ITEC 120 2008fall aaray, ejderrick, ibarland, jmdymacek

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lect06b
No magic numbers; increment syntax

Named Constants

Consider BankAccount fees -- The first few transactions/mo were free (say, the first 4); after that there was a fee (say, $2.50). Our code included the numbers 4 and $2.50 in the middle of the code, perhaps in several different places. What if the bank manager decides to raise the fees? Or change the number of free-transactions?

Well, we an always search-replace, right? Careful — $2.50 might have been used for some other purposes! You need to go through the bank's entire code, and look at each "2.50" (and 2.5 and 5.0/2), and decide whether it th transaction fee, and if so change it. This process is tedious, and error-prone.

The proper solution is to use named constants:

  public static final FREE_TRANSACTIONS_PER_MONTH = 4;
  public static final TRANSACTION_FEE = 2.50;
  public static final ATM_FEE = 2.50;

To see this visually, in BlueJ:

• Make several BankAccount objects on the bench¹.
• Inspect two of the objects (by right-clicking and choosing Inspect).
• You can even make a deposit to just one of them, and verify that the balance field is updated.
• Notice, though, that TRANSACTION_FEE is not shown as a field inside each bank account. (That's correct -- unlike a BankAccount's balance, the TRANSACTION_FEE will never be different for different objects.)
• Now, inspect the class — go to the tan box up in the project window, and right-click Inspect. Now we can see that TRANSACTION_FEE is located inside class BankAccount, and not inside individual BankAccount objects. That's what “static” does for us, and it's exactly what we want!

Another example: Roach population.

class RoachPopulation {
  int numRoaches;

  RoachPopulation( int initialPop ) {
    numRoaches = initialPop;
    }

  void spray() {
    numRoaches = numRoaches/10;
    }

  void grow() {
    numRoaches = numRoaches + numRoaches/10;
    }

  int getNumRoaches() {
    return numRoaches;
    }

  }

Exercise

class PrezElect is used to keep track of presidential election results, assuming there are exactly two candidates.

What fields do we want? What methods? Write 'em!

Extra syntax for incrementing

Introduce/compare:

  n = n+1;
  n += 1;
  ++n;
  n++;

Now, suppose this class is actually used by the electoral college, which has 435+100+3 electors. Make a named constant, to reflect this.

Extra: make further named constants which explain where each part of 435+100+3 comes from! lect06b-soln.html

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©2008, Ian Barland, Radford University Last modified 2008.Oct.13 (Mon)

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