1. 1 Classes Chapter 4 Spring 2005 CS 101 Aaron Bloomfield

2. 2 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 Decisions (if, switch) Loops (while, for, do-while)  Now: Experience what Java is really about Design and implement objects representing information and physical world objects

3. 3 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

4. 4 Known Classes  Classes we’ve seen BigInteger String StringBuffer Scanner System  Classes we’ll be seeing soon BigDecimal

5. 5 The Car class

6. 6 More on classes vs. objects

7. 7 A new example: creating a Car class  What properties does a car have in the real world? Color Position (x,y) Fuel in tank  We will implement these properties in our Car class public class Car { private Color color; private int xpos; private int ypos; private int fuel; //... }

8. 8 Car’s instance variables public class Car { private Color color; private int xpos; private int ypos; private int fuel; //... } + … Car - color - fuel - xpos - ypos

9. 9 Instance variables and attributes  Default initialization If the variable is within a method, Java does NOT initialize it If the variable is within a class, Java initializes it as follows:  Numeric instance variables initialized to 0  Logical instance variables initialized to false  Object instance variables initialized to null + … Car - color = null - fuel = 0 - xpos = 0 - ypos = 0

10. 10 Car behaviors or methods  What can a car do? And what can you do to a car? Move it  Change it’s x and y positions Change it’s color Fill it up with fuel  For our computer simulation, what else do we want the Car class to do? Create a new Car Plot itself on the screen  Each of these behaviors will be written as a method

11. 11 Creating a new car  To create a new Car, we call: Car car = new Car();  Notice this looks like a method You are calling a special method called a constructor A constructor is used to create (or construct) an object  It sets the instance variables to initial values  The constructor: public Car() { fuel = 1000; color = Color.BLUE; }

12. 12 Constructors public Car() { fuel = 1000; color = Color.BLUE; } No return type! EXACT same name as class For now, all constructors are public

13. 13 Our Car class so far public class Car { private Color color; private int xpos; private int ypos; private int fuel; public Car() { fuel = 1000; color = Color.BLUE; } } public class Car { private Color color = Color.BLUE; private int xpos; private int ypos; private int fuel = 1000; public Car() { }

14. 14 Our Car class so far public class Car { private Color color = Color.BLUE; private int xpos = 0; private int ypos = 0; private int fuel = 1000; public Car() { } }  Called the default constructor The default constructor has no parameters If you don’t include one, Java will SOMETIMES put one there automatically + Car() + … Car - color = Color.BLUE - fuel = 1000 - xpos = 0 - ypos = 0

15. 15 Another constructor  Another constructor: public Car (Color c, int x, int y, int f) { color = c; xpos = x; ypos = y; fuel = f; }  This constructor takes in four parameters  The instance variables in the object are set to those parameters  This is called a specific constructor An constructor you provide that takes in parameters is called a specific constructor

16. 16 Our Car class so far public class Car { private Color color = Color.BLUE; private int xpos = 0; private int ypos = 0; private int fuel = 1000; public Car() { } public Car (Color c, int x, int y, int f) { color = c; xpos = x; ypos = y; fuel = f; } } + Car() + Car (Color, int, int, int) + … Car - color = Color.BLUE - fuel = 1000 - xpos = 0 - ypos = 0

17. 17 DeCSS: The program #include typedef unsigned int uint; char ctb[512]="33733b2663236b763e7e362b6e2e667bd393db0643034b96de9ed60b4e0e4\ 69b57175f82c787cf125a1a528fca8ac21fd999d10049094190d898d001480840913d7d35246\ d2d65743c7c34256c2c6475dd9dd5044d0d4594dc9cd4054c0c449559195180c989c11058185\ 081c888c011d797df0247074f92da9ad20f4a0a429f53135b86c383cb165e1e568bce8ec61bb\ 3f3bba6e3a3ebf6befeb6abeeaee6fb37773f2267276f723a7a322f6a2a627fb9f9b1a0e9a9e\ 1f0b8f8b0a1e8a8e0f15d1d5584cd8dc5145c1c5485cc8cc415bdfdb5a4edade5f4bcfcb4a5e\ cace4f539793120692961703878302168286071b7f7bfa2e7a7eff2bafab2afeaaae2ff"; typedef unsigned char uchar;uint tb0[11]={5,0,1,2,3,4,0,1,2,3,4};uchar* F=NULL; uint lf0,lf1,out;void ReadKey(uchar* key){int i;char hst[3]; hst[2]=0;if(F==\ NULL){F=malloc(256);for(i=0;i >2)^(lf0>>16))&1;b=((lf1\ >>12)^(lf1>>20)^(lf1>>21)^(lf1>>24))&1;lf0=(lf0 >1)\ |(a >1)|(b >8)+x+y;} void \ CSSdescramble(uchar *sec,uchar *key){uint i;uchar *end=sec+0x800;uchar KEY[5]; for(i=0;i =0;\ i-- )key[tb0[i+1]]=k[tb0[i+1]]^F[key[tb0[i+1]]]^key[tb0[i]];}void CSStitlekey2\ (uchar *key,uchar *im){uchar k[5];int i;ReadKey(im);for(i=0;i =0;i--)key[tb0[i+1]]=k[tb0[i+1]]^F[key[tb0[i+1]]]^key\ [tb0[i]];}void CSSdecrypttitlekey(uchar *tkey,uchar *dkey){int i;uchar im1[6]; uchar im2[6]={0x51,0x67,0x67,0xc5,0xe0,0x00};for(i=0;i typedef unsigned int uint; char ctb[512]="33733b2663236b763e7e362b6e2e667bd393db0643034b96de9ed60b4e0e4\ 69b57175f82c787cf125a1a528fca8ac21fd999d10049094190d898d001480840913d7d35246\ d2d65743c7c34256c2c6475dd9dd5044d0d4594dc9cd4054c0c449559195180c989c11058185\ 081c888c011d797df0247074f92da9ad20f4a0a429f53135b86c383cb165e1e568bce8ec61bb\ 3f3bba6e3a3ebf6befeb6abeeaee6fb37773f2267276f723a7a322f6a2a627fb9f9b1a0e9a9e\ 1f0b8f8b0a1e8a8e0f15d1d5584cd8dc5145c1c5485cc8cc415bdfdb5a4edade5f4bcfcb4a5e\ cace4f539793120692961703878302168286071b7f7bfa2e7a7eff2bafab2afeaaae2ff"; typedef unsigned char uchar;uint tb0[11]={5,0,1,2,3,4,0,1,2,3,4};uchar* F=NULL; uint lf0,lf1,out;void ReadKey(uchar* key){int i;char hst[3]; hst[2]=0;if(F==\ NULL){F=malloc(256);for(i=0;i >2)^(lf0>>16))&1;b=((lf1\ >>12)^(lf1>>20)^(lf1>>21)^(lf1>>24))&1;lf0=(lf0 >1)\ |(a >1)|(b >8)+x+y;} void \ CSSdescramble(uchar *sec,uchar *key){uint i;uchar *end=sec+0x800;uchar KEY[5]; for(i=0;i =0;\ i-- )key[tb0[i+1]]=k[tb0[i+1]]^F[key[tb0[i+1]]]^key[tb0[i]];}void CSStitlekey2\ (uchar *key,uchar *im){uchar k[5];int i;ReadKey(im);for(i=0;i =0;i--)key[tb0[i+1]]=k[tb0[i+1]]^F[key[tb0[i+1]]]^key\ [tb0[i]];}void CSSdecrypttitlekey(uchar *tkey,uchar *dkey){int i;uchar im1[6]; uchar im2[6]={0x51,0x67,0x67,0xc5,0xe0,0x00};for(i=0;i<6;i++)im1[i]=dkey[i]; CSStitlekey1(im1,im2);CSStitlekey2(tkey,im1);}

