Advanced Inheritance.

[Sean Heise]

So I have an assignment that was due yesterday for my Java class (I know I'm slacking, but Trig seemed more important as I have a test next week and it's kicking the crap out of me at the moment).
The assignment is as follows:
1) For the Account class, create: a) two protected variables for the account number and the account balance. b) two get methods - one for the account number and one for the account balance c) two set methods - one for the account number and one for the balance. d) a constructor that requires an account number and sets the balance to 0. e) a method to override the toString() method which returns a String containing: Account # = < account number> Account Balance = <account balance> where <account number> is the object's account number and <account balance> is the object's account balance. f) an abstract computeInterest() method that takes one integer argument and returns a double. g) Do not allow either the account number or the balance to be set to a negative number. 2) The Checking class will be a subclass of Account. For the Checking class, create: a) a method to override the toString() method which returns a String containing: "Checking" , the account number, and the account balance. Do not print the interest earned as part of toString(). b) a constructor that takes an account number as an argument and calls the superclass constructor passing this account number. c) a method to implement the abstract computeInterest() method of the superclass. This method takes one argument for the interest period and returns the interest earned. The interest earned is 2 % of the balance over $700 times the interest period. The interest period is passed as an argument to the computeInterest() method. For example, the interest on $3000 at 2% for three years is (3000 - 700) times .02 times 3 or 138. 3) The Savings class will be a subclass of Account. For the Savings class, create: a) an additional private variable to hold the interest rate. b) a get and set method for the interest rate. Do not allow the interest rate to be set to a negative number. c) a method named toString() that overrides the toString() method of the Object class. This method should return "Savings", the account number, the account balance and the interest rate. Do not print the interest earned as part of toString(). d) a constructor with two arguments - the account number and the interest rate. This constructor calls the superclass constructor passing the account number. e) a method to implement the abstract computeInterest() method of the superclass. This method takes one argument for the interest period and returns the interest earned. The interest is calculated as (1 + interest rate)period times the balance minus the balance. The interest period is passed as an argument to the computeInterest() method. For example, the interest on $6000 at 2% for three years is (1.02)3 * 6000 - 6000 or 367. 4) For the AccountArray class, create: a) a main() method. b) an array to store 10 objects of either the Savings or Checking class. Use 2% (.02) for the savings account rate. Use account numbers 100 through 109. Use initial balances of 1000 through 10000. c) a for loop that will instantiate 5 objects of the Savings class and 5 objects of the Checking class. Store these objects in a single array created above. d) a for loop that prints the data in all 10 objects of the array using the overridden toString() method and also prints the interest amount computed for each object. Use 3 for the period. The results are displayed in the sample output.

So far I've got:

public abstract class Account 
{
	protected long accountNumber;
	protected double accountBalance;
	public long getAccountNumber()
	{
		return accountNumber;
	}
	public double getAccountBalance()
	{
		return accountBalance;
	}
	public void setAccountNumber()
	{
	}
	public void setAccountBalance()
	{
	}
	public void setUpAccount(long number, double  balance) //look up constructors
	{
		number = accountNumber;
		balance = accountBalance;
	}
	public String toString()
	{
		return "Account Number: " + accountNumber + " Account Balance: " + accountBalance;
	}
	public abstract double computeInterest(int intPeriod);
}


public class Checking extends Account
{
	public String toString()
	{
		return "Checking " + accountNumber + " Account Balance " + accountBalance;
	}
	public Checking(long number)
	{
		number = accountNumber;
	}
	public double computeInterest(int intPeriod) 
	{
		double total = ((accountBalance - 700) * .02) * 3;
		return total;
	}
}


public abstract class Savings extends Account
{
	private double interestRate;
	public double getInterest()
	{
		return interestRate;
	}
	public void setInterest()
	{
		
