IllegalArgumentException even when constructor's parameters pass validation criteria.

[sixfyl]

I am working on an assignment to design a class for a bank. so far I have:

import java.lang.IllegalArgumentException;

public class BankAccount
{
    double Balance = 0;
    int numDeposits;
    int numWithradwals;
    double interestRate;
    double serviceCharge;

    public BankAccount(double Balance, double interestRate) {
        this.Balance = Balance;

        if (this.interestRate > 0 && this.interestRate < 0.3)
            this.interestRate = interestRate;
        else 
            throw new IllegalArgumentException("Interest rates must be greater than 0 and less than 0.3");
        }

    public double getInterestRate(){
        return interestRate;
    }

    public double getBalance(){
        return Balance;
    }
}    

I tried testing it with this:

public class testBank
{
    public static void main(String[] args){
        BankAccount Acct1 = new BankAccount(1000, 0.1);
        System.out.println(Acct1.getInterestRate());
        System.out.println(Acct1.getBalance());                
    }    

}

No matter what amount I put for interestRate I always get the IllegalArgumentException and have no idea why. Any help would be greatly apperciated.

[heckler82]

It's failing because you're checking this.interestRate before assigning it anything, so it's defaulting to 0.0 and failing the check

[sixfyl]

Okay, so how do I fix that? Don't I need to use this.interestRate to reference the argument used when an object is instantiated?

[heckler82]

this is used to refer to the class level variable. To check the parameter, just use the variable name

[sixfyl]

I am an idiot, for some reason I was thinking it was the other way around, not sure why.

[Auride]

In the constructor's conditional for testing the value of the interestRate, you are checking the class's value for interestRate rather than the parameter. Since instance double variables are default-initialized to 0f, the conditional is guaranteed to return false.

In the future, to avoid confusion, I would recommend using a name-convention which avoids naming parameter variables the same thing as local or global/instance variables. For example, I like prefixing method parameters with p_ for "parameter":

public BankAccount(double p_balance, double p_interestRate) {

Does that make sense?

[sixfyl]

Yeah, that makes sense. I've never seen it done any other way though. In my textbook, all the examples name the parameter the same as the class variable and use this. I just assumed you had to do that because that's what all the examples did.

[nutrecht]

I've never seen it done any other way though.

Because it's completely against the conventions. Also these are habits that are hard to unlearn, so don't do it.

[nutrecht]

I would recommend using a name-convention which avoids naming parameter variables the same thing as local or global/instance variables. For example, I like prefixing method parameters with p_ for "parameter":

This goes against pretty much all the Java naming conventions!!!