18. 18 DeCSS: The shirt (and tie!)

19. 19 DeCSS: The poem How to decrypt a DVD: in haiku form. (Thanks, Prof. D. S. T.) ------------------------ (I abandon my exclusive rights to make or perform copies of this work, U. S. Code Title Seventeen, section One Hundred and Six.) Muse! When we learned to count, little did we know all the things we could do some day by shuffling those numbers: Pythagoras said "All is number" long before he saw computers and their effects, or what they could do Table Zero is: Five, zero, one, two, three, four, oh, one, two, three, four. Table One is long: two to the eighth power bytes. Ready? Here they are: Fifty one; then one hundred fifteen; fifty nine; thirty eight; ninety nine; thirty five; one hundred seven; one hundred eighteen; sixty two; one hundred twenty six; fifty four; forty three; one hundred ten; then

20. 20 DeCSS: The number The world’s first illegal prime number: The world’s first illegal prime number: 4856507896573978293098418946942861377074420873513579240196520736686985134010472 3744696879743992611751097377770102744752804905883138403754970998790965395522701 1712157025974666993240226834596619606034851742497735846851885567457025712547499 9648219418465571008411908625971694797079915200486670997592359606132072597379799 3618860631691447358830024533697278181391479795551339994939488289984691783610018 2597890103160196183503434489568705384520853804584241565482488933380474758711283 3959896852232544608408971119771276941207958624405471613210050064598201769617718 0947811362200272344827224932325954723468800292777649790614812984042834572014634 8968547169082354737835661972186224969431622716663939055430241564732924855248991 2257394665486271404821171381243882177176029841255244647445055834628144883356319 0272531959043928387376407391689125792405501562088978716337599910788708490815909 7548019285768451988596305323823490558092032999603234471140776019847163531161713 0785760848622363702835701049612595681846785965333100770179916146744725492728334 8691600064758591746278121269007351830924153010630289329566584366200080047677896 7984382090797619859493646309380586336721469695975027968771205724996666980561453 3820741203159337703099491527469183565937621022200681267982734457609380203044791 2277498091795593838712100058876668925844870047077255249706044465212713040432118 2610103591186476662963858495087448497373476861420880529443

21. 21 DeCSS: The images

22. 22 DeCSS: The recordings All this info from http://www-2.cs.cmu.edu/~dst/DeCSS/Gallery/ All this info from http://www-2.cs.cmu.edu/~dst/DeCSS/Gallery/ Or do a Google search for “decss gallery” Or do a Google search for “decss gallery”

23. 23 DeCSS: The movie

24. 24 Using our Car class  Now we can use both our constructors: Car c1 = new Car(); Car c2 = new Car (Color.BLACK, 1, 2, 500); + Car() + Car (Color, int, int, int) + … Car - color = Color.BLUE - fuel = 1000 - xpos = 0 - ypos = 0 + Car() + Car (Color, int, int, int) + … Car - color = Color.BLACK - fuel = 500 - xpos = 1 - ypos = 2 c1c2

25. 25 So what does private mean?  Consider the following code public class CarSimulation { public static void main (String[] args) { Car c = new Car(); System.out.println (c.fuel); } }  Recall that fuel is a private instance variable in the Car class  Private means that code outside the class CANNOT access the variable For either reading or writing  Java will not compile the above code If fuel were public, the above code would work Note that it’s a different class!

