1. 1 Classes CS 101-E Chapter 4 Aaron Bloomfield

2. 2 Announcements  HWs are being renumbered J1, J2, etc., for Java programming assignments C1, C2, etc., for CodeLab assignments HW1 = J1, HW2 = C1, HW3 = C2, etc.  HWs J2 and J3 assigned this Wednesday (6 Oct) J2 due next Thursday (14 Oct) J3 due following Thursday (21 Oct)  HW J4 will be assigned 18 Oct, and due 29 Oct  Some CodeLab HWs in there as well  Second midterm on 27 Oct  No labs this Sunday Can go to another lab with permission  Lab quiz grades will be entered by the end of this week

3. 3 Preparation  Scene so far has been background material and experience Computing systems and problem solving Variables Types Input and output Expressions Assignments Objects Standard classes and methods  Now: Experience what Java is really about Design and implement objects representing information and physical world objects

4. 4 Object-oriented programming  Basis Create and manipulate objects with attributes and behaviors that the programmer can specify  Mechanism Classes  Benefits An information type is design and implemented once  Reused as needed No need reanalysis and re-justification of the representation

5. 5 First class – ColoredRectangle  Purpose Represent a colored rectangle in a window Introduce the basics of object design and implementation

6. 6 Background  JFrame Principal Java class for representing a titled, bordered graphical window. Standard class  Part of the swing library import javax.swing.* ;

7. 7 Some Java Swing components

8. 8 Example  Consider JFrame w1 = new JFrame("Bigger"); JFrame w2 = new JFrame("Smaller"); w1.setSize(200, 125); w2.setSize(150, 100); w1.setVisible(true); w2.setVisible(true);  Consider JFrame w1 = new JFrame("Bigger"); JFrame w2 = new JFrame("Smaller"); w1.setSize(200, 125); w2.setSize(150, 100); w1.setVisible(true); w2.setVisible(true);  Consider JFrame w1 = new JFrame("Bigger"); JFrame w2 = new JFrame("Smaller"); w1.setSize(200, 125); w2.setSize(150, 100); w1.setVisible(true); w2.setVisible(true);

9. // Purpose: Displays two different windows. import javax.swing.*; public class TwoWindows { // main(): application entry point public static void main (String[] args) { JFrame w1 = new JFrame("Bigger"); JFrame w2 = new JFrame("Smaller"); w1.setSize(200, 125); w2.setSize(150, 100); w1.setVisible(true); w2.setVisible(true); }

10. 10 Class ColoredRectangle – initial version  Purpose Support the display of square window containing a blue filled-in rectangle  Window has side length of 200 pixels  Rectangle is 40 pixels wide and 20 pixels high  Upper left hand corner of rectangle is at (80, 90) Limitations are temporary  Remember BMI.java preceded BMICalculator.java  Lots of concepts to introduce

11. 11 ColoredRectangle in action  Consider ColoredRectangle r1 = new ColoredRectangle(); ColoredRectangle r2 = new ColoredRectangle(); System.out.println("Enter when ready"); Scanner stdin = new Scanner (System.in); stdin.nextLine(); r1.paint(); // draw the window associated with r1 r2.paint(); // draw the window associated with r2  Consider ColoredRectangle r1 = new ColoredRectangle(); ColoredRectangle r2 = new ColoredRectangle(); System.out.println("Enter when ready"); Scanner stdin = new Scanner (System.in); stdin.nextLine(); r1.paint(); // draw the window associated with r1 r2.paint(); // draw the window associated with r2  Consider ColoredRectangle r1 = new ColoredRectangle(); ColoredRectangle r2 = new ColoredRectangle(); System.out.println("Enter when ready"); Scanner stdin = new Scanner (System.in); stdin.nextLine(); r1.paint(); // draw the window associated with r1 r2.paint(); // draw the window associated with r2  Consider ColoredRectangle r1 = new ColoredRectangle(); ColoredRectangle r2 = new ColoredRectangle(); System.out.println("Enter when ready"); Scanner stdin = new Scanner (System.in); stdin.nextLine(); r1.paint(); // draw the window associated with r1 r2.paint(); // draw the window associated with r2

12. // Purpose: Create two windows containing colored rectangles. import java.util.*; public class BoxFun { //main(): application entry point public static void main (String[] args) { ColoredRectangle r1 = new ColoredRectangle(); ColoredRectangle r2 = new ColoredRectangle(); System.out.println("Enter when ready"); Scanner stdin = new Scanner (System.in); stdin.nextLine(); r1.paint(); // draw the window associated with r1 r2.paint(); // draw the window associated with r2 }

