Static methods

[Woot4Moo]

Hi guys I am working on a program and I cant seem to figure out where
it is I am going wrong.
It is involving Bank accounts and PINs. I have a Bank class and a
BankAccount class.

In my BankAccount class I have a variable declared as private static
int pin.
as well as:
public static int checkPin()
{
return pin;
}

In my Bank class I have
public double getBalance(String nameOfOwner, int finalPin)
{
int pin = BankAccount.checkPin();
if(pin != finalPin) return -1;
// rest of code to get balance, which works properly.
}

My question is how do I make it so the pin can be referenced for each
account and also so that pin does NOT get over written everytime a new
account is created. Because currently what happens is the pin is only
stored for the last account created.

Thanks

 

[Joe Attardi]

In my BankAccount class I have a variable declared as private static
int pin.
...
}My question is how do I make it so the pin can be referenced for each
account and also so that pin does NOT get over written everytime a new
account is created. Because currently what happens is the pin is only
stored for the last account created.

A static variable has the same value for every instance of the class.
So you are basically assigning the same PIN to every account. You want
this to be an instance variable, i.e.

private int pin;

public int getPin() {
return pin;
}

This will allow each instance to have its own PIN, which is what you
want.

 

[Flo 'Irian' Schaetz]

And thus spoke (e-mail address removed)...

In my BankAccount class I have a variable declared as private static
int pin.
as well as:
public static int checkPin()
{
return pin;
}

This means, that only ONE Pin does exist, no matter how many accounts
you create. Every account will access THIS ONE Pin. Are you really sure
that you want to do that?

My question is how do I make it so the pin can be referenced for each
account and also so that pin does NOT get over written everytime a new
account is created. Because currently what happens is the pin is only
stored for the last account created.

That's because the variable Pin exists only ONE time. It does NOT exist
one time for each account. Static means exactly that: Not every account
has it's own pin, there is only one "global" pin instead.

So if you want every account to have it's own pin, remove the static.
And please read some good tutorials about Java.

Flo

 

[Eric Sosman]

Hi guys I am working on a program and I cant seem to figure out where
it is I am going wrong.
It is involving Bank accounts and PINs. I have a Bank class and a
BankAccount class.

In my BankAccount class I have a variable declared as private static
int pin.
as well as:
public static int checkPin()
{
return pin;
}

In my Bank class I have
public double getBalance(String nameOfOwner, int finalPin)
{
int pin = BankAccount.checkPin();
if(pin != finalPin) return -1;
// rest of code to get balance, which works properly.
}

My question is how do I make it so the pin can be referenced for each
account and also so that pin does NOT get over written everytime a new
account is created. Because currently what happens is the pin is only
stored for the last account created.

Get rid of `static' (in both places). A `static' variable
belongs to the class as a whole, rather than to any particular
instance of the class. No matter how many BankAccount objects
you create, there will be only one `static int pin' and it will
hold only one value at a time. Without `static', each BankAccount
will have its own pin variable, and the pin values of two different
BankAccount objects can be different.

 

[Woot4Moo]

Originally that is what I had but when I tried to check the PIN in my
Bank class it was giving me problems about it being non-static. So
that is why I had changed it originally.

 

[Eric Sosman]

Originally that is what I had but when I tried to check the PIN in my
Bank class it was giving me problems about it being non-static. So
that is why I had changed it originally.

(Please quote enough context to make your message
understandable on its own.)

For those who haven't seen what's gone before, Woot4Moo
has a BankAccount class that contains a `private static int pin'
variable and a `static int checkPin()' method. He's wondering
why all BankAccounts have the same pin value, and he's been
advised (by me and others) to get rid of `static'.

Back to the action: Woot4Moo, I suggested that you get
rid of `static' "in both places," meaning both on the declaration
of the pin variable and on the checkPin() method. From your
vague description of the problem, I suspect you have eliminated
only one occurrence of `static'.

If you have eliminated both and are still having trouble,
please post a *complete* self-contained example of the code
and the *exact* text of any error messages you receive. We're
not mind readers.

 

[Flo 'Irian' Schaetz]

And thus spoke (e-mail address removed)...