26. 26 End of lecture on 12 October 2005

27. 27 So how do we get the fuel of a Car?  Via accessor methods in the Car class: public int getFuel() { return fuel; } public Color getColor() { return color; }  As these methods are within the Car class, they can read the private instance variables  As the methods are public, anybody can call them public int getYPos() { return ypos; } public int getXPos() { return xpos; }

28. 28 So how do we set the fuel of a Car?  Via mutator methods in the Car class: public void setFuel (int f) { fuel = f; } public void setColor (Color c) { color = c; }  As these methods are within the Car class, they can read the private instance variables  As the methods are public, anybody can call them public void setXPos (int x) { xpos = x; } public void setYPos (int y) { ypos = y; }

29. 29 Why use all this?  These methods are called a get/set pair Used with private variables  We’ll see why one uses these later in this slide set  Our Car so far: + Car() + Car (Color, int, int, int) + void setXPos (int x) + void setYPos (int y) + void setPos (int x, int y) + void setColor (Color c) + void setFuel (int f) + int getFuel() + int getXPos() + int getYPos() + Color getColor() + … Car - color = Color.BLUE - fuel = 1000 - xpos = 0 - ypos = 0

30. 30 A solution to commenting your code The commentator: http://www.cenqua.com/commentator/ The commentator: http://www.cenqua.com/commentator/ http://www.cenqua.com/commentator/

31. 31 Back to our specific constructor public class Car { private Color color = Color.BLUE; private int xpos = 0; private int ypos = 0; private int fuel = 1000; public Car (Color c, int x, int y, int f) { color = c; xpos = x; ypos = y; fuel = f; } } public class Car { private Color color = Color.BLUE; private int xpos = 0; private int ypos = 0; private int fuel = 1000; public Car (Color c, int x, int y, int f) { setColor (c); setXPos (x); setYPos (y); setFuel (f); }

32. 32 Back to our specific constructor  Using the mutator methods (i.e. the ‘set’ methods) is the preferred way to modify instance variables in a constructor We’ll see why later

33. 33 So what’s left to add to our Car class?  What else we should add: A mutator that sets both the x and y positions at the same time A means to “use” the Car’s fuel A method to paint itself on the screen  Let’s do the first: public void setPos (int x, int y) { setXPos (x); setYPos (y); }  Notice that it calls the mutator methods

34. 34 Using the Car’s fuel  Whenever the Car moves, it should burn some of the fuel For each pixel it moves, it uses one unit of fuel  We could make this more realistic, but this is simpler public void setXPos (int x) { xpos = x; } public void setYPos (int y) { ypos = y; } public void setXPos (int x) { fuel -= Math.abs (getXPos()-x); xpos = x; } public void setYPos (int y) { fuel -= Math.abs (getYPos()-y); ypos = y; }

35. 35 Using the Car’s fuel  Notice that to access the instance variables, the accessor methods are used  Math.abs() gets the absolute value of the passed parameter public void setPos (int x, int y) { setXPos(x); setYPos(y); } public void setPos (int x, int y) { setXPos(x); setYPos(y); }

36. 36 Drawing the Car  The simple way to have the Car draw itself: public void paint (Graphics g) { g.setColor (color); g.fillRect (xpos-50, ypos-100, 100, 200); }  This draws a single rectangle that is 100 by 200 pixels in size  Lets use constants for the car’s height and width...

37. 37 Drawing the Car  A better version: private final int CAR_WIDTH = 100; private final int CAR_HEIGHT = 200; public void paint (Graphics g) { g.setColor (color); g.fillRect (getXPos()-CAR_WIDTH/2, getYPos()-CAR_HEIGHT/2, CAR_WIDTH, CAR_HEIGHT); }  This makes it easier to change the car size We could have made the car size instance variables and set them via mutators  Lets add tires!

38. 38 Drawing the Car private final int CAR_WIDTH = 100; private final int CAR_HEIGHT = 200; private final int TIRE_WIDTH = 20; private final int TIRE_HEIGHT = 40; private final int TIRE_OFFSET = 20; public void paint (Graphics g) { g.setColor (color); g.fillRect (getXPos()-CAR_WIDTH/2, getYPos()-CAR_HEIGHT/2, CAR_WIDTH, CAR_HEIGHT); // Draw the tires g.setColor (Color.BLACK); g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH), getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH), getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()+(CAR_WIDTH/2), getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()+(CAR_WIDTH/2), getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT); } Don’t worry about this – just know that it draws four tires

39. 39 What happens when the car runs out of fuel?  We could do a number of things: Not allow the car to move anymore Print out a message saying, “fill me up!” We’ll color the car red  We’ll insert the following code at the beginning of the paint() method: if ( fuel < 0 ) { color = Color.RED; }

40. 40 Drawing the Car private final int CAR_WIDTH = 100; private final int CAR_HEIGHT = 200; private final int TIRE_WIDTH = 20; private final int TIRE_HEIGHT = 40; private final int TIRE_OFFSET = 20; public void paint (Graphics g) { if ( fuel < 0 ) { color = Color.RED; } g.setColor (color); g.fillRect (getXPos()-CAR_WIDTH/2, getYPos()-CAR_HEIGHT/2, CAR_WIDTH, CAR_HEIGHT); // Draw the tires g.setColor (Color.BLACK); g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH), getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH), getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()+(CAR_WIDTH/2), getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()+(CAR_WIDTH/2), getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT); }

41. 41 Our car in action CarGUI.java CarGUI.java

42. 42 End of lecture on 24 Oct 2005  Started from the beginning of the Car class

43. 43 Miscellaneous Stuff

44. 44 What I’m not expecting you to know yet…  What the static keyword means And why the main() method is And why other methods are not  Why you should always call the mutator methods, instead of setting the field directly Just know that it’s a good programming practice, and follow it We’ll see why later  Why instance variables are supposed to be private Just know that it’s a good programming practice, and follow it Again, we’ll see why soon

45. 45 Terminology  An attribute of a class can be called: Instance or class variable (we’ll see the difference later) Static variable (or static field)  Synonymous with class variable Field  Generally means either type Variable  Also means either type Attribute Property  Argh! I will generally use the terms variable or field when I am not differentiating between the two And instance variable and class variable when I am

46. 46 The main() method  Consider a class with many methods: public class WhereToStart { public static void foo (int x) { //... } public static void bar () { //... } public static void main (String[] args) { //... } }  Where does Java start executing the program? Always at the beginning of the main() method!