	}
	public String toString()
	{
		return "Savings " + accountNumber + " Account Balance "
		   + accountBalance + " Interest Rate " + interestRate;
	}
	public Savings(long number, double interest)
	{
		number = accountNumber;
		interest = interestRate;
	}
	public double computeInterest(int intPeriod) 
	{
		double total = Math.pow((1 + interestRate), intPeriod) * accountBalance - accountBalance;
		return total;	
	}
}



public class AccountArray 
{
	public static void main(String[] args)
	{
		Account[] refAccount = new Account[10];
		refAccount[0] = new Checking(100);
		refAccount[1] = new Checking(101);
		refAccount[2] = new Checking(102);
		refAccount[3] = new Checking(103);
		refAccount[4] = new Checking(104);
		refAccount[5] = new Savings(105, .02);
		refAccount[6] = new Savings(106, .02);
		refAccount[7] = new Savings(107, .02);
		refAccount[8] = new Savings(108, .02);
		refAccount[9] = new Savings(109, .02);
		
	}

}
}

My main questions are on steps:

1d, 2b, 3d, these are for the constructors which I must have one in each class except the AccountArray class. I believe I've set them up correctly, but I'm not 100% sure. I know I should know this, but I guess I'm just not seeing how it ties into the rest of the program.

also on 1e, 2a, 3c... which is creating the toString method overrides again, I'm not quite sure if I'm doing this correctly (first time I've learned about or utilized them.) Being that the output is supposed to look like this:

Checking
Account Number: 100
Account Balance: 1000.0
Interest Earned: 18.0

I imagine I should have \n in there between each line I want to create?

I'm also having problems with the set methods for some reason. I can't for the life of me think of what I should have in their bodies, being that the program is set up the way it is.


Aaaaaand lastly basically the entire last class I'm not quite sure what I should do. I've got the array set up for ten place holders but other than that I'm not quite sure what I should do next. Should I instantiate a checking and savings for each place holder? So checking would take the first five place holders and savings would take the last five? Then how do I work in the constructors?

Anyway I guess I'm going to keep plugging away at it.

Any help would be appreciated. As I've written this, I think I've come up with an idea for figuring out how to put together the last class. We'll see I suppose. Thanks in advance.

Edit: As I said I think I might have gotten the last class down, although I do still need some help. I just want to make sure that everything is correct.

[Enkita mody]

UseCodeTags (<---click)

[Sean Heise]

Enkita mody wrote:UseCodeTags (<---click)

Yea, sorry about that, I'm used to being on another forum and accidently used the incorrect tag for it. But I fixed it.

[Henry Wong]

Sean Heise wrote:
1d, 2b, 3d, these are for the constructors which I must have one in each class except the AccountArray class. I believe I've set them up correctly, but I'm not 100% sure. I know I should know this, but I guess I'm just not seeing how it ties into the rest of the program.

IMHO, that is a big issue with these "color by numbers" homework assignments. You know what you are doing, but in many cases, you don't know *why* you are doing it.

For this question, one way to think about it is -- a constructor is used to initialize an instance as it is being instantiated. You need to think about (1) what the parameters are for, in order to enable the caller to pass you configuration details, and (2) what it needs to implement that initialization, including all the components of the class, and its super class. Hopefully, this helps, but if not, then you need to step back, look at the instructions again, and try to see what it is doing -- try to see the "big picture" from the "color by numbers" instructions.

Henry

[Campbell Ritchie]

Another problem about what Henry calls “colour by numbers” is that you don’t know when something would be better done otherwise. Because you are told to use protected fields, for example, you never learn how much better private fields would have been.

[Sean Heise]

That's exactly how I feel. I've never used constructors this way.

But I think I've figured it out.
However, the balance is now what I'm pondering, how it ties into the rest of the program.