Originally that is what I had but when I tried to check the PIN in my
Bank class it was giving me problems about it being non-static. So
that is why I had changed it originally.

public double getBalance(String nameOfOwner, int finalPin)
{
int pin = BankAccount.checkPin();

THIS won't work, of course, if you make the pin non-static (as it should
be). You will need something like...

public double getBalance(Account account, int finalPin)

....or...

public double getBalance(String nameOfOwner, int finalPin)
{
BankAccount account = ... // Somehow get the account of the owner
int pin = account.checkPin();

Oh, and btw: As your method "checkPin" doesn't really "check" the pin,
justs gets it, I would call it "getPin" instead. checkPin would be more
something like...

boolean checkPin(int accountPin, int givenPin) {
if (accountPin == givenPin) {
return true;
}
return false;
}

Flo

 

[jupiter]

And thus spoke (e-mail address removed)...


This means, that only ONE Pin does exist, no matter how many
accounts
you create. Every account will access THIS ONE Pin. Are you
really sure
that you want to do that?

What's the matter with that? There's only one customer at his
lousy bank.

 

[Stefan Ram]

What's the matter with that? There's only one customer at his
lousy bank.

/* Why? Here is a class with a static field: */

public class Scan { static int i = 0; public static int get(){ return i++; }}

/* Let's create two instances of it: */

public class Main
{ public static void main( java.lang.String[] args ) throws java.lang.Exception
{ final java.lang.Class instance1 = new ScanLoader().loadClass( "Scan" );
final java.lang.Class instance2 = new ScanLoader().loadClass( "Scan" );
final java.lang.Class[] c = new java.lang.Class[]{};
final java.lang.Object[] o = new java.lang.Object[]{};
java.lang.System.out.println( instance1.getMethod( "get", c ).invoke( null, o ));
java.lang.System.out.println( instance1.getMethod( "get", c ).invoke( null, o ));
java.lang.System.out.println( instance2.getMethod( "get", c ).invoke( null, o ));
java.lang.System.out.println( instance2.getMethod( "get", c ).invoke( null, o )); }}

/* There are really two different static fields, as they can
count independently:

0
1
0
1

The rest of the code: */

class ScanLoader extends ClassLoader
{ public ScanLoader()
{ super(ScanLoader.class.getClassLoader()); }
public java.lang.Class<?> loadClass( final java.lang.String className)
throws ClassNotFoundException
{ return findClass( className ); }
public java.lang.Class<?> findClass( final java.lang.String className )
{ byte classByte[];
java.lang.Class<?> result = null;
if( "java.lang.Object".equals( className ))return java.lang.Object.class;
try
{ java.lang.String classPath =
((java.lang.String)ClassLoader.getSystemResource
( className.replace( '.', java.io.File.separatorChar ) + ".class" ).getFile() ).
substring( 1 );
classByte = loadClassData(classPath);
result = defineClass(className,classByte,0,classByte.length,null);
return result; }
catch(final java.lang.Exception e){ throw new java.lang.RuntimeException(e); }}
private byte[] loadClassData
( final java.lang.String className )
throws java.io.IOException
{ final java.io.File f = new java.io.File( className );
final int size =( int )f.length();
final byte buff[] = new byte[ size ];
{ java.io.FileInputStream fis = new java.io.FileInputStream( f );
{ java.io.DataInputStream dis = new java.io.DataInputStream( fis );
dis.readFully( buff );
dis.close(); }}
return buff; }}

 

[Woot4Moo]

Hi guys I am working on a program and I cant seem to figure out where
it is I am going wrong.
It is involving Bank accounts and PINs. I have a Bank class and a
BankAccount class.

In my BankAccount class I have a variable declared as private static
int pin.
as well as:
public static int checkPin()
{
return pin;
}

In my Bank class I have
public double getBalance(String nameOfOwner, int finalPin)
{
int pin = BankAccount.checkPin();
if(pin != finalPin) return -1;
// rest of code to get balance, which works properly.
}

My question is how do I make it so the pin can be referenced for each
account and also so that pin does NOT get over written everytime a new
account is created. Because currently what happens is the pin is only
stored for the last account created.

