cs lesson: creating your first class in java

January 28, 2003 233 Katrin Becker Slide 1

Creating Your first Class


What do we Need?

• MAIN

• One or more other classes.


What's the Difference (main vs others)?

MAIN CLASS OTHER CLASS(ES)

only one per application as many as we need

must contain function called main

must NOT contain function called main*

all functions & "globals" must be static

use static only when we need class (as opposed

to object) members

NEVER gets instantiated - depends -

main called implicitly by run-time system

functions explicitly called as necessary

* for now


What's the Difference (primitive types vs arrays & classes)?

Primitive Types Arrays & Classes

automatically allocated dynamically allocated

space for them created when they are declared

int i;

space for a reference made when they are declared

String S;

that space set when initialized

int i = 5;

new object created when initialized

String S = "Cordie";

- NA - new instance created with new


Java has ONLY pass by value:

413int i

public void F( int other) { other = 17;}

413

String S

public void F( String other) { other = "Fred";}

String: Cordie

↢ ↢

int other

41317

String: Fred

↢↢

F(i);

F(S);


But......

MyInt i;

public void F( MyInt other) { other .set( 247 );}

MyInt: v: 413

↢ ↢F(i);

public class MyInt {int v;

public set(int n) {v = n; }

}

v = n;247


Back to classes......


What's in a (main) Class? [ MAIN.java ]

//Attributes:static int i;static String S;

// Methods:static int f1() { return 0; }

public static void main( String[] args) {}

class MAIN {

} // end MAIN

// "wrapper"


What's in a (other) Class? [ Other.java ]

//Attributes:int i;String S;

// Methods:public Other () { // CONSTRUCTOR}public String junk() { return "Hi! Y'all!";}

class Other {

} // end Other

// "wrapper"


Access

• global within the class

• local within the class

• public

• private

int i;

void F() {int i;

}public int i;

int i;


Classes can "own" (have)

• primitive variables (like int, boolean)

• references (like String S;)

Can an object OWN another object?

(is that slavery?)not really, not in Java, although....If I'm the only one who "knows" this object,

I can be said to own it. (there's a difference between knowing, owning, and being)


Kinds ofMethods / Messages

• access / get / status – see a private member

• modify / set – change a private member

• forwarding – call a member's function


Creating a Class

• decide what the class should KNOW– these become the attributes (data

members)

• decide what the class should Do– these become the behaviours (methods /

functions / operations)


Account ClassQuestions to ask:

• What does an object of this sort need to remember (what are its attributes)?

• current balance [type?]• overdraft limit [type?]• account name [type?]• is account open? [type?] "status"


Account ClassQuestions to ask:

• What does an object of this sort need to do (what are its behaviours)?– access (the "Gets")

– open / close this account

– print (display) the balance

– accept a deposit

– process a withdrawal


The "Gets"

• Balance

• Limit

• Name

• isOpen


Open & Close

• change the "status"

to open: just change status

to close: must be empty first


print

• do some output


accept a deposit

• make sure we're open

• ask user for it

• check value of deposit

• add to balance


process a withdrawal

• make sure we're open

• ask user for amount

• check value of withdrawal– (check for overdraft)

• subtract from balance


Do a transfer?

• How is this different from a deposit / withdrawal?

• We only ask user for one amount...

• treat one side as a withdrawal ('cause we already have the code for that)

• treat the other side as a deposit but we shouldn't ask the user for the amount, so we need another function


What about constructors?

public Account (int ident)

{

id = ident;

}

public Account (int ident, int OD)

{

id = ident;

overdraft = OD;

}TWO?


What does that leave for the main class?

• in main function:// declare an array for 5 accounts

Account[] list = new Account[MAX];

// fill the array with the actual accounts

for (int i = 0; i < MAX; i++)

list[i] = new Account(i);


Figuring out which Account:

acctno = chooseAccount(NEW, list);

//or

acctno = chooseAccount(OLD, list);

• this will need it's own little menu


once we have an account:

case 'w' :

System.out.print

("Doing Withdrawal....\n");

amount = list[acctno].doWithdrawal();

break;

case 'o' :

System.out.print("New Account.\n");

list[acctno].openAcct();

break;


Doing a transfer - 1

System.out.print

("Account to transfer FROM:\n");

acctfrom = chooseAccount(OLD, list);

System.out.print

("Account to transfer INTO:\n");

acctto = chooseAccount(OLD, list);


Doing a transfer - 2

amount = list[acctfrom].doWithdrawal();

if (amount!= 0.0)

result =

list[acctto].doTransfer(amount);

if (result != 0.0)

System.out.print

("Transfer Successful.\n\n");

else

/* didn't work... put it back */

list[acctfrom].doTransfer(amount);


Putting it together: the Account:

public class Account

{

// the vars

// the constructor(s)

// the methods

}


Putting it together: the Bank:

public class Bank{ // the static vars// the static methodspublic static void main(String[] args){ Account list[] = new Account[MAX]; for (int i = 0; i < MAX; i++) list[i] = new Account(i); // do stuff with them}// end main

}// end Bank


The Classes:Bank Class

chooseAccount

main

doTransfer balance

openAccount

the "Gets"

Account Class

closeAccount

showBalance

doDeposit

doWithdrawal

doTransfer

open?

limit

nameAccount(s)

cs lesson: creating your first class in java

Technology

balancejanuary233 katrinslide

newjanuary233 katrinslide

listjanuary233 katrinslide

isopenjanuary233 katrinslide

fredjanuary233 katrinslide

nowjanuary233 katrinslide

outputjanuary233 katrinslide

classjanuary233 katrinslide