47. 47 Running a class without a main() method  Consider the Car class It had no main() method! The main() method was in the CarSimulation (or CarGUI) class  So let’s try running it…

48. 48 Program Demo Car.java Car.java

49. 49 Variable scoping  A variable is visible within the block it is declared in Called the “scope” of the variable public class Scoping { int z public static void foo (int x) { //... } public static void bar () { //... } public static void main (String[] args) { int y; //... } } This local variable is visible until the end of the main() method This instance variable is visible anywhere in the Scoping class This parameter is visible only in the foo() method

50. 50 Variable initialization  A local variable is NOT initialized to a default value This is any variable declared within a method  Or within a block within a method This is pretty stupid, in my opinion  Parameters are initialized to whatever value they are passed  Instance and class variables are initialized to default values Numbers to zero, booleans to false, references to null This means any field in a class  Either class variables or instance variables

51. 51 Motivational posters…

52. 52 Rational class

53. 53 What we’ve seen so far  An example of creating a class Car  Up next: another example Rational  Represents rational numbers  A rational number is any number that can be expressed as a fraction  Both the numerator and denominator must be integers!  Discussed in section 4.8 of the textbook

54. 54 What properties should our Rational class have?  The numerator (top part of the fraction)  The denominator (bottom part of the fraction)  Not much else…

55. 55 What do we want our Rational class to do?  Obviously, the ability to create new Rational objects  Setting the numerator and denominator  Getting the values of the numerator and denominator  Perform basic operations with rational numbers: + - * /  Ability to print to the screen

56. 56 Our first take at our Rational class  Our first take public class Rational { private int numerator; private int denominator; //... }  This does not represent a valid Rational number! Why not?  Java initializes instance variables to zero Both the numerator and denominator are thus set to zero 0/0 is not a valid number! + … Rational - numerator = 0- denominator = 0

57. 57 Our next take at our Rational class  Our next take public class Rational { private int numerator = 0; private int denominator = 1; //... }  We’ve defined the attributes of our class  Next up: the behaviors + … Rational - numerator = 0- denominator = 1

58. 58 The default constructor  Ready? public Rational() { }  Yawn!  Note that we could have initialized the instance variables here instead  The default constructor is called that because, if you don’t specify ANY constructors, then Java includes one by default  Default constructors do not take parameters + Rational() + … Rational - numerator = 0- denominator = 1

59. 59 The specific constructor  Called the specific constructor because it is one that the user specifies They take one or more parameters public Rational (int num, int denom) { setNumerator (num); setDenominator (denom); }  Note that the specific constructor calls the mutator methods instead of setting the instance variables directly We’ll see why later + Rational() + Rational (int num, int denom) + … Rational - numerator = 0- denominator = 1

60. 60 Accessor methods  Our two accessor methods: public int getNumerator () { return numerator; } public int getDenominator () { return denominator; } + Rational() + Rational (int num, int denom) + int getNumerator() + int getDemonimator() + … Rational - numerator = 0- denominator = 1

61. 61 Mutator methods  Our two mutator methods: public void setNumerator (int towhat) { numerator = towhat; } public void setDenominator (int towhat) { denominator = towhat; }

62. 62 Rational addition  How to do Rational addition:  Our add() method: public Rational add (Rational other) { }

63. 63 The this keyword + Rational () + Rational (int n, int d) + Rational add (Rational other) + … Rational - numerator = 1- denominator = 2 this + Rational () + Rational (int n, int d) + Rational add (other ) + … Rational - numerator = 1- denominator = 2 + Rational () + Rational (int n, int d) + Rational add (Rational other) + … Rational - numerator = 1- denominator = 3 + Rational () + Rational (int n, int d) + Rational add (Rational other) + … Rational - numerator = 5- denominator = 6 Returns:

64. 64 The this keyword  this is a reference to whatever object we are currently in  Will not work in static methods We’ll see why later Note that the main() method is a static method  While we’re at it, when defining a class, note that NONE of the methods so far were static

65. 65 Rational addition  How to do Rational addition:  Our add() method: public Rational add (Rational other) { int a = this.getNumerator(); int b = this.getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational (a*d+b*c, b*d); }

66. 66 Rational addition  The following method is equivalent:  Our add() method: public Rational add (Rational other) { int a = getNumerator(); int b = getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational (a*d+b*c, b*d); }

67. 67 Rational addition  The following method is equivalent, but not preferred:  Our add() method: public Rational add (Rational other) { int a = numerator; int b = denominator; int c = other.numerator; int d = other.nenominator; return new Rational (a*d+b*c, b*d); }

68. 68 Rational addition  The following method is equivalent, but not preferred:  Our add() method: public Rational add (Rational other) { int a = this.numerator; int b = this.denominator; int c = other.numerator; int d = other.nenominator; return new Rational (a*d+b*c, b*d); }

69. 69 Today’s demotivators

70. 70 Rational subtraction  How to do Rational subtraction:  Our subtract() method: public Rational subtract (Rational other) { int a = this.getNumerator(); int b = this.getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational (a*d-b*c, b*d); }

71. 71 Rational multiplication  How to do Rational multiplication:  Our multiply() method: public Rational multiply (Rational other) { int a = this.getNumerator(); int b = this.getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational (a*c, b*d); }

72. 72 Rational division  How to do Rational division:  Our divide() method: public Rational divide (Rational other) { int a = this.getNumerator(); int b = this.getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational (a*d, b*c); }

73. 73 Printing it to the screen  If we try printing a Rational object to the screen: Rational r = new Rational (1,2); System.out.println (r);  We get the following: Rational@82ba41  Ideally, we’d like something more informative printed to the screen  The question is: how does Java know how to print a custom class to the screen?