13. 13 ColoredRectangle.java outline import javax.swing.*; import java.awt.*; public class ColoredRectangle { // instance variables for holding object attributes private int width; private int height; private int x; private int y; private JFrame window; private Color color; // ColoredRectangle(): default constructor public ColoredRectangle() { //... } // paint(): display the rectangle in its window public void paint() { //... }

14. 14 Instance variables and attributes  Data field Java term for an object attribute  Instance variable Symbolic name for a data field Usually has private access  Assists in information hiding by encapsulating the object’s attributes Default initialization  Numeric instance variables initialized to 0  Logical instance variables initialized to false  Object instance variables initialized to null

15. public class ColoredRectangle { // instance variables for holding object attributes private int width; private int x; private int height; private int y; private JFrame window; private Color color; // ColoredRectangle(): default constructor public ColoredRectangle() { window = new JFrame("Box Fun"); window.setSize(200, 200); width = 40; x = 80; height = 20;y = 90; color = Color.BLUE; window.setVisible(true); } // paint(): display the rectangle in its window public void paint() { Graphics g = window.getGraphics(); g.setColor(color); g.fillRect(x, y, width, height); }

16. 16 ColoredRectangle default constructor

17. public class ColoredRectangle { // instance variables for holding object attributes private int width; private int x; private int height; private int y; private JFrame window; private Color color; // ColoredRectangle(): default constructor public ColoredRectangle() { window = new JFrame("Box Fun"); window.setSize(200, 200); width = 40; x = 80; height = 20;y = 90; color = Color.BLUE; window.setVisible(true); } // paint(): display the rectangle in its window public void paint() { Graphics g = window.getGraphics(); g.setColor(color); g.fillRect(x, y, width, height); }

18. 18 Color constants  Color.BLACK  Color.BLUE  Color.CYAN  Color.DARK_GRAY  Color.GRAY  Color.GREEN  Color.LIGHT_GRAY  Color.MAGENTA  Color.ORANGE  Color.PINK  Color.RED  Color.WHITE  Color.YELLOW

19. r The value of a ColoredRectangle variable is a reference to a ColoredRectangle object + paint() : void ColorRectangle - width = 40 - height = 20 - x = 80 - y = 90 - window = - color = Color - color = -... + brighter() : Color +... String - text = "Box Fun" -... + length() : int +... + setVisible( boolean status) : void +... JFrame - width = 200 - height = 200 - title = -... ColoredRectangle r = new ColoredRectangle();

20. 20 Another possible Constructor public class ColoredRectangle { // instance variables for holding object attributes private int width = 40; private int x = 80; private int height = 80; private int y = 90; private JFrame window; private Color color = Color.BLUE; // ColoredRectangle(): default constructor public ColoredRectangle() { window = new JFrame("Box Fun"); window.setSize(200, 200); window.setVisible(true); }

21. public class ColoredRectangle { // instance variables for holding object attributes private int width; private int x; private int height; private int y; private JFrame window; private Color color; // ColoredRectangle(): default constructor public ColoredRectangle() { window = new JFrame("Box Fun"); window.setSize(200, 200); width = 40; x = 80; height = 20;y = 90; color = Color.BLUE; window.setVisible(true); } // paint(): display the rectangle in its window public void paint() { Graphics g = window.getGraphics(); g.setColor(color); g.fillRect(x, y, width, height); }

22. 22 Graphical context  Graphics Defined in java.awt.Graphics Represents the information for a rendering request  Color  Component  Font …… Provides methods Text drawing  Line drawing  Shape drawing Rectangles Ovals Polygons

23. 23 Java coordinate system

24. 24 Method invocation  Consider r1.paint(); // display window associated with r1 r2.paint(); // display window associated with r2  Observe When an instance method is being executed, the attributes of the object associated with the invocation are accessed and manipulated Important that you understand what object is being manipulated

25. 25 Method invocation public class ColoredRectangle{ // instance variables to describe object attributes... // paint(): display the rectangle in its window publicvoid paint() { window.setVisible( true); Graphics g = window.getGraphics(); g.setColor(color); g.fillRect(x, y, width, height); }... } Instance variable window references the JFrame attribute of the object that caused the invocation. The values of these instance variables are also from the ColoredRectangle object Typo in book: p. 149 claims paint() is static; it’s not