Thanks

And now
public class BankAccount
{
private final String nameOfOwner;
private double balance;
private int pin;
}
public BankAccount( String nameOfOwner)
{
this(nameOfOwner,0,0) ;
}
public BankAccount(String nameOfOwner, double balance, int pin)
{
if(balance < 0 )
throw new IllegalArgumentException("Balance must not be negative.
");
this.nameOfOwner = nameOfOwner;
this.balance = balance;
this.pin = pin;
}

public String getNameOfOwner()
{
return nameOfOwner;
}

public double getBalance()
{
return balance;
}

private String secureToString()
{
return "Balance : $ "+ balance + "Name of owner : " + nameOfOwner;
}

public int checkPin()
{
return pin;
}
}

public class Bank
{
private ArrayList(BankAccount) accounts;
}

public Bank()
{
accounts = new ArrayList(BankAccount)();
}

public boolean addAccount(String nameOfOwner, double initialBalance,
int pin)
{
if( initialBalance < 0 ) return false;
int index = getAccountIndex(nameOfOwner);
if( index != -1) return false;
accounts.add( new BankAccount( nameOfOwner,initialBalance, pin ));
return true;
}

public double getBalance(String nameOfOwner, int finalPin)
{
int pin = BankAccount.checkPin();
if(pin != finalPin) return -1;
int index = getAccountIndex(nameOfOwner);
if( index == -1) return -1;
double finalBalance = accounts.get(index).getBalance();
return finalBalance;
}

private int mostRecentIndex = -1;

public int getAccountIndex(String nameOfOwner)
{
if( ( 0 <= mostRecentIndex && mostRecentIndex < accounts.size() )
&& nameOfOwner.equalsIgnoreCase(
accounts.get(mostRecentIndex).getNameOfOwner() ) )
return mostRecentIndex;
for( int i = 0; i < accounts.size(); i++)
{
if( nameOfOwner.equalsIgnoreCase(
accounts.get(i).getNameOfOwner() ) )
{
mostRecentIndex = i;
return i;
}
return -1;
}
}

There ya go guys and the error I get is
non-static method getPin() cannot be referenced from a static context

int pin = BankAccount.checkPin();
^

 

[Eric Sosman]

And now
public class BankAccount
{
private final String nameOfOwner;
private double balance;
private int pin;
}

... and right there at the closing } is where the BankAccount
class ends. Nothing else you've provided is part of BankAccount
(in fact, it's just a bunch of syntax errors caused by placing
must-be-inside-a-class things outside of a class).

Woot4Moo, if you want help debugging some code you must
provide the *actual* *code* and not some half-hearted rough-
and-ready close-enough-for-jazz paraphrase. When you visit the
doctor because you broke your right arm, do you present your
left foot for his examination?

Oh, well -- I have a pretty good idea what's wrong, despite
all your attempts to obfuscate the matter, and in the interests
of Just Getting It Over With ...

> non-static method getPin() cannot be referenced from a static context
>
> int pin = BankAccount.checkPin();

On various people's advice you've made pin non-static, so
each BankAccount instance can have its own value. And you've
made checkPin() non-static so it can work on "this" BankAccount
instance instead of working on the BankAccount class. Fine.

But: Which BankAccount instance owns the pin that you are
trying to check? If you walk into a large bank and ask "Is
this PIN valid?" the likely answer is "Probably." (The bank
has 100000 accounts and there are only 10000 possible PIN
values, so chances are pretty good that any PIN you choose is
valid for *some* account.)

Instead, you need to ask "Is this PIN valid for account
number such-and-such?" You must ask the question in relation
to some particular BankAccount, not in relation to the "schema"
from which all BankAccounts are created. So: You start with
the name of the account owner, from that you must find the
BankAccount he owns, and *then* you can ask *of that BankAccount*
whether the pin is correct.

 

[Woot4Moo]

Most appreciated even though I did think that those were 2 seperate
classes my eyes must really be going.

 

[John Ersatznom]

(e-mail address removed) wrote:
[snip]

I find it interesting that nobody has commented on:
a) The suspicious resemblance to a homework assignment (I haven't had a
comp sci class that *didn't* involve a "Bank and BankAccount" example or
exercise, unless it didn't involve *either* a) OO programming or b) DMBS
*at all*.)
b) The use of "double" to store dollar amounts. Rather than e.g. int
balanceInCents; and methods like getDollars, getChange,
getTotalBalanceInCents ... do you *really* want to sock $499.99 in the
bank and withdraw $499.98999909125?

 

[Woot4Moo]

(e-mail address removed) wrote:
[snip]

I find it interesting that nobody has commented on:
a) The suspicious resemblance to a homework assignment (I haven't had a
comp sci class that *didn't* involve a "Bank and BankAccount" example or
exercise, unless it didn't involve *either* a) OO programming or b) DMBS
*at all*.)
b) The use of "double" to store dollar amounts. Rather than e.g. int
balanceInCents; and methods like getDollars, getChange,
getTotalBalanceInCents ... do you *really* want to sock $499.99 in the
bank and withdraw $499.98999909125?

