Question

I am attempting to convert arrays of primitive double values to objects. As a result I am getting a “type mismatch error”

private double[]purchases;

to

private CreditCard[]purchases;

then when I try to add a value to the array

public void purchase(double amount)throws Exception
{
    if (numPurchases<purchases.length)
        if (amount >0 )
            if(amount+balance<=creditLimit)
                if( GregorianCalendar.getInstance().getTimeInMillis()<=expDate.getTimeInMillis())
                    {
                     balance+=amount;

                     purchases[numPurchases]= amount;
                     numPurchases++;

                  }
                else
                {
                    throw new Exception("card expired");
                }
            else{
                throw new Exception("insufficient credit");
                }
        else{
            throw new Exception("invalid amount");

        }
    else{
        throw new Exception("exceeded number of allowed purchases");
    }


}

the error message says type mismatch for amount “cannot convert from double to CreditCard
how can I correct the code to allow me to add purchase amounts to the array?

Answer 1

The general point here is that you’ve defined purchases so that it must only contain CreditCard instances:

private CreditCard[] purchases;

The type you specify here controls what you’re allowed to place in it later. You then attempt to place a double into the array:

purchases[numPurchases] = amount;

But you just told the compiler that purchases is only allowed to contain CreditCards!

You need to wrap your double in a CreditCard instance first.

Imagine you have the following class:

public class CreditCard {
    private double amount;
    public CreditCard(double amount) {
        this.amount = amount;
    }
}

Now you can do this:

purchases[numPurchases] = new CreditCard(amount);

…because the thing you’re putting into the array has the correct type.

On a side note, consider renaming your class to CreditCardPurchase, if that’s what it really represents. The name of your class should say something about what it is. If it’s going into a purchases array, then it’s probably a purchase, not the credit card itself.