74. 74 The toString() method  When an object is put into a print statement: Rational r = new Rational (1,2); System.out.println (r);  Java will try to call the toString() method to covert the object to a String If the toString() method is not found, a default one is included  Hence the Rational@82ba41 from the previous slide  So let’s include our own toString() method

75. 75 The toString() method  Our toString() method is defined as follows: public String toString () { return getNumerator() + "/" + getDenominator(); }  Note that the prototype must ALWAYS be defined as shown The prototype is the ‘public String toString()’

76. 76 Printing it to the screen  Now, when we try printing a Rational object to the screen: Rational r = new Rational (1,2); System.out.println (r);  We get the following: 1/2  Which is what we wanted!  Note that the following two lines are (mostly) equivalent: System.out.println (r); System.out.println (r.toString());

77. 77 Our full Rational class + Rational() + Rational (int num, int denom) + int getNumerator() + int getDemonimator() + void setNumerator (int num) + void setDenominator (int denom) + Rational add (Rational other) + Rational subtract (Rational other) + Rational multiply (Rational other) + Rational divide (Rational other) + String toString() Rational - numerator = 0- denominator = 1

78. 78 Our Rational class in use, part 1 of 4  This code is in a main() method of a RationalDemo class  First, we extract the values for our first Rational object: Scanner stdin = new Scanner(System.in); System.out.println(); // extract values for rationals r and s Rational r = new Rational(); System.out.print("Enter numerator of a rational number: "); int a = stdin.nextInt(); System.out.print("Enter denominator of a rational number: "); int b = stdin.nextInt(); r.setNumerator(a); r.setDenominator(b);

79. 79 Our Rational class in use, part 2 of 4  Next, we extract the values for our second Rational object: Rational s = new Rational(); System.out.print("Enter numerator of a rational number: "); int c = stdin.nextInt(); System.out.print("Enter denominator of a rational number: “); int d = stdin.nextInt(); s.setNumerator(c); s.setDenominator(d);  Notice that I didn’t create another Scanner object! Doing so would be bad I used the same one

80. 80 Our Rational class in use, part 3 of 4  Next, we do the arithmetic: // operate on r and s Rational sum = r.add(s); Rational difference = r.subtract(s); Rational product = r.multiply(s); Rational quotient = r.divide(s);

81. 81 Our Rational class in use, part 4 of 4  Lastly, we print the results // display operation results System.out.println("For r = " + r.toString() + " and s = " + s.toString()); System.out.println(" r + s = " + sum.toString()); System.out.println(" r - s = " + difference.toString()); System.out.println(" r * s = " + product.toString()); System.out.println(" r / s = " + quotient.toString()); System.out.println();

82. 82 A demo of our Rational class RationalDemo.java RationalDemo.java

83. 83 Other things we might want to add to our Rational class  The ability to reduce the fraction So that 2/4 becomes 1/2 Not as easy as it sounds!  More complicated arithmetic Such as exponents, etc.  Invert Switches the numerator and denominator  Negate Changes the rational number into its (additive) negation  We won’t see any of that here

84. 84 These images are not animated…

85. 85 End of lecture on 26 Oct 2005  Also spent time:  Discussing issues with HW J5  Introducing HW J6  Introducing Lab 8

86. 86 More on methods

87. 87 Calling a method  Consider two Strings: String s = “foo”; String t = “bar”;  Calling s.substring(2) is different than calling t.substring(2)  Why?  Because of the object it is being called out of The method works the same in both cases (returns the substring) But it returns different results  Whenever we are calling a method, we also need to know which object we are calling it out of

88. 88 Return values  Many methods return a value Math.cos() String.valueOf()  Consider: double d = Math.cos (90 * Math.PI/180.0);  Let’s consider the Math.cos() method public double cos (double a) { double c = 0.0; // compute cos somehow into a variable c return c; }  The value c in the cos() method is copied into d

89. 89 The return keyword  The return keyword does a few things: Immediately terminate the current method Pass the value back to whoever called the method  You can have a return anywhere you want Inside loops, ifs, etc.  You can have as may returns as you want as well: public String foo (int x) { if ( x == 1 ) return “one”; else if ( x == 2 ) return “two”; else return “other”; }

90. 90 More on returns  Consider this class: public class Foo { // Default constructor omitted on this slide public String bar (String s) { String t = “CS 101” + “ ” + s; return t; } }  And the code to invoke it: Foo w = new Foo(); String x = “rules”; String y = foo.bar (x); System.out.println (y);  What happens in memory?

91. 91 Foo w = new Foo(); String x = “rules”; String y = w.bar (x); System.out.println (y); Foo w = new Foo(); String x = “rules”; String y = w.bar (x); System.out.println (y); public String bar (String s) { String t = “CS 101” + “ ” + s; return t; } public String bar (String s) { String t = “CS 101” + “ ” + s; return t; Foo “rules"“CS 101 rules"wx t y this + Foo() + bar (String s): String + … s

92. 92 Returning an object from a method  We could rewrite our bar() method a number of ways: public String bar (String s) { String t = “CS 101” + “ ” + s; return t; } public String bar (String s) { return new String (“CS 101” + “ ” + s); } public String bar (String s) { return “CS 101” + “ ” + s; }

93. 93 Returning a non-object from a method  In other words, returning a primitive type from a method public foo () { //... return x + y; }  This method evaluates x+y, then returns that value to the caller

94. 94 A bit of humor…

95. 95 The Circle class Introducing static-ness, visibilities, etc.

