Programming with methods and classes

1. Programming withmethods and classes Chapter 7 Fall 2005 CS 101 Aaron Bloomfield

2. Static vs. non-static

3. Methods • Instance (or member) method • Operates on a object (i.e., and instance of the class) String s = new String("Help every cow reach its " + "potential!"); int n = s.length(); • Class (i.e. static) method • Service provided by a class and it is not associated with a particular object String t = String.valueOf(n); Instance method Class method

4. Variables • Instance variable and instance constants • Attribute of a particular object • Usually a variable Point p = new Point(5, 5); int px = p.x; • Class variables and constants • Collective information that is not specific to individual objects of the class • Usually a constant Color favoriteColor = Color.MAGENTA; double favoriteNumber = Math.PI - Math.E; Instance variable Class constants

5. static and non-static rules • Member/instance (i.e. non-static) fields and methods can ONLY be accessed by the object name • Class (i.e. static) fields and methods can be accessed by Either the class name or the object name • Non-static methods can refer to BOTH class (i.e. static) variables and member/instance (i.e. non-static) variables • Class (i.e. static) methods can ONLY access class (i.e. static) variables

6. Static vs. non-static • Consider the following code: public class Staticness { private int a = 0; private static int b = 0; public void increment() { a++; b++; } public String toString() { return "(a=" + a + ",b=" + b + ")"; } }

7. Static vs. non-static • And the code to run it: public class StaticTest { public static void main (String[] args) { Staticness s = new Staticness(); Staticness t = new Staticness(); s.increment(); t.increment(); t.increment(); System.out.println (s); System.out.println (t); } }

8. Static vs. non-static • Execution of the code… • Output is: (a=1,b=3) (a=2,b=3)

9. t s Staticness Staticness Staticness Staticness Staticness • a = 0 • a = 1 • a = 1 • a = 2 • a = 0 Static vs. non-static: memory diagram Staticness s = new Staticness(); Staticness t = new Staticness(); s.increment(); t.increment(); t.increment(); System.out.println (s); System.out.println (t); Staticness s = new Staticness(); Staticness t = new Staticness(); s.increment(); t.increment(); t.increment(); System.out.println (s); System.out.println (t); 1 0 2 3 b

10. Program demo • StaticTest.java

11. Yale vs. Harvard • Web references: http://www.harvardsucks.org/, http://www.yaledailynews.com/article.asp?AID=27506

12. Conversion.java

13. Task – Conversion.java • Support conversion between English and metric values • d degrees Fahrenheit = (d – 32)/1.8 degrees Celsius • 1 mile = 1.609344 kilometers • 1 gallon = 3.785411784 liters • 1 ounce (avdp) = 28.349523125 grams • 1 acre = 0.0015625 square miles = 0.40468564 hectares

14. Conversion Implementation public class Conversion { // conversion equivalencies private static final double KILOMETERS_PER_MILE = 1.609344; private static final double LITERS_PER_GALLON = 3.785411784; private static final double GRAMS_PER_OUNCE = 28.349523125; private static final double HECTARES_PER_ACRE = 0.40468564;

15. Modifier public indicates other classes can use the method Modifier static indicates the method is a class method No use of member/instance variables!!! Conversion implementation public static double fahrenheitToCelsius (double f) { return (f - 32) / 1.8; } }

16. Conversion Implementation // temperature conversions methods public static double fahrenheitToCelsius(double f) { return (f - 32) / 1.8; } public static double celsiusToFahrenheit(double c) { return 1.8 * c + 32; } // length conversions methods public static double kilometersToMiles(double km) { return km / KILOMETERS_PER_MILE; }

17. Conversion Implementation // mass conversions methods public static double litersToGallons(double liters) { return liters / LITERS_PER_GALLON; } public static double gallonsToLiters(double gallons) { return gallons * LITERS_PER_GALLON; } public static double gramsToOunces(double grams) { return grams / GRAMS_PER_OUNCE; } public static double ouncesToGrams(double ounces) { return ounces * GRAMS_PER_OUNCE; }

18. Conversion Implementation // area conversions methods public static double hectaresToAcres(double hectares) { return hectares / HECTARES_PER_ACRE; } public static double acresToHectares(double acres) { return acres * HECTARES_PER_ACRE; }

19. Conversion use Scanner stdin = new Scanner (System.in); System.out.print("Enter a length in kilometers: "); double kilometers = stdin.nextDouble(); double miles = Conversion.kilometersToMiles(kilometers); System.out.print("Enter a mass in liters: "); double liters = stdin.nextDouble(); double gallons = Conversion.litersToGallons(liters); System.out.print("Enter a mass in grams: "); double grams = stdin.nextDouble(); double ounces = Conversion.gramsToOunces(grams); System.out.print("Enter an area in hectares: "); double hectares = stdin.nextDouble(); double acres = Conversion.hectaresToAcres(hectares);