Here you see I've set up my array of ten account objects and have instantiated new objects of each class and used their constructor in order to fill in the information as per the instructions.

public class AccountArray   
{  
    public static void main(String[] args)  
    {  
        Account[] refAccount = new Account[10];  
        refAccount[0] = new Checking(100);  
        refAccount[1] = new Checking(101);  
        refAccount[2] = new Checking(102);  
        refAccount[3] = new Checking(103);  
        refAccount[4] = new Checking(104);  
        refAccount[5] = new Savings(105, .02);  
        refAccount[6] = new Savings(106, .02);  
        refAccount[7] = new Savings(107, .02);  
        refAccount[8] = new Savings(108, .02);  
        refAccount[9] = new Savings(109, .02);  
          
    }  
  
}  
}  

So I've got my five Checking and five savings as per the assignment instructions. But now I'm supposed to work in the balance of 1000 - 10000, I'm not seeing where this is supposed to be input. The only constructor that asks for a balance is the Account class. Being that the Account class is the super and checking and savings are both subclasses, I thought perhaps I'd be able to input the balance directly into the constructors being that the constructor would have been inherited (so maybe that the constructors for savings/checking might have inherited the spot for balance into their own constructors), but this isn't the case.

I really kind of wish we could get more practice. We're able to do the "you do it" portions of the text book, but being that this is simply a "copy it out of the book" type of practice it doesn't really help me. There are vague descriptions that go along with it that I try to decipher, but don't usually get very far with them.

[Sean Heise]

Campbell Ritchie wrote:Another problem about what Henry calls “colour by numbers” is that you don’t know when something would be better done otherwise. Because you are told to use protected fields, for example, you never learn how much better private fields would have been.

This is also true, however we do go over them in class.
But you're correct, not just for field modifiers. I understand that in inheritance you can't use private variables/methods in any class other than the class that it's called in. And for protected variables/methods you can't use them outside of the super/subclasses... etc.
I atleast understand some principles.

I kind of wish we'd have multiple little programs that would work up to the final weekly program, so as to drill home the principles used in each chapter.

[Campbell Ritchie]

Ask your instructor what protected means. A lot of people thing it means accessible in the class and its subclasses, but that is not correct.

[Winston Gutkowski]

Sean Heise wrote:Here you see I've set up my array of ten account objects

Just FYI, you can save yourself a bit of typing by declaring and setting your array all in one go, viz:

Account[] refAccount = new Account[] {  // the array size is the number of elements assigned
   new Checking(100),  
   new Checking(101),  
   new Checking(102),  
   ...
   new Savings(108, .02),  
   new Savings(109, .02) // no ','  
}; // Note: DON'T forget the final ';'

So I've got my five Checking and five savings as per the assignment instructions. But now I'm supposed to work in the balance of 1000 - 10000, I'm not seeing where this is supposed to be input.

Well, the normal way would be to use your setter method, eg something like:
refAccount[3].setBalance(3500.0);

I really kind of wish we could get more practice. We're able to do the "you do it" portions of the text book, but being that this is simply a "copy it out of the book" type of practice it doesn't really help me.

And this is what Henry was talking about. Programming is NOT simple, and you can only learn so much by rote.

My advice: StopCoding (←click), and think about what a Bank Account actually is. All banks have them; and they probably have the different varieties your exercise is trying to model (eg Checking, Savings etc.).

What do they all have in common? (a 'balance' sounds likely; quite possibly a 'customer' or 'owner' as well)

What DON'T they have in common? (for example: does a Checking account pay interest?)

Writing down this stuff will probably help you to sort out what the exercise is asking you to do.

Lastly, you might want to look at the tutorials (and specifically this chapter) to see how constructors work.

The main thing to know is that when you have a hierarchy of classes, ALL of them get called; so your Savings class constructor will need to call the Account constructor, which in turn calls Object's constructor (remember: ALL classes extend Object, even if you don't explicitly state it); and furthermore, the call to the superclass constructor must be the first statement in a constructor.
If you don't put it in yourself, the compiler will add a call to super() for you, which may NOT be what you want (and in your case, it probably isn't).

