cmsc 150 methods and classes

CMSC 150METHODS AND

CLASSES

CS 150: Wed 25 Jan 2012

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

2

Why write a method?3

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.

Why write a method?4

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

Writing methods5

public static <return-type> method-name ( parameters ) { statements;

return <expression>; // Details later}

Writing methods6



What “kind” of Method this is – More on this later.

Writing methods7



Does this method producea value, and if so, what type?

Writing methods8



What name is used to “invoke”this method?

Writing methods9



What information is expected from the caller in order to run this method? (Can be empty.)

Writing methods10



The first line of a method is calledthe “method header” or “method signature”

Writing methods11



What’s inside is called the “method body”

Writing methods12



Statements executed when this method is called.

Writing methods13



If the method “returns a value,”this is how we make that happen.

Example: Back to FunBrain…

14


15

Hmm… looks suspiciouslylike a method…


16

Check the guess againstthe secret number and generate a message tothe user. Let’s make itinto a method.

Compare The Two Implementations

17

Parameter passing18

Formal parameter list

Parameter passing19

Actual parameter list

Parameter passing20

Can be any expressionthat evaluates to thetype of the matchingformal parameter.

What happens in memory?21

… = checkGuess( userGuess, randomNumber, guesses );

userGuess

randomNumber

guesses

25

47

2

…

Memory used forvariables in

main()



public static String checkGuess ( int currentGuess, int secret, int numGuesses ){ …

userGuess

randomNumber

guesses

25

47

2

…

currentGuess

secret

numGuesses

25

47

2

Memory used for formal

parametersin checkGuess




userGuess

randomNumber

guesses

25

47

2

…

currentGuess

secret

numGuesses

25

47

2

When method call is made,values stored in the

arguments…

…are copied into the formal parameters




userGuess

randomNumber

guesses

25

47

2

…

currentGuess

secret

numGuesses

25

47

2

Arguments & formal parameters

initially have the same values…

but occupy different spacesin memory

Method facts25

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”)

Classes in Java26

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

Example: String27

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(int pos) indexOf(char c) substring(int begin, int pastEnd) … it goes on and on …

Methods with different jobs28

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

In order to use a class…29

… 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

http://docs.oracle.com/javase/6/docs/api/

In lab, who you gonna call?30

Lab setup31

What you are given: 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

Your methods32

public void setupGame() Sets up the initial game configuration (similar to Wumpus)

public boolean handleMove( 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 boolean checkForLoss() Determine if the Ghostbuster encountered Slimer or a portal

public void checkForGhostTrap() Determine if the Ghostbuster found a desirable ghost trap

Ghostbusters: Two Classes Provided

33

public class GameBoard{ …

int playerRow; int playerCol; int score; Room[][] gridOfRooms;

public GameBoard getGame()

public void setPlayerRow(int row) public void setPlayerCow(int col) public int getPlayerRow() public int getPlayerCol()

public int getScore() public void updateScore(int score)

public void updateMessage(String m)

public Room getRoom(int row, int col)

…

}

public class Room { …

public boolean containsSlimer() public boolean containsPortal() public boolean containsGhostTrap()

public void showPlayerEnters() public void showPlayerExits()

public void addSlimer() public void showSlimer()

public void addPortal() public void showPortal()

public void addGhostTrap() public void removeGhostTrap() public void showGhostTrap()

public void showPlayerSlimed() public void showSlimerCaptured() …

}




public void setPlayerRow(int row) public void setPlayerCol(int col) public int getPlayerRow() public int getPlayerCol()



public Room getRoom(int row,int col)

…

}

34

GameBoard








…

}

35

GameBoard

Keeps track of certain data related to the

game, but you cannot assign these values

directly.

You must use methods provided by the class.








…

}

36

GameBoard



directly.









…

}

37

GameBoard



directly.









…

}

38

GameBoardTo change anything with respect to the

GameBoard,your program must have a variable of type

GameBoard:

GameBoard game = GameBoard.getGame();








…

}

39

GameBoardThen you can use that variable to call methods within the GameBoard class:

game.setPlayerRow( randomRow );game.setPlayerCol( randomCol );int theScore = game.getScore();








}

40

Room








}

41

Room








}

42

Room

To change anything with respect to a particular room, you must have a variable

of the Room type.

You get at a particular room via the GameBoard.

GameBoard game = GameBoard.getGame();Room firstRoom = game.getRoom(0,0);








}

43

Room

Then you can use that Room variable to call methods within the

Room class:

firstRoom.addPortal(); firstRoom.showPortal();

if (firstRoom.containsPortal()) …




public void setPlayerRow(int row) public void setPlayerRow(int col) public int getPlayerRow() public int getPlayerCol()



public Room getRoom()

…

}

public class Ghostbusters { …

public void setupGame()

public boolean handleMove(String dir) public void handleFire(String dir)

public boolean checkForLoss() public void checkForGhostTrap() …

}

44

Ghostbusters

Your setupGame() method is called by the GameBoard class when the

game is first started.








…

}





}

45

Ghostbusters

And your method is responsible for putting things onto the game board

appropriately…








…

}





}

46

Ghostbusters

Your handleMove(…) method is called by the GameBoard class whenever the user clicks an arrow button to

move








…

}





}

47

Ghostbusters

Your handleFire(…) method is called by the GameBoard class whenever

the user clicks a red arrow button to fire





}

48

Ghostbusters

Your checkForLoss() and checkForGhostTrap() methods are

called by your own handleMove(…) method

cmsc 150 methods and classes

Documents

length method

method definition

invokethis method

kind of method

method useful code

method producea value

calledthe method header

repeated code