96. 96 A Circle class  We are going to develop a Circle class Perhaps for use in a graphics program  Why? Partly to review creating classes Go over some topics that were a bit fuzzy  Constructors and creating objects Show why one uses the get/set methods instead of directly modifying the instance variables Discuss visibilities (public, private, etc.) Discuss the static keyword

97. 97 Circle class properties  What properties does a circle have? Radius PI = 3.141592653589793234 Color (if plotting in a graphics program) (x,y) location  These properties will become instance variables We are only going to play with the first two (radius and PI) in this example Thus, we are ignoring the color and location

98. 98 Our Circle class Circle c = new Circle(); public class Circle { double radius; double PI = 3.1415926536; } Note the fields are not static Note the radius field is not initialized by us We’re ignoring the public for now c Circle - radius = 0.0 - PI = 3.14159… - … + …

99. 99 Accessing our Circle object  Any variable or method in an object can be accessed by using a period The period means ‘follow the reference’ Example: System.in Example: System.out.println (c.radius); Example: c.PI = 4; This is bad – PI should have been declared final (this will be done later) c Circle - radius = 0.0 - PI = 3.14159… - … + …

100. 100 What’s the output? public class Circle { double radius; double PI = 3.1415926536; } public class CircleTest { public static void main (String[] args) { int x; Circle c = new Circle(); System.out.println (x); } }  When a variable is declared as part of a method, Java does not initialize it to a default value Java will give a “variable not initialized” error

101. 101 What’s the output now? public class Circle { double radius; double PI = 3.1415926536; } public class CircleTest { public static void main (String[] args) { int x; Circle c = new Circle(); System.out.println (c.radius); } }  When a variable is declared as part of a class, Java does initialize it to a default value Java outputs 0.0!

102. 102 What’s going on?  A (method) variable needs to be initialized before it is used Usually called a local variable  A instance variable is automatically initialized by Java All numbers are initialized to 0, booleans to false, etc.  This is a bit counter-intuitive…

103. 103 Circle class behaviors  What do we want to do with (and to) our Circle class? Create circles Modify circles (mutators) Find out about our circles’ properties (accessors) Find the area of the circle Plot it on the screen (or printer) A few others…  These will be implemented as methods

104. 104 Another optical illusion

105. 105 Calling the Circle constructor  To create a Circle object: Circle c1 = new Circle();  This does four things: Creates the c1 reference Creates the Circle object Makes the c1 reference point to the Circle object Calls the constructor with no parameters (the ‘default’ constructor)  The constructor is always the first method called when creating (or ‘constructing’) an object c1 Circle - radius = 0.0 - PI = 3.14159… - … + Circle() + Circle (double r) + …

106. 106 Calling the Circle constructor  To create a Circle object: Circle c1 = new Circle(2.0);  This does four things: Creates the c1 reference Creates the Circle object Makes the c1 reference point to the Circle object Calls the constructor with 1 double parameters (the ‘specific’ constructor)  The constructor is always the first method called when creating (or ‘constructing’) an object c1 Circle - radius = 0.0 - PI = 3.14159… - … + Circle() + Circle (double r) + …

107. 107 Constructors  Remember, the purpose of the constructor is to initialize the instance variables PI is already set, so only radius needs setting public Circle() { radius = 1.0; } public Circle (double r) { radius = r; } Note there is no return type for constructors Note that the constructor name is the EXACT same as the class name Note that there are two “methods” with the same name!

108. 108 What happens in memory  Consider: Circle c = new Circle();  A double takes up 8 bytes in memory  Thus, a Circle object takes up 16 bytes of memory As it contains two doubles c Circle - radius = 1.0 - PI = 3.1415926536 - … + Circle() + Circle (double r) + … Circle - radius = 1.0 - PI = 3.14159 c Shorthand representation

109. 109 Consider the following code public class CircleTest { public static void main (String[] args) { Circle c1 = new Circle(); Circle c2 = new Circle(); Circle c3 = new Circle(); Circle c4 = new Circle(); } }

110. 110 What happens in memory  There are 4 Circle objects in memory Taking up a total of 4*16 = 64 bytes of memory Circle - radius = 1.0 - PI = 3.14159 c1 Circle - radius = 1.0 - PI = 3.14159 c2 Circle - radius = 1.0 - PI = 3.14159 c3 Circle - radius = 1.0 - PI = 3.14159 c4

111. 111 Consider the following code public class CircleTest { public static void main (String[] args) { Circle c1 = new Circle(); //... Circle c1000000 = new Circle(); } } This program creates 1 million Circle objects!

