Generics K&B Page 638 Question 5 Code

[Harikrishna Gorrepati]

Hi, Here is the code from Generics K&B Page 638 Question 5

 
12. public class AccountManager {
13.     private Map accountTotals = new HashMap();
14.     private int retirementFund;
15.
16.     public int getBalance(String accountName) {
17.         Integer total = (Integer) accountTotals.get(accountName);
18.         if (total == null)
19.             total = Integer.valueOf(0);
20.             return total.intValue();
21.     }
23.     public void setBalance(String accountName, int amount) {
24.         accountTotals.put(accountName, Integer.valueOf(amount));
25.     } 
    }       

This class is to be updated to make use of appropriate generic types, with no changes in behavior
(for better or worse). Which of these steps could be performed? (Choose three.)
Correct answers are
B. Replace line 13 with
private Map<String, Integer> accountTotals = new HashMap<String, Integer>();
E. Replace lines 17–20 with
Integer total = accountTotals.get(accountName); //Type mismatch: cannot convert from Object to Integer
if (total == null) total = 0;
return total;

How come Option "E" is correct..I am getting Type mismatch: cannot convert from Object to Integer error..Please advice

[Duc Vo]

Have you done step B yet?

[Harikrishna Gorrepati]

I did not do it because question is not clear. If I keep B step, I will not get error for Option E..Please let me know why should I keep Step "B"

Duc Vo wrote:Have you done step B yet?

[Duc Vo]

It's actually quite clear. From the question:

Which of these steps could be performed? (Choose three.)

Which means you need to perform all three steps.

[Harikrishna Gorrepati]

I thought I need to insert the 3 options independently..Thanks Duc

[Glen Iris]

Unrelated query on the same question:

The book says:

"D is wrong because an int cannot autobox to null".

FYI - option d:

int total = accountTotals.get(accountName);
if (total == null) total = 0;
return total;

I accept that this is correct as we are unboxing an Integer to an int. However, I was not aware that this cannot be null. can somebody please explain why. Is this true for all unboxing?

[dennis deems]

Glen Iris wrote:Unrelated query on the same question:

The book says:

"D is wrong because an int cannot autobox to null".

FYI - option d:

int total = accountTotals.get(accountName);
if (total == null) total = 0;
return total;

I accept that this is correct as we are unboxing an Integer to an int. However, I was not aware that this cannot be null. can somebody please explain why. Is this true for all unboxing?

An Integer can be null, but not an int. If we attempt an autoboxing assignment using a null reference, an exception will be thrown. This is true for all wrappers.

[Glen Iris]

Again Dennis, thank you. Its something simple that I didnt know as I had never tried it in practice.

[Bert Bates]

There is an informal standard on the real exam that we try to follow in the book:

If the question wants you to try only one answer at a time, like inserting a line of code, it should say something like:

"Which fragment, inserted independently at line X, compiles?"

The key phrase to look for is the "inserted independently". This isn't always present, but it usually is.

hth,

Bert