26. 26  Medicine  Physics  Public Health  Chemistry  Engineering  Literature  Psychology  Economics  Peace  Biology The Ig Nobel Prizes "The Effect of Country Music on Suicide.“ For explaining the dynamics of hula-hooping Investigating the scientific validity of the Five-Second Rule The Coca-Cola Company of Great Britain For the patent of the combover The American Nudist Research Library It’s easy to overlook things – even a man in a gorilla suit. The Vatican, for outsourcing prayers to India The invention of karaoke, thereby providing an entirely new way for people to learn to tolerate each other For showing that herrings apparently communicate by farting

27. 27  Computing grades so far  HW J2 assigned today, due next Wednesday  Lab this week! Must be done by Sunday at 8:30  Sunday labs are cancelled due to fall break If you want, send me an e-mail and you can show up to another lab session  No office hours Friday  Grades so far: use formula  HISTORICALLY, the grade range has been: A: 90+, B: 80-89, C: 60-79, D/F: <60 Wednesday, 6 October, 2004

28. 28 Improving ColoredRectangle  Analysis A ColoredRectangle object should  Be able to have any color  Be positionable anywhere within its window  Have no restrictions on its width and height  Accessible attributes  Updateable attributes

29. 29 Improving ColoredRectangle  Additional constructions and behaviors Specific construction  Construct a rectangle representation using supplied values for its attributes Accessors  Supply the values of the attributes  Individual methods for providing the width, height, x- coordinate position, y-coordinate position, color, or window of the associated rectangle Mutators  Manage requests for changing attributes  Ensure objects always have sensible values  Individual methods for setting the width, height, x- coordinate position, y-coordinate position, color, or window of the associated rectangle to a given value

30. 30 A mutator method  Definition // setWidth(): width mutator public void setWidth(int w) { width = w; }  Usage ColoredRectangle s = new ColoredRectangle(); s.setWidth(80); public void setWidth(intw) {... } Initial value of the formal parameter comes from the actual parameter Changes to the formal parameter do not affect the actual parameter Object to be manipulated is the one referenced by s

31. 31 Mutator setWidth() evaluation ColoredRectangle s = new ColoredRectangle(); s.setWidth(80); publicclass ColoredRectangle {... // setWidth(): width mutator public void setWidth( int w) { width = w; }... } The invocation sends a message to the ColoredRectangle referenced by s to modify its width attribute. To do so, there is a temporary transfer of flow of control to setWidth(). The value of the actual parameter is 80 Method setWidth() sets the instance variable width of its ColoredRectangle. For this invocation, width is set to 80 and the ColoredRectangle is the one referenced by s For this invocation of method setWidth(), w is initialized to 80. The object being referenced within the method body is the object referenced by s Method setWidth() is completed. Control is transferred back to the statement that invoked setWidth()

32. 32 A bit of humor…

33. 33 Java parameter passing  The value is copied to the method  Any changes to the parameter are forgotten when the method returns

34. 34 Java parameter passing  Consider the following code: static void foobar (int y) { y = 7; } public static void main (String[] args) { int x = 5; foobar (x); System.out.println(x); }  What gets printed? formal parameter actual parameter

35. 35 Java parameter passing  Consider the following code: static void foobar (String y) { y = “7”; } public static void main (String[] args) { String x = “5”; foobar (x); System.out.println(x); }  What gets printed? formal parameter actual parameter

36. 36 Java parameter passing  Consider the following code: static void foobar (ColoredRectangle y) { y.setWidth (10); } public static void main (String[] args) { ColoredRectangle x = new ColoredRectangle(); foobar (x); System.out.println(y.getWidth()); }  What gets printed? formal parameter actual parameter

37. 37 Java parameter passing  Consider the following code: static void foobar (ColoredRectangle y) { y = new ColoredRectangle(); y.setWidth (10); } public static void main (String[] args) { ColoredRectangle x = new ColoredRectangle(); foobar (x); System.out.println(y.getWidth()); }  What gets printed? formal parameter actual parameter

38. 38 Java parameter passing  The value of the actual parameter gets copied to the formal parameter This is called pass-by-value C/C++ is also pass-by-value Other languages have other parameter passing types  Any changes to the formal parameter are forgotten when the method returns  However, if the parameter is a reference to an object, that object can be modified Similar to how the object a final reference points to can be modified

39. 39 Subtleties  Consider ColoredRectangle r = new ColoredRectangle(); r.paint(); r.setWidth(80); r.paint();  What is the width is the rectangle on the screen after the mutator executes?