112. 112 What happens in memory  There are 1 million Circle objects in memory Taking up a total of 1,000,000*16 ≈ 16 Mb of memory … Note that the final PI field is repeated 1 million times Circle - radius = 1.0 - PI = 3.14159 c1 Circle - radius = 1.0 - PI = 3.14159 c2 Circle - radius = 1.0 - PI = 3.14159 c1000000

113. 113 Total memory usage: 8 Mb + 8 bytes (1,000,000+1=1,000,001 doubles) The use of static for fields  If a variable is static, then there is only ONE of that variable for ALL the objects That variable is shared by all the objects Total memory usage: 16 bytes (1+1=2 doubles) Total memory usage: 40 bytes (4+1=5 doubles) … Circle - radius = 1.0 c1 Circle - radius = 1.0 c2 c1000000 Circle - radius = 1.0 c3 c4 Circle - radius = 1.0 PI 3.1415926536

114. 114 More on static fields  What does the following print Note that PI is not final Circle c1 = new Circle(); Circle c2 = new Circle(); Circle c3 = new Circle(); Circle c4 = new Circle(); c1.PI = 4.3; System.out.println (c2.PI);  It prints 4.3 Note you can refer to static fields by object.variable

115. 115 Even more on static fields  There is only one copy of a static field no matter how many objects are declared in memory Even if there are zero objects declared! The one field is “common” to all the objects  Static variables are called class variables As there is one such variable for all the objects of the class Whereas non-static variables are called instance variables  Thus, you can refer to a static field by using the class name: Circle.PI

116. 116 Even even more on static fields  This program also prints 4.3: Circle c1 = new Circle(); Circle c2 = new Circle(); Circle c3 = new Circle(); Circle c4 = new Circle(); Circle.PI = 4.3; System.out.println (c2.PI);

117. 117 Even even even more on static fields  We’ve seen static fields used with their class names: System.in(type: InputStream) System.out(type: OutputStream) Math.PI(type: double) Integer.MAX_VALUE(type: int)

118. 118 Back to our Circle class public class Circle { double radius; final static double PI = 3.1415926536; public Circle() { radius = 1.0; } public Circle (double r) { radius = r; } }  But it doesn’t do much! Note that PI is now final and static

119. 119 End of lecture on 31 Oct 2005

120. 120 Adding a method public class Circle { double radius; final static double PI = 3.1415926536; // Constructors... double computeArea () { return PI*radius*radius; } Note that a (non-static) method can use both instance and class variables

121. 121 Using that method public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); c.radius = 2.0; double area = c.computeArea(); System.out.println (area); } } Prints 12.566370614356

122. 122 Circle - radius = 0.0 - PI = 3.14159… - … + Circle() + Circle (double r) + computeArea() + … What happens when that method is called public class Circle { double radius; final static double PI = 3.1415926536; public Circle() { radius = 1.0; } // other constructor double computeArea () { return PI*radius*radius; } } public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); c.radius = 2.0; double area = c.computeArea(); System.out.println (area); } } c Circle - radius = 1.0 - PI = 3.14159… - … + Circle() + Circle (double r) + computeArea() + … area 12.566 Circle - radius = 2.0 - PI = 3.14159… - … + Circle() + Circle (double r) + computeArea() + …

123. 123 Review of our Circle class public class Circle { double radius; final static double PI = 3.1415926536; public Circle() { } public Circle (double r) { radius = r; } double computeArea () { return PI*radius*radius; } Slight change from before

124. 124 A note about methods/variable order  Within a method, a variable must be declared before it is used  In a class, methods and variables can be declared in any order This is different than C++

125. 125 Adding another method double oneOverRadius() { return 1.0/radius; }  I couldn’t think of a good reason to divide something by the radius…

126. 126 What happens now?  Code in class CircleTest’s main() method Circle c = new Circle(); // c.radius is now 0.0 System.out.println (c.oneOverRadius());  Java won’t crash, but many other programming languages (C and C++, in particular) will So we’ll call this a ‘crash’ for the sake of this lecture  Java prints “Infinity” Not what we wanted, though!

127. 127 One way to fix this… public class Circle { double radius = 1.0; final static double PI = 3.1415926536; // Constructors... double computeArea () { return PI*radius*radius; } double oneOverRadius() { return 1.0/radius; } Note that the radius variable is now initialized to 1.0

128. 128 Back to our program…  This code will now run properly: Circle c = new Circle(); // c.radius = 1.0 System.out.println (c.oneOverRadius());  But this code will “crash”: Circle c = new Circle(); // c.radius = 1.0 c.radius = 0.0; System.out.println (c.oneOverRadius());

129. 129 Where the “crash” occurs public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); // c.radius = 1.0 c.radius = 0.0; System.out.println (c.oneOverRadius()); } public class Circle { double radius = 1.0; final static double PI = 3.1415926536; double computeArea () { return PI*radius*radius; } double oneOverRadius() { return 1.0/radius; } Here is the badly written code Here is where the “crash” occurs

130. 130 Motivation for private fields  Problem: We do not want people using our Circle class to be able to modify the fields on their own  Solution: Don’t allow other code to modify the radius field Give it private visibility  private means that only code within the class can modify the field

131. 131 One way to fix this… public class Circle { private double radius = 1.0; final static double PI = 3.1415926536; // Constructors... double computeArea () { return PI*radius*radius; } double oneOverRadius() { return 1.0/radius; } Note that the radius variable is now private

132. 132 Back to our program…  This code will now not compile: Circle c = new Circle(); // c.radius = 1.0 c.radius = 0.0; System.out.println (c.oneOverRadius());  Java will give a compile-time error: radius has private access in Circle

133. 133 Back to our program…  This code will also not compile: Circle c = new Circle(); // c.radius = 1.0 System.out.println (c.radius);  Java will give the same compile-time error: radius has private access in Circle

134. 134 What we wish computers could do

135. 135 The problem now…  But now you can’t have a Circle with a radius other than 1.0!  Solution: Use a get/set methods in Circle:  A mutator method: void setRadius (double r) { radius = r; }  An accessor method: double getRadius () { return radius; }

136. 136 Our Circle class so far public class Circle { private double radius = 1.0; final static double PI = 3.1415926536; // Constructors... double computeArea () { return PI*radius*radius; } double oneOverRadius() { return 1.0/radius; } void setRadius (double r) { radius = r; } double getRadius () { return radius; }

