Lab 7: Generics

The goal of this lab is for you to practice writing generic classes. This will also give the ocasion to write an anonymous inner class. Other (near-cliche) items will be reviewed (again): linked structures, maps, and iterators.

In fact, this lab is really a revisinting of project 3, the homemade list map. The difference is that this time you will be making a list map (ie, a class that implements the Map ADT using a linked list) but with the following twists:

• The map will be generic in both the key type and the value type
• Every place where you can use a nested class, you will.
• Your map will be a sorted list map, meaning it will keep the nodes sorted by their key type.
• The key type won't necessarily be Comparable; instead, the sorted list map must have an object of type Comparator, that is, an object that will make comparisons between keys to determine their order.

The Comparator interface

Sicne Comparators are something that was introduced for the first time briefly in class Tuesday, they deserve a more careful explanation here.

The other morning at breakfast my daughter was talking about how she would put the children in her class in alphabetical order by first name (Annika, Brendan, Calla, Drake...). I asked her how she would put them in order if instead she alphabetized them by the ends of their names rather than the beginnings. Since most of the girls in her class have names that end in a, that's a harder task. Then I asked her how she would sort them by the number of letters in their name. Then by height...

The point is, there can be many ways in which complex data can be sorted---or, more generally, compared. If a class implements the Comparable interface, then we need to provide exactly one way to compare two instances of that class, as coded in the compareTo() method.

What if, instead, we want to have several options for how we order the instances of a class? Then we can encode the logic of each way-of-ordering in a class that implements Comparator, that is, make "comparator" classes (and objects) to control the ordering decisions.

(The mathy explanation, for those of you with such tastes: Just as a class defines a type, and, therefore, a set, we can have several relations on that set. By luck, for those of you also taking CSCI 243 right now, we're starting the Relation chapter the very day we have this lab! A comparator defines a total order relation for that set. Total orders, along with partial orders, will be covered in DMFP on Oct 27. If any of you DMFPers ever lose sight of why all that stuff is relavant for real programming, you can read the Java API entry for Comparator and see how they use the idea of a total order to explain what the interface is supposed to mean. Ok, I'm done talking math stuff.)

As a simple example, here's a comparator that will order Strings based on their length:

class StringLengthComparator implements Comparator {
   public int compare(String o1, String o2) {
     int len1 = o1.length(),
         len2 = o2.length();
     if (len1 < len2) return -1;
     else if (len1 == len2) return 0;
     else return 1; 
   }
}

(If you see a line below that reads "Notice: Undefined index: name in /web/tvandrun/cs245/prelab-generics.php on line 103", please ignore it. Something funny is happening in the PHP code I use for having people indicate they read the pre-lab. It should still work, though.)