Abstract classes doubt

[Johan Mena]

I got stuck in this exercise about abstract classes. These are 4 different exercises. The problem is in number 13 (although if you spot any other error do let me know).

/**
	 * 10. Create an abstract class DiscountPolicy. It should have a single abstract method computeDiscount 
	 * that will return the discount for the purchase of a given number of a single item. The method 
	 * has two parameters, count and itemCost.
	 */

public abstract class DiscountPolicy
{	
	 public abstract double computeDiscount(int count, double itemCost);
}	

/** 11. Derive a class BulkDiscount from DiscountPolicy, as described in the previous exercise. It should 
 * have a constructor that has two parameters, minimum and percent. It should define the method 
 * computeDiscount so that if the quantity purchased of an item is more than minimum, the discount
 * is "percent" percent.
 * **/

public class BulkDiscount extends DiscountPolicy
{	
	private double percent;
	private double minimum;
	
	public BulkDiscount(int min, double per)
	{
		int minimum = min;
		double percent = per;
	}
	
	@Override
	public double computeDiscount(int count, double itemCost)
	{	
		//no discount by default
		double discount = 1;
		
		 if (count > minimum)
		 {
			 discount = percent;
		 }
		 
		 return percent*itemCost;
	}
}

/** 12. Derive BuyNItemsGetOneFree from DiscountPolicy, as described in Exercise 10.
 * The class should have a constructor that has a single parameter n. In
 * addition, the class should define the method computeDiscount so that every 
 * nth item is free.
 */

public class BuyNItemsGetOneFree extends DiscountPolicy
{

	private int numberOfItems;
	
	public BuyNItemsGetOneFree(int n)
	{
		numberOfItems = n;
	}
	
	@Override
	public double computeDiscount(int count, double itemCost)
	{
		double discount = 0;
		
		if(count > numberOfItems)
			discount = (count/numberOfItems)*itemCost;
		
		return discount;
	}
}

Now, here I don't know if I'm misunderstanding what the author wants but I'm not sure how to approach this.

In CombinedDiscount's version of computeDiscount, should I call BulkDiscount and BuyNGetOneFree version of computeDiscount and then return the largest discount? If so, how would I call those methods from this sibling (?) class? Will I have to create a temp object from each of those classes and then call the methods using those objects?

/** 13. Derive CombinedDiscount from DiscountPolicy, as described in Exercise 10. It should have 
 * a constructor that has two parameters of type DiscountPolicy. It should define the 
 * method computeDiscount to return the maximum value returned by computeDiscount for 
 * each of its two private discount policies.: BulkDiscount and BuyNItemsGetOneFree.
 */
public class CombinedDiscount extends DiscountPolicy
{
	private DiscountPolicy first, second;
	
	public CombinedDiscount(DiscountPolicy firstDiscount, DiscountPolicy secondDiscount)
	{
		first = firstDiscount;
		second = secondDiscount;
	}
	
	@Override
	public double computeDiscount(int count, double itemCost)
	{
		???
	}

}

Thank you in advance guys.

[Jesper de Jong]

Johan Mena wrote:In CombinedDiscount's version of computeDiscount, should I call BulkDiscount and BuyNGetOneFree version of computeDiscount and then return the largest discount?

Yes, that's exactly what the instructions say:

Johan Mena wrote:Derive CombinedDiscount from DiscountPolicy, as described in Exercise 10. It should have a constructor that has two parameters of type DiscountPolicy. It should define the method computeDiscount to return the maximum value returned by computeDiscount for each of its two private discount policies.: BulkDiscount and BuyNItemsGetOneFree.

Johan Mena wrote:If so, how would I call those methods from this sibling (?) class? Will I have to create a temp object from each of those classes and then call the methods using those objects?

No, why do you think you have to create a temporary object? You already have references to the two other discounts, in the member variables first and second. You can just call the computeDiscount method on those two, and then return the largest of the two answers.

Do you understand how to call a method on an object? It's just: variable name, dot, method name, arguments:

double discount = first.computeDiscount(count, itemCost);

[Johan Mena]

Jesper de Jong wrote:You already have references to the two other discounts, in the member variables first and second. You can just call the computeDiscount method on those two, and then return the largest of the two answers.

For some reason I wasn't realizing this. Thanks for pointing it out. Solved.

[Campbell Ritchie]

In the bogof (=buy one, get one free) class, why have you got the > test? What does it achieve which would not happen in its absence? I think it is unnecessary.
If you want some sort of test, maybe a >= 0 test would be more useful, to prevent anybody trying to calculate a discount on -1 items.

[Johan Mena]

Campbell Ritchie wrote:In the bogof (=buy one, get one free) class, why have you got the > test? What does it achieve which would not happen in its absence? I think it is unnecessary.
If you want some sort of test, maybe a >= 0 test would be more useful, to prevent anybody trying to calculate a discount on -1 items.

Buy N items, get one free.

count (the actual number of items the user buys) has to be greater than the predefined numberOfItems in order to get a discount every time numberOfItems are bought.

[Campbell Ritchie]

No, it doesn’t. If you buy 4 items and the amount for discount is 5, your formula will calculate 4 / 5 and get 0. See, you had it right already.

[Johan Mena]

Campbell Ritchie wrote:No, it doesn’t. If you buy 4 items and the amount for discount is 5, your formula will calculate 4 / 5 and get 0. See, you had it right already.

If 4 items are bought and the amount for discount is 5, the wouldn't even execute and the default discount amount, 0, would be returned.

[Ryan Sykes]

public BulkDiscount(int min, double per)  
    {  
        int minimum = min;  
        double percent = per;  
    } 

This should probably be giving you some error. Declaring the type as you have done above, creates local variables which end up hiding your class variables of the same name.

[Johan Mena]

Ryan Sykes wrote:

public BulkDiscount(int min, double per)  
    {  
        int minimum = min;  
        double percent = per;  
    } 

This should probably be giving you some error. Declaring the type as you have done above, creates local variables which end up hiding your class variables of the same name.

Thank you. Changed the type of minimum to int and used them instead of creating new local variables.