About Question enthuware.ocajp.i.v8.2.1447 :

[yndingo]

public void addUsage(int bytesUsed) {    
if (bytesUsed > 0) {       
totalUsage = totalUsage + bytesUsed;       
totalBill = totalBill + bytesUsed * costPerByte;  // !=  totalBill = totalUsage*costPerByte
}
}

totalBill is always equal to totalUsage*costPerByte

[admin]

Sorry but what is your question?

[catang]

I think what he is trying to say is the same thing for which I came here to discuss.
The question / business logic says that "totalBill is always equal to totalUsage*costPerByte", but the correct answer does not reflect this requirement.
Can you please detail why is the first option the correct option in spite of this requirement?

[admin]

There are two requirements - 1. all the bandwidth used by a User is reflected by the totalUsage field and 2. totalBill is always equal to totalUsage*costPerByte.

So, yes, you could do totalBill = totalUsage*costPerByte; but option 3, which implements it this way is private. You don't want the method to be private because the code in User class invokes bw.addUsage(bytesUsed);. This call will fail to compile if you make addUsage private. That is why option 3 is incorrect.

Option 1 is good because totalBill = totalBill + bytesUsed*costPerByte; still manages to satisfy the required constraint, which is, totalCost must be equal to totalUsage*costPerByte.

totalBill = totalBill + bytesUsed*costPerByte;
=> (bytes used earlier)*costPerByte + (bytes used now) *costPerByte;
=> (bytes used earlier + bytes used now) *costPerByte;
=> (totalUsage + bytesUsed )*costPerByte;
=> totalUsage*costPerByte; //because totalUsage = totalUsage + bytesUsed;

[catang]

Thank you for your quick reply.

I understand better now what was required to be answered, but practically this is a trick question, because of the way it is presented.
Option 3 was clearly not an option because of the private access modifier.

I do feel like option 4 still might be a good answer because:
1. it clearly uses the requirement mentioned in the business logic: totalBill = totalUsage*costPerByte;
2. the question suggests that more than one method can be created in order to comply with all requirements; "Your goal is to implement a method addUsage (and other methods, if required)"
3. the second method updateTotalBill() could be called in the User class instead of " ... other irrelevant code";

[admin]

No, option 4 is absolutely incorrect. What if someone calls addUsage but doesn't call updateTotalBill? This leaves the consistency of the Bandwidth objects at the mercy of other people. Completely undesirable as per the problem statement.

[rlobato]

Well, obviously if the method is being made private, the code in the consume method does not compile. But, on the other side, if it is being made public, the user can temper with bytesUsed.. that's why I chose 3

I must admit it is tricky.

Cheers

[giginar]

"Further, that a User should not be able to tamper with the totalBill value" - the answer does not meet with this req, user can tamper with the totalBill value. This question is confusing.

[admin]

"To tamper" means to alter in an objectionable manner. The answer doesn't tamper. It modifies the value in accordance with the requirements presented in the previous sentence i.e., "Your goal is to implement a method addUsage (and other methods, if required) in Bandwidth class such that all the bandwidth used by a User is reflected by the totalUsage field and totalBill is always equal to totalUsage*costPerByte. "

[raphaelzintec]

totalBill is always equal to totalUsage*costPerByte

but you write: totalBill = totalBill + bytesUsed*costPerByte;

why not saying totalBill is always equal to totalBill+totalUsage*costPerByte

totalUsage*costPerByte != totalBill+totalUsage*costPerByte

[admin]

totalUsage*costPerByte is the functional requirement.
totalBill+totalUsage*costPerByte is an implementation that satisfies the requirement.

[raphaelzintec]

ok thanks