20. A Conversion use System.out.println(kilometers + " kilometers = " + miles + " miles "); System.out.println(liters + " liters = " + gallons + " gallons"); System.out.println(grams + " grams = " + ounces + " ounces"); System.out.println(hectares + " hectares = " + acres + " acres"); 2.0 kilometers = 1.242742384474668 miles 3.0 liters = 0.7925161570744452 gallons 4.0 grams = 0.14109584779832166 ounces 5.0 hectares = 12.355269141746666 acres

21. A preferred Conversion use Part of java.text NumberFormat style = NumberFormat.getNumberInstance(); style.setMaximumFractionDigits(2); style.setMinimumFractionDigits(2); System.out.println(kilometers + " kilometers = " + style.format(miles) + " miles "); System.out.println(liters + " liters = " + style.format(gallons) + " gallons"); System.out.println(grams + " grams = " + style.format(ounces) + " ounces"); System.out.println(hectares + " hectares = " + style.format(acres) + " acres"); 2.0 kilometers = 1.24 miles 3.0 liters = 0.79 gallons 4.0 grams = 0.14 ounces 5.0 hectares = 12.36 acres

22. Program Demo • Conversion.java

23. Fractals

24. Parameter passing

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

26. 5 5 7 y x y 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

27. y x “5" “7" 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

28. y x width = 10 width = 0 Java parameter passing • Consider the following code: static void foobar (Rectangle y) { y.setWidth (10); } public static void main (String[] args) { Rectangle x = new Rectangle(); foobar (x); System.out.println(x.getWidth()); } • What gets printed? formal parameter actual parameter

29. y x width = 10 width = 0 width = 0 Java parameter passing • Consider the following code: static void foobar (Rectangle y) { y = new Rectangle(); y.setWidth (10); } public static void main (String[] args) { Rectangle x = new Rectangle(); foobar (x); System.out.println(x.getWidth()); } • What gets printed? formal parameter actual parameter

30. 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

31. Method invocations • Actual parameters provide information that is otherwise unavailable to a method • When a method is invoked • Java sets aside memory for that particular invocation • Called the activation record • Activation record stores, among other things, the values of the formal parameters • Formal parameters initialized with values of the actual parameters • After initialization, the actual parameters and formal parameters are independent of each other • Flow of control is transferred temporarily to that method

32. Value parameter passing demonstration public class ParameterDemo { public static double add(double x, double y) { double result = x + y; return result; } public static double multiply(double x, double y) { x = x * y; return x; } public static void main(String[] args) { double a = 8, b = 11; double sum = add(a, b); System.out.println(a + " + " + b + " = " + sum); double product = multiply(a, b); System.out.println(a + " * " + b + " = " + product); } }

33. Value parameter passing demonstration • The file/class is actually called ParameterDemo.java

34. Program demo • ParameterDemo.java

35. ParameterDemo.java walkthrough double sum = add(a, b); Initial values of formal parameters come from the actual parameters public static double add (double x, double y) { double result = x + y; return result; }

36. ParameterDemo.java walkthrough double multiply = multiply(a, b); Initial values of formal parameters come from the actual parameters public static double multiply(double x, double y) { x = x * y; return x; }

37. PassingReferences.java

38. PassingReferences.java import java.awt.*; public class PassingReferences { public static void f(Point v) { v = new Point(0, 0); } public static void g(Point v) { v.setLocation(0, 0); } public static void main(String[] args) { Point p = new Point(10, 10); System.out.println(p); f(p); System.out.println(p); g(p); System.out.println(p); } }

39. PassingReferences.java run g() can change the attributes of the object to which p refers

40. Program demo • PassingReferences.java

41. PassingReferences.java public static void main(String[] args) { Point p = new Point(10, 10); System.out.println(p); f(p); java.awt.Point[x=10,y=10]

42. PassingReferences.java public static void f(Point v) { v = new Point(0, 0); }

43. PassingReferences.java public static void main(String[] args) { Point p = new Point(10, 10); System.out.println(p); f(p); System.out.println(p); g(p); java.awt.Point[x=10,y=10] java.awt.Point[x=10,y=10]

44. PassingReferences.java public static void g(Point v) { v.setLocation(0, 0); }

45. PassingReferences.java public static void main(String[] args) { Point p = new Point(10, 10); System.out.println(p); f(p); System.out.println(p); g(p); System.out.println(p); java.awt.Point[x=10,y=10] java.awt.Point[x=10,y=10] java.awt.Point[x=0,y=0]

46. Star Wars Episode 3 Trailer

47. Star Wars Episode 3 Trailer • That was a edited version • I changed the PG-rated trailer to a G-rated trailer • The original one can be found at http://www.sequentialpictures.com/ • Or Google for “star wars parody”

48. Casting

49. 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?

50. 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…