CS205 Notes 6 (4 September 2006)

cs205 Monday 4 September 2006

• Today: remaining reading for ps2: Chapter 4, Chapter 10 through 10.2, and Chapter 5 through section 5.2
• Friday, 8 September: Problem Set 2

Exceptions

public class Quiz {
   static public int mode (int[] a) 
       // REQUIRES: a has at least one element
       // EFFECTS: Returns a value m such that m is an element of a,
       //    and no other value appears more often than m in a.
   {
      int best = a[0];
      int bestcount = 1;

      for (int i = 0; i < a.length; i++) {
         int val = a[i];
         int count = 0;
         for (int j = 0; j < a.length; j++) {
            if (a[j] == val) {
               count++;
            }
         }
         if (count > bestcount) {
            best = val;
            bestcount = count;
         }
      }
      return best;
   }

   static public void main (String [] args) {
      int [] tarray1 = { 1, 2, 2, 3, 2, 5 };
      int [] tarray2 = { };

      System.out.println ("Mode tarray1: " + mode(tarray1));
      System.out.println ("Mode tarray2: " + mode(tarray2));
   }
}

What are advantages of using Exceptions (and specifying exceptional behaviors in the EFFECTS clause) over using REQUIRES clauses?

What are advantages of REQUIRES clauses over using Exceptions?

static public int mode(int[] a) throws NoModeException 
   // EFFECTS: If a does not contain at least one element, throws
   //    NoModeException.  Otherwise, returns a value v such 
   //    that v is an element of a, and no other value appears 
   //    in a more frequently than v does.
 
{
   if (a == null || a.length == 0) {
       throw new NoModeException ();
   }
   ... // same as before
}

   ...
   try {
      System.out.println("Mode tarray1: " + mode(tarray1));
   } catch (NoModeException nme) {
      System.err.println("Error: " + nme);
   }

   try {
      System.out.println("Mode tarray2: " + mode(tarray2));
   } catch (NoModeException nme) {
      System.err.println("Error: " + nme);
   }

  ...
   try {
      System.out.println("Mode tarray1: " + mode(tarray1));
      System.out.println("Mode tarray2: " + mode(tarray2));
   } catch (NoModeException nme) {
      System.err.println("Error: " + nme);
   }