Well John Ill give that much to you, quite close in saying that its
a hw assignment it was something given to finish out the semester which
ended 2 weeks ago. Grades are in and such Im just trying to wrap my
mind around it at this point before I have a second semester of java.
I really am just quite stumped on this, I failed this program,
obviously. So all in all I cant really have it change my grade, but I
would like to have the concept under my belt.

 

[Chris Smith]

Most appreciated even though I did think that those were 2 seperate
classes my eyes must really be going.

I'm not trying to drag this out unnecessarily, but copy and paste is a
great thing. It means that even if you don't notice something about
your own code, you'll still manage to post the right code in your
newsgroup message, so that someone else can spot it for you. (It's also
a heck of a lot easier than retyping everything all the time!)

 

[jupiter]

/* Why? Here is a class with a static field: */

Sorry, I was only making a joke (about the bank only having one
customer), Stephan.

Nevertheless, you provided a nice example. Thanks, I've never
stepped through it like that.

public class Scan { static int i = 0; public static int get(){
return i++; }}

/* Let's create two instances of it: */

public class Main
{ public static void main( java.lang.String[] args ) throws
java.lang.Exception
{ final java.lang.Class instance1 = new
ScanLoader().loadClass( "Scan" );
final java.lang.Class instance2 = new
ScanLoader().loadClass( "Scan" );
final java.lang.Class[] c = new java.lang.Class[]{};
final java.lang.Object[] o = new java.lang.Object[]{};
java.lang.System.out.println( instance1.getMethod( "get",
c ).invoke( null, o ));
java.lang.System.out.println( instance1.getMethod( "get",
c ).invoke( null, o ));
java.lang.System.out.println( instance2.getMethod( "get",
c ).invoke( null, o ));
java.lang.System.out.println( instance2.getMethod( "get",
c ).invoke( null, o )); }}

/* There are really two different static fields, as they can
count independently:

0
1
0
1

The rest of the code: */

class ScanLoader extends ClassLoader
{ public ScanLoader()
{ super(ScanLoader.class.getClassLoader()); }
public java.lang.Class<?> loadClass( final java.lang.String
className)
throws ClassNotFoundException
{ return findClass( className ); }
public java.lang.Class<?> findClass( final java.lang.String
className )
{ byte classByte[];
java.lang.Class<?> result = null;
if( "java.lang.Object".equals( className ))return
java.lang.Object.class;
try
{ java.lang.String classPath =
((java.lang.String)ClassLoader.getSystemResource
( className.replace( '.', java.io.File.separatorChar ) +
".class" ).getFile() ).
substring( 1 );
classByte = loadClassData(classPath);
result =
defineClass(className,classByte,0,classByte.length,null);
return result; }
catch(final java.lang.Exception e){ throw new
java.lang.RuntimeException(e); }}
private byte[] loadClassData
( final java.lang.String className )
throws java.io.IOException
{ final java.io.File f = new java.io.File( className );
final int size =( int )f.length();
final byte buff[] = new byte[ size ];
{ java.io.FileInputStream fis = new java.io.FileInputStream(
f );
{ java.io.DataInputStream dis = new
java.io.DataInputStream( fis );
dis.readFully( buff );
dis.close(); }}
return buff; }}

 

[Juha Laiho]

(e-mail address removed) said:

Hi guys I am working on a program and I cant seem to figure out where
it is I am going wrong.
It is involving Bank accounts and PINs. I have a Bank class and a
BankAccount class.

In my BankAccount class I have a variable declared as private static
int pin.
as well as:
public static int checkPin()
{
return pin;
}

In my Bank class I have
public double getBalance(String nameOfOwner, int finalPin)
{
int pin = BankAccount.checkPin();
if(pin != finalPin) return -1;
// rest of code to get balance, which works properly.
}

You already have gotten advise on some other aspects, but I'd like to
comment on your naming of things.

When you look at the "checkPin" method, does it really check a given pin?
No; what it does is that it gets the pin. Actually you check the pin in your
Bank.getBalance method (the "pin != finalPin" expression).

