1. CMSC 150 CLASSES CS 150: Mon 6 Feb 2012 Images: Library of Congress

2. First: quick review of using methods  Remember types of methods:  Function: creates and “returns” a value int score = game.getScore(); System.out.println( “You get “ + game.numPointsForWin() ); int newScore = game.getScore() – game.numPointsPerMove(); Room playerRoom = game.getRoom( playerRow, playerCol );  Remember to either:  Save the returned value in a variable of the appropriate type, or  Use the returned value in an expression.

3. First: quick review of using methods  Remember types of methods:  Procedure: does work & returns no value (i.e., void return type) game.updateScore(newScore); game.updateMessage( “It’s the StayPuft Marshmallow Man!”); roomForSlimer.addSlimer();  Remember:  methods with void return type don’t create a value  illogical to use them in an assignment or expression

4. Anatomy of a method call  game.updateScore( newScore );  An object reference is always needed to call a method Object to call the method with.

5. Anatomy of a method call  game.updateScore( newScore );  An object reference is always needed to call a method Name of the method to call.

6. Anatomy of a method call  game.updateScore( newScore );  An object reference is always needed to call a method Parameters – input values for the method.

7. One Exception public class GhostBusters { … public boolean handleMove( ) { … boolean result = checkForLoss( ); } public boolean checkForLoss( ) { … } }  EXCEPT: if the method definition and the method call are in the same class, the object reference is “implicit”  Supplied automatically by Java

8. One other exception  Static methods are a little different int jenny = Integer.parseInt(“8675309”); This is the name of a class, not a reference to an object.

9. What is a Class?  A “blueprint” of your entity to be implemented  Defines and encapsulates your entity’s  Characteristics (data)  Behaviors (methods)  Example:

10. Example: Dog  What characteristics do dogs have in general?   Your class’s data (instance variables)  What behaviors do dogs exhibit?   Your class’s methods  But do these identify a specific dog?

11. Example: Dog  Do these identify a specific dog?

12. Example: Dog

13.  Class: Dog  Characteristics: breed, eye color, weight  Behaviors: fetch(), lick(), sit(), stay(), poop()  Science Lab Lilly: an instance (object) of class Dog  chocolate lab, golden eyes, 65 lbs.  can perform Dog behaviors  Darth Bailey: an instance (object) of class Dog  black lab, brown eyes, 50 lbs.  can perform Dog behaviors

14. In Summary  Write the class “blueprint” once  Create a specific object of that class  Create another object of that class  And another…  Consider a familiar example

15. Write Our Own String Class public class SimpleString { // instance variables (data) // methods }  Notice no main() method  Only when you want to directly execute that class

16. Writing Your Own String Class public class SimpleString { // instance variables (data) private char myFirstCharacter; private char mySecondCharacter; private int myLength; // methods }

17. Methods  Constructor  must have same name as class  syntax: public ClassName( parameters… )  creates the object of this class type SimpleString str = new SimpleString(‘Z’,’a’);  no return type in method definition (implicit)  All other methods

18. Writing Your Own String Class public class SimpleString { // instance variables (data) private char myFirstCharacter; private char mySecondCharacter; private int myLength; // constructor public SimpleString( char charOne, char charTwo ) { myFirstCharacter = charOne; mySecondCharacter = charTwo; myLength = 2; }

19. In Another Class… public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString( ‘O’, ‘y’ ); SimpleString str2 = new SimpleString( ‘Z’, ‘a’ ); }

20. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java

21. public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java Variables are of the class type SimpleString public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; }

22. public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java Objects will be constructed using the constructor from that class public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; }

23. The Two Programs Together  SimpleString.java:  defines blueprint for a generic "SimpleString" object  we do not directly run this  Tester.java:  we run this (because it contains main() )  uses SimpleString as a variable type  creates objects of the class SimpleString  Let's see what happens in memory as we run Tester…

24. public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java str1 declare… public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } 127 128 129 130 131 132 133 134 135 136 137 138

25. public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java ‘O’ ‘y’ character literals stored elsewhere automatically public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } str1 127 128 129 130 131 132 133 134 135 136 137 138

26. public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java reserve enough space for a SimpleString object… public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138

27. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java reserve enough space for a SimpleString object… mySecondChar myLength DATA METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138

28. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 invoke the constructor…

29. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 which reserves space for the parameters (like variables)… which reserves space for the parameters (like variables)… char1 char2

30. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 char1 char2 parameters arguments ‘O’

31. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 char1 char2 parameters arguments ‘O’ ‘y’

32. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 char1 char2 ‘O’ ‘y’ execute the first statement in the constructor… ‘O’

33. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 char1 char2 ‘O’ ‘y’ execute the next statement in the constructor… ‘O’ ‘y’

34. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 char1 char2 ‘O’ ‘y’ execute the next statement in the constructor… ‘O’ ‘y’ 2

35. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 as the constructor finishes, space for the parameters is reclaimed… ‘O’ ‘y’ 2

36. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 constructor call returns the memory address of the new object, which is assigned to str1… ‘O’ ‘y’ 2 130

37. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 str1 variable now “references” the SimpleString object ‘O’ ‘y’ 2 130

38. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar ‘O’ ‘y’ str1 127 128 129 130 131 132 133 134 135 136 137 138 ‘O’ ‘y’ 2 130 object (instance) of class SimpleString variable of type SimpleString (containing reference) variable of type SimpleString (containing reference)

39. public class SimpleString { private char myFirstChar; private char mySecondChar; private int myLength; public SimpleString(char char1, char char2) { myFirstChar = char1; mySecondChar = char2; myLength = 2; } public class Tester { public static void main( String[] args ) { SimpleString str1 = new SimpleString(‘O’,’y’); SimpleString str2 = new SimpleString(‘Z’,’a’); } SimpleString.java Tester.java mySecondChar myLength METHOD STUFF myFirstChar str2 337 338 339 340 341 342 343 344 345 346 347 348 ‘Z’ ‘a’ 2 340 next statement results in similar picture elsewhere in memory

40. public class MyString { // instance variables (data) private char myFirstCharacter; private char mySecondCharacter; private int myLength; // default constructor public SimpleString() { myFirstCharacter = ‘ ‘; mySecondCharacter = ‘ ‘; myLength = 0; } // constructor with two parameters public SimpleString( char charOne, char charTwo ) { myFirstCharacter = charOne; mySecondCharacter = charTwo; myLength = 2; }

41. Methods  Constructor  must have same name as class  syntax: public ClassName( arguments… )  creates the object of this class type MyString str = new MyString();  no return type (implicit)  All other methods  use any non-reserved name  two types: perform action, return a value

42. Methods  All other methods  use any non-reserved name  two types: perform action, return a value 1. perform an action: don't return anything public void doSomething() { … public void doSomethingElse() { … 2. return a value: provide a specific return type public int length() { … public String substring( int …

43.  All other methods  use any non-reserved name  two types: perform action, return a value 1. perform an action: don't return anything public void doSomething( … ) { … public void doSomethingElse( … ) { … 2. return a value: provide a specific return type public int length() { … public String substring( int … Methods

44. public class SimpleString { … // these methods do something without // needing to return any sort of value public void printTheString() { System.out.println( “” + myFirstCharacter + mySecondCharacter ); } public void clearTheString() { myFirstCharacter = ‘ ‘; mySecondCharacter = ‘ ‘; myLength = 0; }

45. return type of method public class SimpleString { … // these methods do something without // needing to return any sort of value public void printTheString() { System.out.println( “” + myFirstCharacter + mySecondCharacter ); } public void clearTheString() { myFirstCharacter = ‘ ‘; mySecondCharacter = ‘ ‘; myLength = 0; }

46. public class SimpleString { … // these methods do something without // needing to return any sort of value public void printTheString() { System.out.println( “” + myFirstCharacter + mySecondCharacter ); } public void clearTheString() { myFirstCharacter = ‘ ‘; mySecondCharacter = ‘ ‘; myLength = 0; } void: no return statement

47. public class SimpleString { … // these methods return some sort of info public int length() { return myLength; // return instance variable } public char charAt(int index) { char returnChar = ‘ ‘; if (index == 0) { returnChar = myFirstCharacter; } else if (index == 1) { returnChar = mySecondCharacter; } return returnChar; }

48. public class SimpleString { … // these methods return some sort of info public int length() { return myLength; // return instance variable } public char charAt(int index) { char returnChar = ‘ ‘; if (index == 0) { returnChar = myFirstCharacter; } else if (index == 1) { returnChar = mySecondCharacter; } return returnChar; } return type of method

49. public class SimpleString { … // these methods return some sort of info public int length() { return myLength; // return instance variable } public char charAt(int index) { char returnChar = ‘ ‘; if (index == 0) { returnChar = myFirstCharacter; } else if (index == 1) { returnChar = mySecondCharacter; } return returnChar; } return statement using a variable of type int return statement using a variable of type int return statement using a variable of type char return statement using a variable of type char