CMSC 150 Methods and Classes

1. CMSC 150Methods and Classes CS 150: Wed 25 Jan 2012

2. Remember the length “method”? String message = “Hello, Watson”; System.out.println( message.length() ); // prints 13 • Method – useful code packaged with a name • In Java, methods belong to classes • With an object like message, use the object variable to “call” the method • works on information inside the object message refers to

3. Why write a method? • Code can get very long and unwieldy • Methods let us break up code into logical chunks • Readability – with appropriately named methods, your code is easier to read and follow • Reuse – make repeated code into a method. Write once, use many times.

4. Why write a method? • Code can get very long and unwieldy • Methods let us break up code into logical chunks • Readability – with appropriately named methods, your code is easier to read and follow • Reuse – make repeated code into a method. Write once, use many times. MODULARITY

5. Writing methods public static <return-type> method-name ( parameters ) { statements; return <expression>; // Details later }

6. Writing methods public static <return-type> method-name ( parameters ) { statements; return <expression>; // Details later } What “kind” of Method this is – More on this later.

7. Writing methods public static <return-type> method-name ( parameters ) { statements; return <expression>; // Details later } Does this method produce a value, and if so, what type?

8. Writing methods public static <return-type> method-name ( parameters ) { statements; return <expression>; // Details later } What name is used to “invoke” this method?

9. Writing methods public static <return-type> method-name ( parameters ) { statements; return <expression>; // Details later } What information is expected from the caller in order to run this method? (Can be empty.)

10. Writing methods public static <return-type> method-name ( parameters ) { statements; return <expression>; // Details later } The first line of a method is called the “method header” or “method signature”

11. Writing methods public static <return-type> method-name ( parameters ) { statements; return <expression>; // Details later } What’s inside is called the “method body”

12. Writing methods public static <return-type> method-name ( parameters ) { statements; return <expression>; // Details later } Statements executed when this method is called.

13. Writing methods public static <return-type> method-name ( parameters ) { statements; return <expression>; // Details later } If the method “returns a value,” this is how we make that happen.

14. Example: Back to FunBrain…

15. Example: Back to FunBrain… Hmm… looks suspiciously like a method…

16. Example: Back to FunBrain… Check the guess against the secret number and generate a message to the user. Let’s make it into a method.

17. Parameter passing Formal parameter list

18. Parameter passing Actual parameter list

19. Parameter passing Can be any expression that evaluates to the type of the matching formal parameter.

20. What happens in memory? 25 userGuess … = checkGuess( userGuess, randomNumber, guesses ); 47 randomNumber 2 guesses … Memory used for variables in main()

21. What happens in memory? 25 userGuess … = checkGuess( userGuess, randomNumber, guesses ); public static String checkGuess ( intcurrentGuess, int secret, intnumGuesses ){ … 47 randomNumber 2 guesses Memory used for formal parameters in checkGuess … 25 currentGuess secret 47 numGuesses 2

22. What happens in memory? 25 userGuess … = checkGuess( userGuess, randomNumber, guesses ); public static String checkGuess ( intcurrentGuess, int secret, intnumGuesses ){ … 47 randomNumber 2 guesses When method call is made, values stored in the arguments… …are copied into the formal parameters … 25 currentGuess secret 47 numGuesses 2

23. What happens in memory? 25 userGuess … = checkGuess( userGuess, randomNumber, guesses ); public static String checkGuess ( intcurrentGuess, int secret, intnumGuesses ){ … 47 randomNumber 2 guesses Arguments & formal parameters initially have the same values… but occupy different spaces in memory … 25 currentGuess secret 47 numGuesses 2

24. Method facts • In Java, parameters are “passed by value” • (AKA “pass by copy”) • Each call to a method executes in its own memory space • Function: a method that creates and “returns” a value • Procedure: a method that only does work and returns no value (i.e., return type “void”)

25. Classes in Java • Classes contain two things: • methods (also called “behavior”) • data (also called “state”) • Encapsulate related data & methods • A good way of structuring large programs • e.g,. the Java libraries • String class • Random class • Scanner class

26. Example: String • What’s the data for a String object? • The sequence of characters it was initialized with • The length of the sequence • What behavior is available? • length() • charAt(intpos) • indexOf(char c) • substring(int begin, intpastEnd) • … it goes on and on …

27. Example: String • What’s the data for a String object? • The sequence of characters it was initialized with • The length of the sequence • What behavior is available? • length() • charAt(intpos) • indexOf(char c) • substring(int begin, intpastEnd) • … it goes on and on …

28. Methods with different jobs • Some methods let you request information from the class • “Accessor” methods • names often start with “get” (but not always) • e.g., charAt method in String class • Some methods cause a change to the state (i.e., data) held by the class • “Mutator” methods • names may start with “set” (but not always) • e.g., setSeed method in Random class

29. In order to use a class… • … need to know what methods it provides • … need to know what parameters they expect • … need to know how the methods behave • … don’t need to know how they were written • Application Programming Interface (API) • Again, need a variable of that class type • Use that variable to call methods in the class

30. Next lab, who you gonna call?

31. Lab setup • What we will give you: • GameBoard Class • Room Class • Ghostbusters skeleton • API documentation for GameBoard and Room • What you will write: • Several methods in Ghostbusters needed by GameBoard to run the game • Your methods will use methods from GameBoard and Room

32. Your methods • public void setupGame() • Sets up the initial game configuration (similar to Wumpus) • public booleanhandleMove( String direction ) • Logic to control Ghostbuster moving from room to room • public void handleFire( String direction ) • Logic to control Ghostbuster firing proton pack @ Slimer • public booleancheckForLoss() • Determine if the Ghostbuster encountered Slimer or a portal • public void checkForGhostTrap() • Determine if the Ghostbuster found a desirable ghost trap