40. 40 Other mutators public void setHeight(int h) { height = h; } public void setX(int ulx) { x = ulx; } public void setY(int uly) { y = uly; } public void setWindow(JFrame f) { window = f; } public void setColor(Color c) { color = c; }

41. 41

42. 42 Mutator usage ColoredRectangle u = new ColoredRectangle(); ColoredRectangle v = new ColoredRectangle(); u.setHeight(100); u.setColor(Color.PINK); v.setX(25); v.setY(50); JFrame display = new JFrame("Fun"); v.setWindow(display); Sends a message to v's Colored- Rectangle to modify its window attribute to display's JFrame Sends a message to u's ColoredRectangle to modify its height attribute to 100 Sends a message to u's Colored- Rectangle to modify its color attribute to pink Sends a message to v's Colored- Rectangle to modify its y-axis position to 50 Sends a message to v's Colored- Rectangle to modify its x-axis position to 25

43. 43 Accessors  Properties Do not require parameters Each accessor execution produces a return value  Return value is the value of the invocation public int getWidth() { return width; } The method return type precedes the name of the method in the method definition For method getWidth(), the return value is the value of the width attribute for the ColoredRectangle associated with the invocation. In invocation t.getWidth(), the return value is the value of the instance variable width for the ColoredRectangle referenced by t

44. 44 Accessor usage ColoredRectangle t = new ColoredRectangle(); intw = t.getWidth(); publicclass ColoredRectangle {... // getWidth(): accessor public int getWidth() { return width; }... } Invocation sends a message to the ColoredRectangle referenced by t to return the value of its width. To do so, there is a temporary transfer of flow of control to getWidth() The return expression evaluates to 40 (the width attribute of the ColoredRectangle object referenced by t ) Method getWidth() starts executing. For this invocation, the object being referenced is the object referenced by t Method completes by supplying its return value (40) to the invoking statement. Also, invoking statement regains the flow of control. From there variable w is initialized with the return value of the invocatio

45. 45 Specific construction public ColoredRectangle(int w, int h, int ulx, int uly, JFrame f, Color c) { setWidth(w); setHeight(h); setX(ulx); setY(uly); setWindow(f); setColor(c); }  Requires values for each of the attributes JFrame display = new JFrame("Even more fun"); display.setSize(400, 400); ColoredRectangle w = new ColoredRectangle(60, 80, 20, 20, display, Color.YELLOW);

46. 46 Specific construction public ColoredRectangle(int w, int h, int ulx, int uly, JFrame f, Color c) { setWidth(w); setHeight(h); setX(ulx); setY(uly); setWindow(f); setColor(c); }  Advantages to using mutators Readability Less error prone Facilitates enhancements through localization

47. 47 Seeing double import java.awt.*; public class SeeingDouble { public static void main(String[] args) { ColoredRectangle r = new ColoredRectangle(); System.out.println("Enter when ready"); Scanner stdin = new Scanner (System.in); stdin.nextLine(); r.paint(); r.setY(50); r.setColor(Color.RED); r.paint(); }

48. 48 Seeing double

49. 49 An optical illusion

50. 50 Casting  We’ve seen casting before: double d = (double) 3; int x = (int) d;  Aside: duplicating an object String s = “foo”; String t = s.clone();  Causes an error: “inconvertible types”  (Causes another error, but we will ignore that one) What caused this?

51. 51 Casting, take 2 .clone() returns an object of class Object (sic) More confusion: You can also have an object of class Class  Thus, you can have an Object class and a Class object  Got it? We know it’s a String (as it cloned a String) Thus, we need to tell Java it’s a String via casting  Revised code: String s = “foo”; String t = (String) s.clone();  Still causes that “other” error, but we are still willfully ignoring it…

52. 52 Casting, take 3  That “other” error is because String does not have a.clone() method Not all classes do! We just haven’t seen any classes that do have.clone() yet  Check in the documentation if the object you want to copy has a.clone() method  A class that does: java.util.Vector Vector s = new Vector(); Vector t = s.clone(); Vector u = (Vector) s.clone(); Causes the “inconvertible types” error

53. 53 Casting, take 4  What happens with the following code? Vector v = new Vector(); String s = (String) v;  Java will encounter a compile-time error “inconvertible types”  What happens with the following code? Vector v = new Vector(); String s = (String) v.clone();  Java will encounter a RUN-time error ClassCastException