HIH

Winston

[Sean Heise]

Ok, so I think I've gotten it down to the very last step (I think, and hope lol), here's what I've gotten so far.

public abstract class Account 
{
	protected long accountNumber;
	protected double accountBalance;
	public long getAccountNumber()
	{
		return accountNumber;
	}
	public double getAccountBalance()
	{
		return accountBalance;
	}
	public void setAccountNumber(long number)
	{
		number = accountNumber;
	}
	public void setAccountBalance(double balance)
	{
		balance = accountBalance;
	}
	public void setUpAccount(long number, double  balance) //look up constructors
	{
		number = accountNumber;
		balance = accountBalance;
	}
	public String toString()
	{
		return "Account Number: " + accountNumber + " Account Balance: " + accountBalance;
	}
	public abstract double computeInterest(int intPeriod);
}

public class Checking extends Account
{
	public String toString()
	{
		return "Checking " + accountNumber + " Account Balance " + accountBalance;
	}
	public Checking(long number)
	{
		number = accountNumber;
	}
	public double computeInterest(int intPeriod) 
	{
		double total = ((accountBalance - 700) * .02) * 3;
		return total;
	}
}

public class Savings extends Account
{
	private double interestRate;
	public double getInterest()
	{
		return interestRate;
	}
	public void setInterest(double inter)
	{
		inter = interestRate;
	}
	public String toString()
	{
		return "Savings " + accountNumber + " Account Balance " +
                                         accountBalance + " Interest Rate " + interestRate;
	}
	public Savings(long number, double interest)
	{
		number = accountNumber;
		interest = interestRate;
	}
	public double computeInterest(int intPeriod) 
	{
		double total = Math.pow((1 + interestRate), intPeriod) * 
                                                            accountBalance - accountBalance;
		return total;	
	}
}

public class AccountArray 
{	
	public static void main(String[] args)
	{
		Account[] refAccount = new Account[10];

		for(int inc = 0; inc < 10; inc++ )
		{
			refAccount[0] = new Checking(100);
			refAccount[1] = new Checking(101);
			refAccount[2] = new Checking(102);
			refAccount[3] = new Checking(103);
			refAccount[4] = new Checking(104);
			refAccount[5] = new Savings(105, .02);
			refAccount[6] = new Savings(106, .02);
			refAccount[7] = new Savings(107, .02);
			refAccount[8] = new Savings(108, .02);
			refAccount[9] = new Savings(109, .02);
			
			refAccount[0].setAccountBalance(1000.0);
			refAccount[1].setAccountBalance(2000.0);
			refAccount[2].setAccountBalance(3000.0);
			refAccount[3].setAccountBalance(4000.0);
			refAccount[4].setAccountBalance(5000.0);
			refAccount[5].setAccountBalance(6000.0);
			refAccount[6].setAccountBalance(7000.0);
			refAccount[7].setAccountBalance(8000.0);
			refAccount[8].setAccountBalance(9000.0);
			refAccount[9].setAccountBalance(10000.0);
		}
		for(int ctr  = 0; ctr < 10; ctr++)
		{
			
		}
	}

}

from what I'm gathering from the instructions as I re-read them. I should be using the first for loop to instantiate five objects of the savings class and five objects of the checking class. I've probably gone about it in an inefficient manner, but it's what I've got so far.

My last step is what's blocking me at the moment. I'm not quite sure how to access the toString() methods in each of the other classes to get them to run and display in the for loop. Can anyone assist me, I greatly appreciate it.

Also, if you read the instructions and notice something amiss, I'd greatly appreciate that too.

[Campbell Ritchie]

Sorry for the delay in replying.

Sean Heise wrote: . . . I understand that in inheritance you can't use private variables/methods in any class other than the class that it's called in.

If you have a well‑designed class with toString, equals, hashCode, getXXX and maybe setXXX methods, then the fields can be as private and inaccessible as you like.

Because you don’t need any more access. Those methods provide everything you need.

If you are not adding any fields, you can often create a subclass which contains a constructor and nothing else.

And for protected variables/methods you can't use them outside of the super/subclasses...

That is exactly the same mistake I told you to check about earlier. Actually you can’t use a protected member of a class in its superclasses.