Class help

[Shaolin]

Hey Guys,

I'm defining my class for my mini java project. I'm very new to all of this. I'm trying to figure out how the following code works and whether it is correct:

private int accountbalance;

      public deposit( double amount )
      {
      	return accountbalance = accountbalance + amount ;
      }

If the amount is $20 and I want to return the total in "accountbalance", is the above code correct ? I don't know if it should be:

return accountbalance = accountbalance + amount ;

-OR-

return accountbalance + amount ;

[JT4NK3D]

The correct return statement would be return accountbalance + amount;
alternatively i think you could either declare a new variable and assign accountbalance + amount, or you could initialize accountbalance to be accountbalance + amount and return that ( use shorthand accountbalance += amount, shorthand for accountbalance = acountbalance + amount)

personally, i'd stick with return accountbalance + amount.

hope that helped :p

[Shaolin]

Thanks. I've done that. Here is the updated code:

      public int deposit( double amount )
      {
      	return accountbalance + amount ;
      }

But I keep getting this error when I build the class:

possible loss of precision
found   : double
required: int
        return accountbalance + amount ;

[JT4NK3D]

hmm.. the argument amount is a double, so any number would be a double.. unless you declared an int variable, and called the method with the name of the int variable for the argument. but then it would have been found int required double, so i'm guessing its the problem with accountbalance.. i would have to see the rest of the class(es) to figure anything else out.

[Shaolin]

accountBalance is double.

Do I need to change:

public int deposit( double amt )

-TO-

public double deposit( double amt )


??

[JT4NK3D]

yes thats it... you said it returns a int in the method signature thing but with return statement returned a double ( even if just account balance or just amount is double, value will be superior primitive - double.) change that to a public double right on

[JT4NK3D]

and remember if you ever declare a method without void or a return type, its automatically void.

[Shaolin]

Thanks, I don't fully understand what you meant by your last post.

I have another question, I want to use a boolean, here is my function:

      public boolean isOpen(boolean i)
      {
      	if(i == false) 
      	{
      		return false; /*Not Open*/
      	} 
      		else 
      			{
      				return true; /*Is Open*/
      			}
      }

My question is, on the declaration line, do I need to declare a bool between the brackets ?

public boolean isOpen(boolean i)

-OR-

public boolean isOpen()

Which one is correct ? Or the better practise ?

[blackstone]

Yes, you do need to have the boolean in the parentheses.

public boolean isOpen(boolean i)

would be the correct way to do it. The boolean inside the parentheses passes the argument (or parameter, whatever you like to call it) to the method so that the method can use that variable. Without it there, you wouldn't be able to use i in your method.

[Shaolin]

Thanks. Yet another question. Is the following code ok:

      public double takeMoneyOut( double amount )
      {
      	double totalBalance;
      	totalBalance = balance + overdraftLimit ;
      	
      	if( amount > totalBalance ) {
      		// error message
      	}
      	else
      	{
      		// return totalBalance 
      	}
      }

[blackstone]

Sure! It doesn't do a whole lot at the moment, but everything in the code is good. The only thing I noticed was that there is a possibility that you don't return anything, which would cause an error. Since you said that the method would return a double, it has to eventually return a double.

The case where it wouldn't return a double would be if amount > totalBalance. In that case, you say it is going to give an error message. It might be better to return something like a -1 (or some other obscure value that it shouldn't be returning) and then handle the error outside of the method, wherever you call it.

Erm... does that make sense? The main thing I'm trying to say is that if you say you are going to return a double, you have to return a double no matter what.

[Shaolin]

So its not wise to output error messages in classes ? If yes, how come ?

I might return 0 or something.

[blackstone]

Not necessarily. I'm not exactly an expert on what is right and what is wrong, but I'm guessing that in some cases it would be perfectly fine and even logical to print errors messages in the class. In your case, however, you need to return a double. If you wanted to print out an error and then return the double as well, that might work.

0 might not work, if the amount being taken out is exactly the same as the amount in the balance.

[Shaolin]

No problem. Ok Im trying to do something else. From my function "takeMoneyOut" I want it to call another boolean function which will check if the dollars being taken out is over the accountbalance limit and thus return a true or false. But I dont know how I can do this. See code:

      public double takeMoneyOut( double amount )
      {
      	double totalBalance;
      	totalBalance = balance + overdraftLimit ;      	

      	if( CheckWithdrawal(amount) == false ) {
      		// error message
      	}
      	else
      	{
      		// return totalBalance 
      	}
      }

      public boolean CheckWithdrawal(boolean checkWithdraw)
      {
      	if(checkWithdraw > balance )
      	{
      		return true;
      	}
      	else
      	{
      		return false;
      	}
      }

This code doesnt work though.

