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

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

return <expression>; // Details later}

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

Writing methods7

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

return <expression>; // Details later}

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

Writing methods8

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

return <expression>; // Details later}

What name is used to “invoke”this method?

Writing methods9

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.)

Writing methods10

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

return <expression>; // Details later}

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

Writing methods11

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

return <expression>; // Details later}

What’s inside is called the “method body”

Writing methods12

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

return <expression>; // Details later}

Statements executed when this method is called.

Writing methods13

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.

Example: Back to FunBrain…

14

Example: Back to FunBrain…

15

Hmm… looks suspiciouslylike a method…

Example: Back to FunBrain…

16

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

Parameter passing17

Formal parameter list

Parameter passing18

Actual parameter list

Parameter passing19

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

What happens in memory?20

… = checkGuess( userGuess, randomNumber, guesses );

userGuess

randomNumber

guesses

25

47

2

…

Memory used forvariables in

main()

What happens in memory?21

… = checkGuess( userGuess, randomNumber, guesses );

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

What happens in memory?22

… = checkGuess( userGuess, randomNumber, guesses );

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

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

What happens in memory?23

… = checkGuess( userGuess, randomNumber, guesses );

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

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 facts24

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 Java25

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: String26

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 …

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

Next lab, who you gonna call?

30

Lab setup31

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

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