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