[JT4NK3D]

i think it will work after changing these things:

public double takeMoneyOut( double amount )
      {
      	double totalBalance;
      	totalBalance = balance + overdraftLimit ;      	

      	if( CheckWithdrawal(amount) == false ) {
      		[B]System.err.println("You tried to withdraw more than you have.");[/B]
      	}
      	else
      	{
      		[B]return totalBalance[/B] 
      	}
      }

      public boolean CheckWithdrawal(boolean checkWithdraw)
      {
      	if(checkWithdraw > balance )
      	{
      		return true;
      	}
      	else
      	{
      		return false;
      	}
      }

if that doesn't work at the if statement instead of if checkwithdrawal(amount) == false try making a boolean variable and assigning yourObject.CheckWithdrawal(amount); then doing if ( yourbool == false ) code.
btw u could do the error with system.out, err just means its an error

[Shaolin]

Thanks, I get the following errors when I compile it:


code: if( CheckWithdrawal(amount) == false )

error: CheckWithdrawal(boolean) in Account cannot be applied to (double)
if( CheckWithdrawal(amount) == false )


code: if(check > balance )

error: operator > cannot be applied to boolean,double
if(check > balance )

In essence what I'm doing is comparing a boolean against a value. How can I use the object one ? would I have to call it from main() ?

[hardwired]

public double takeMoneyOut( double amount )
{
    double totalBalance;
    totalBalance = balance + overdraftLimit;      	

    if( CheckWithdrawal(amount) == false ) {
        // error message
    }
    else
    {
        // return totalBalance 
    }
}

public boolean CheckWithdrawal(boolean checkWithdraw)
{
    if(checkWithdraw > balance )
    {
        return true;
    }
    else
    {
        return false;
    }
}

In the takeMoneyOut method the argument "amount" is of type double. The if statement in this method is calling the CheckWithdrawal method sending the value of the "amount" variable. So this method call is to a (n apparantly non-existent) method with this type signature:

boolean CheckWithdrawal(double value)

But the type signature of your CheckWithdrawal method is this:

boolean CheckWithdrawal(boolean checkWithdraw)

So java cannot find a method with the type signature that your takeMoneyOut method is calling, ie, java cannot find a takeMoneyOut method that takes a (type) double argument. This causes the first compile error:

error: CheckWithdrawal(boolean) in Account cannot be applied to (double)
if( CheckWithdrawal(amount) == false )

In the CheckWithdrawal method the argument "checkWithdraw" is (primitive type) boolean. In the if statement the condition has a type mis-match:

//   boolean       double
if(checkWithdraw > balance )

which results in the second compile error

error: operator > cannot be applied to boolean,double
if(check > balance )

To compare these two variables they must be the same type. Java is very prissy/picky about variable types. This is to avoid confusion: so everyone is very clear about what types are being passed around/manipulated by the jvm (java virtual machine).
So how to fix this?
Change this

public boolean CheckWithdrawal(boolean checkWithdraw)

to this

public boolean CheckWithdrawal(double checkWithdraw)

[Shaolin]

Excellent, fixed!

I have some more questions.

1. what does the ++ mean when it is infront of a variable e.g.

int n;
n = ++MyAccount;

Also, a conversion constructor:

1. What is it and whats its purpose ?
2. If I wanted to create one to deal with the customers withdrawal/deposits how would I do that ?

[hardwired]

what does the ++ mean when it is infront of a variable
Look at the last two paragraphs on this page Assignment, Arithmetic, and Unary Operators.
conversion constructor
Seems to be a C++ term; I'm not familiar with it in java. If you can give some
more detail we might be able to come up with a java solution for what you want to do.

[Shaolin]

Thanks. I guess my friend coined up that term then. I will ask him what he meant by that.

If I have the following line in main():

trans t1 = new Trans( account1, Operation.WITHDRAWAL, 100 ) ;

and in the class I have

public class trans {

	private Operation op ; /*Deposit or Withdrawal*/
	private Account acc ;  /*account for which a transaction*/	
private double amt ;    /*amount of money involved in that transaction, deposit or withdrawal*/

}

If I want to proccess the values from the line from main() in the trans class, how would I do that ?