Rather, you could change the method to really check a pin, by changing
the return value into boolean - like
public boolean checkPin(int pinInput)
{
return (pin == pinInput);
}

.... which you would then use like

public double getBalance(String nameOfOwner, int finalPin)
{
BankAccount account;
// somehow fetch the account for the owner, then check the pin
if(account.checkPin(finalPin)) return -1;
// rest of code to get balance, which works properly.
}

Of course, to continue with this trend, I'm not certain whether
checking the pin belongs into a method called getBalance.

 

[Matt Humphrey]

(e-mail address removed) wrote:
[snip]

I find it interesting that nobody has commented on:
a) The suspicious resemblance to a homework assignment (I haven't had a
comp sci class that *didn't* involve a "Bank and BankAccount" example or
exercise, unless it didn't involve *either* a) OO programming or b) DMBS
*at all*.)

I'm sure everyone here recognized this as homework, but sidestepped the
issue because the OP politely asked for help on a specific question, not for
people to do his homework for him. Homework is not verboten, but must be
asked and answered more carefully. (Guidelines are available on the web.)

b) The use of "double" to store dollar amounts. Rather than e.g. int
balanceInCents; and methods like getDollars, getChange,
getTotalBalanceInCents ... do you *really* want to sock $499.99 in the
bank and withdraw $499.98999909125?

Someone must make the comment first--I guess it's you.

Matt Humphrey (e-mail address removed) http://www.iviz.com/

 

[Patricia Shanahan]

(e-mail address removed) wrote:
[snip]

I find it interesting that nobody has commented on:
a) The suspicious resemblance to a homework assignment (I haven't had a
comp sci class that *didn't* involve a "Bank and BankAccount" example or
exercise, unless it didn't involve *either* a) OO programming or b) DMBS
*at all*.)
b) The use of "double" to store dollar amounts. Rather than e.g. int
balanceInCents; and methods like getDollars, getChange,
getTotalBalanceInCents ... do you *really* want to sock $499.99 in the
bank and withdraw $499.98999909125?

Well John Ill give that much to you, quite close in saying that its
a hw assignment it was something given to finish out the semester which
ended 2 weeks ago. Grades are in and such Im just trying to wrap my
mind around it at this point before I have a second semester of java.
I really am just quite stumped on this, I failed this program,
obviously. So all in all I cant really have it change my grade, but I
would like to have the concept under my belt.

You seem to have grasped the most important point, that learning to
program is a cumulative process, and you will need to understand the
first semester material in order to understand what follows.

Perhaps you should try applying the advice so far, and post you new best
effort, working or not. Do copy-paste, without retyping, so that we get
to see exactly what you are doing.

The newsgroup, collectively, has a lot of experience at working out what
concepts, if any, someone is missing from their code attempts.

Patricia

 

[Woot4Moo]

(e-mail address removed) wrote:
[snip]

I find it interesting that nobody has commented on:
a) The suspicious resemblance to a homework assignment (I haven't had a
comp sci class that *didn't* involve a "Bank and BankAccount" example or
exercise, unless it didn't involve *either* a) OO programming or b) DMBS
*at all*.)
b) The use of "double" to store dollar amounts. Rather than e.g. int
balanceInCents; and methods like getDollars, getChange,
getTotalBalanceInCents ... do you *really* want to sock $499.99 in the
bank and withdraw $499.98999909125?

Well John Ill give that much to you, quite close in saying that its
a hw assignment it was something given to finish out the semester which
ended 2 weeks ago. Grades are in and such Im just trying to wrap my
mind around it at this point before I have a second semester of java.
I really am just quite stumped on this, I failed this program,
obviously. So all in all I cant really have it change my grade, but I
would like to have the concept under my belt.

You seem to have grasped the most important point, that learning to
program is a cumulative process, and you will need to understand the
first semester material in order to understand what follows.

Perhaps you should try applying the advice so far, and post you new best
effort, working or not. Do copy-paste, without retyping, so that we get
to see exactly what you are doing.

The newsgroup, collectively, has a lot of experience at working out what
concepts, if any, someone is missing from their code attempts.

Patricia

Well it really does come down to me trying to reference a method from
my BankAccount class into my Bank class, the getPin(). I honestly
cant seem to figure out how to get around the whole static issue.
Maybe I am overanalyzing or perhaps even underanalyzing my situation.
Then again maybe I am just so far lost I should take some time away
from this and come back to it after a day or so of not thinking about
it. I do appreciate all the help though.