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Methods

Methods. Overview. Class/Library/ static vs. Message/non- static return and void Parameters and arguments. Dot and Parenthesis. x . y means “the y inside x ” Similar to: ’s System.out.println means “The println inside the out that is inside System”

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Methods

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  1. Methods

  2. Overview • Class/Library/static vs. Message/non-static • return and void • Parameters and arguments

  3. Dot and Parenthesis • x.y means “the y inside x” • Similar to: ’s • System.out.println means “The println inside the out that is inside System” • x() means “x is a method and I want to run it”

  4. What is a class? • A class can be (any mix of) 3 things: • A program • If and only if it has “public static void main( String[] )” • A library of methods and constants • Examples: Math, JOptionPane • A description of a kind of object • … We’ll get into this in several weeks • Examples: Scanner, Random

  5. static and non-static • Consider x.y() • If x is a class, y must be static • If x is an Object, y should not be static • Math.cos(): static because Math is a class • stdin.nextInt(): non-static because stdin is an object (in particular, a Scanner).

  6. Method Definition and Invocation • A method is a named piece of code • Defining a method = writing a how-to • Invoking a method = following that how-to • Pressing “run” just invokes main()

  7. Parameters and Arguments • A formal parameter is what a method writer assumes an invoker will bring to the table • An actual argument is the thing the method invoker brings • … people are sloppy with these names …

  8. Parameters and Arguments • How to fry an egg • Crack the egg into a pan • Add heat • Parameters: what I call things I assume will exist when defining a method • Fry this ostrich egg • Fry this chicken egg • Arguments: the values I use for the parameters when invoking a method

  9. Parameters and Arguments public class Example { public static void f( int x ) { System.out.println( x + 2 ); } } public classExampleUser { p … id main( String[] args ) { Example.f( 3 ); int y = 17; Example.f( y - 2 ); } }

  10. Return • When I ask you “what’s sqrt(4),” you return “2” to me • return is how methods give back values • return is also a way to halt a method

  11. Carbon Dating • Archeologists can date artifacts by the relative concentration of carbon-14 in a sample compared to concentration in the air • The formula is age = ln( relative concentration ) × −8260

  12. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } }

  13. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Run invokes method main Dater.main() String args

  14. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Create a variable x in main Dater.main() String args intx

  15. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke carbonDate with argument 0.05 Dater.main() String args intx

  16. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke carbonDate with argument 0.05 Dater.main() Dater.carbonDate() String args double concentration 0.05 int x

  17. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Create a variable age in carbonDate Dater.main() Dater.carbonDate() String args double concentration 0.05 int x double age

  18. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke Math’s log method with argument 0.05 Dater.main() Dater.carbonDate() Math.log() String args double concentration 0.05 double x 0.05 int x double age

  19. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke Math’s log method with argument 0.05 Dater.main() Dater.carbonDate() Math.log() String args double concentration 0.05 int x double age -2.99573…

  20. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke Math’s log method with argument 0.05 Dater.main() Dater.carbonDate() String args double concentration 0.05 int x double age Math.log() -2.99573…

  21. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke Math’s log method with argument 0.05 Dater.main() Dater.carbonDate() String args double concentration 0.05 int x double age -2.99573…

  22. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Compute the value (24744.748…) and store it in age Dater.main() Dater.carbonDate() String args double concentration 0.05 int x double age 24744.74 -2.99573…

  23. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Compute the return value (24744) Dater.main() Dater.carbonDate() String args double concentration 0.05 int x double age 24744.74

  24. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } return and store the result Dater.main() Dater.carbonDate() String args int x 24744

  25. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } return and store the result Dater.main() Dater.carbonDate() String args 24744 intx

  26. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } return and store the result Dater.main() 24744 String args intx 24744

  27. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } After returning, carbonDate’s variables disappear Dater.main() String args intx 24744

  28. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Compute the string to print: "0.05 means 24744 years old" Dater.main() String args intx 24744

  29. "0.05 means 24744 years old" publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke System.out’sprintln method Dater.main() PrintWriter.println() String args String thingToPrint intx 24744

  30. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke carbonDate with argument 0.10 Dater.main() String args intx 24744

  31. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke carbonDate with argument 0.10 Dater.main() Dater.carbonDate() String args double concentration 0.10 int x

  32. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Create a variable age in carbonDate Dater.main() Dater.carbonDate() String args double concentration 0.10 int x double age

  33. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke Math’s log method with argument 0.10 Dater.main() Dater.carbonDate() Math.log() String args double concentration 0.10 double x 0.10 int x double age

  34. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke Math’s log method with argument 0.10 Dater.main() Dater.carbonDate() String args double concentration 0.10 int x double age Math.log() -2.30259…

  35. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke Math’s log method with argument 0.10 Dater.main() Dater.carbonDate() String args double concentration 0.10 int x double age -2.30259…

  36. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Compute the value (19019.352…) and store it in age Dater.main() Dater.carbonDate() String args double concentration 0.10 int x double age 19019.35 -2.30259…

  37. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Compute the return value (19019) Dater.main() Dater.carbonDate() String args double concentration 0.10 int x double age 19019.35

  38. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } return and use the result Dater.main() Dater.carbonDate() String args int x 19019

  39. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } return and use the result Dater.main() String args Dater.carbonDate() intx 24744 19019

  40. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } return and use the result Dater.main() String args intx 24744 19019

  41. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } After returning, carbonDate’s variables disappear Dater.main() String args intx 24744 19019

  42. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Compute the string to print: "0.10 means 19019 years old" Dater.main() String args intx 24744 19019

  43. "0.10 means 19019 years old" publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } Invoke System.out’sprintln method Dater.main() PrintWriter.println() String args String thingToPrint intx 24744 19019

  44. publicclass Dater { publicstaticvoid main( String[] args ) { int x = Dater.carbonDate( 0.05 ); System.out.println( "0.05 means " + x + " years old" ); System.out.println( "0.10 means " + Dater.carbonDate( 0.10 ) + " years old" ); } publicstaticintcarbonDate( double concentration ) { double age = Math.log( concentration ) * -8260; return (int)(age); } } After main ends, its variables disappear

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