new FlowLayout() ); contentPane.add(salutationLabel); contentPane.add(nameLabel); frame.setSize(200, 100); frame.setVisible(true); } 70

71 Application Deconstructed 71

72 Application Deconstructed package labelsappletdemo; import javax.swing.JLabel; import javax.swing.JApplet; import java.awt.Container; import java.awt.FlowLayout; import java.awt.Graphics; public class LabelsAppletDemo extends JApplet { private void drawString(Graphics g, String string, int x, int y) { g.drawString(string, x, y); }// end drawString() 72

73 Application public void init() { JLabel salutationLabel = new JLabel("Hello there,"); JLabel nameLabel = new JLabel("Mr Magoo!"); Container contentPane = getContentPane(); contentPane.setLayout( new FlowLayout() ); contentPane.add(salutationLabel); contentPane.add(nameLabel); }// end init() 73 The init() method executes once before applet starts. It displays the two labels, but they do not remain visible for long.

74 Application public void paint(Graphics g) { drawString(g, "Hello to you too!", 50, 100); }// end paint() }// end LabelsAppletDemo 74 The paint() method is then executed and repaints the applet's content pane and draws the string. Notice how the labels never have a chance!


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