137. 137 Using the get/set methods public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); c.setRadius (1.0); System.out.println (c.computeArea()); System.out.println (c.getRadius()); } public class Circle { private double radius = 1.0; final static double PI = 3.1415926536; double computeArea () { return PI*radius*radius; } double oneOverRadius() { return 1.0/radius; } void setRadius (double r) { radius = r; } double getRadius () { return radius; } Here a method is invoked Here the change to radius occurs

138. 138 Wait! Another problem! public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); c.setRadius (0.0); System.out.println (c.oneOverRadius()); } } Here is the problem now…

139. 139 This problem is easily fixed  Change the setRadius method to the following void setRadius (double r) { if ( r > 0.0 ) radius = r; else radius = 1.0; }  Now there is (almost) no way for code outside the Circle class to change the radius to zero

140. 140 Visibilities in Java  There are four visibilities: private : Only code within the same class can access the field or method  Note: “access” means reading or writing the field, or invoking the method public : Any code, anywhere, can access the field or method protected : Used with inheritance  We won’t get to that this semester default : Almost the same as public  This is the default (duh!)  Note that it can’t be specified like the others  Also called ‘package’

141. 141 A few notes on visibilities  You can NOT specify visibilities for method variables Any method variable can only be accessed within that method  Think of it as public within the method (after it’s defined) and private outside the method  You can also specify visibilities for methods and classes We won’t get to that in this course

142. 142 Overriding methods (and constructors)  Consider the following code: Circle c1 = new Circle (); Circle c2 = new Circle (2.0);  Java knows which constructor to call by the list of parameters This is called “overloading” Meaning it means multiple things, depending on the context  We’ve seen overloading before: 3+4Performs integer addition 3.0+4.0Performs floating-point addition “3”+”4”Performs string concatenation  The ‘+’ operator is overloaded Creates a Circle of radius 1.0 Creates a Circle of radius 2.0

143. 143 Overriding methods (and constructors), take 2  The following Circle constructors would not be allowed: We are assuming PI is not final for this example public Circle() { radius = 1.0; } public Circle (double r) { radius = r; } public Circle (double p) { PI = p; } When Circle(1.0) is called, which one is meant?

144. 144 Using mutators in the constructor  Our second constructor has a problem: public Circle (double r) { radius = r; }  Consider the following code: Circle c = new Circle (0.0); System.out.println (c.oneOverRadius()); The method is dividing by zero (again)

145. 145 Using mutators in the constructor  This is easily fixed!  Our revised constructors: public Circle() { setRadius (1.0); } public Circle (double r) { setRadius (r); }  The mutator will properly set the radius (and won’t set it to zero)

146. 146 Why we always use the mutators  Consider a modified version of our circle class: class Circle { double radius; double diameter; String size; //...  Our mutator now looks like this:  That’s a lot of code to copy if you decide not to call the mutator! void setRadius (double r) { if ( radius <= 0.0 ) radius = 1.0; else radius = r; diameter = 2*radius; if ( radius < 1.0 ) size = “small”; else if ( radius < 5.0 ) size = “medium”; else if ( radius < 10.0 ) size = “large”; else size = “huge”; }

147. 147 Today’s demotivators

148. 148 Back to the static discussion  Remember that there is one (and only one) static PI field, regardless of how many objects are declared  Consider the following method: double getPI() { return PI; }  It doesn’t read or modify the “state” of any object In this example, it doesn’t read/write the radius  In fact, that particular method doesn’t care anything about the objects declared It’s only accessing a static field

149. 149 Make getPI() static  Consider the following: static double getPI() { return PI; }  As the method is static, it can ONLY access static fields  A static method does not care about the “state” of an object Examples: Math.sin(), Math.tan(), Math.cos()  They don’t care about the state of any Math object  They only perform the computation

150. 150 Invoking static methods  As with static fields, they can be called using either an object or the class name: Circle c = new Circle(); System.out.println (c.getPI()); System.out.println (Circle.getPI());  Static methods are also called class methods

151. 151 static methods and non- static fields  Consider the following (illegal) Circle method: static double getRadius() { return radius; }  And the code to invoke it: public static void main (String[] args) { Circle c1 = new Circle(); Circle c2 = new Circle(); Circle c3 = new Circle(); Circle c4 = new Circle(); System.out.println (Circle.getRadius()); }

152. 152 What happening in memory  There are 4 Circle objects in memory  Which radius field does Circle.getRadius() want?  There are 1 million Circle objects in memory  There are no Circle objects in memory … Circle - radius = 1.0 c1 Circle - radius = 1.0 c2 c1000000 Circle - radius = 1.0 c3 c4 Circle - radius = 1.0 PI 3.1415926536

153. 153 The main static lesson  A static method cannot access or modify the state of the object it is a part of  If you remember nothing else about static methods, remember this!

154. 154 static and non- static rules  Non-static fields and methods can ONLY be accessed by the object name  Static fields and methods can be accessed by EITHER the class name or the object name  Non-static methods can refer to BOTH static and non-static fields  Static methods can ONLY access static fields of the class they are part of

155. 155 Back to our main() method public static void main (String[] args) Any code anywhere can call this method It’s a static method: Can’t access non-static fields or methods directly Can be called only by the class name The method does not return a value We’ll learn about arrays in chapter 8

156. 156 Implications of main() being static  It can call other static methods within the same class class StaticMethods { static void method1() { System.out.println (“hi!”); } public static void main (String args[]) { method1(); } }  Note that we didn’t have to prefix method1() with a object Java assumes that it is in the same class

157. 157 Today’s demotivators

158. 158 End of lecture on 2 Nov 2005