Lab 7: Generics

1. Introduction

The goal of this lab is to practice writing generic classes. In class we have seen examples of generic linked lists and a generic ArrayList-like class. In this lab we will write a generic class that will be something similar to a HashMap.

A map (also called a dictionary or associative array) associates keys with values. See the Map in Java's API as an example of the functionality.

We will use a simplified version of this interface in the lab. Your main task in this lab is to implement this interface.

2. Set up

As usual, make a directory for this lab. Copy the following code from the public directory.

cp /homes/tvandrun/Public/cs245/lab7/*

Open Eclipse and make a new project from the existing source in your directory.

Next, look at the driver program, SLMDriver.java, so that you see what the map will be used for. Basically it will be used to tally the frequencies of words in a file. You may remember this example from Lab 4.

3. Details

On the inside, your class will be a linked list. Think about how you would implement such a think as a linked list, consider how it would need to be different to the linked lists we've already seen, and how the specific operations would be defined.

This map, however, will have a special feature. Java's HashMap orders associations in an unpredictable order--- if you iterated through the key set, you would not get the keys in the same order that you added them, nor would there be any clear reason for the order that they would be iterated through. Your map, however, will take a Comparator<K> object in its constructor---that is, a comparator with the same parameterized type as the key type. This will determine how the associations are ordered and what order the keys come when using keyIterator().

Oh, and resist the temptation to refer to your class as a hash map. The "hash" in "hashmap" refers to a specific way to implement a map (or set). We are not doing hashing in this lab. But we will learn about hashing later in the semester.

Finally, before you move on discuss with your partner and decide the basic structure of your classes. How will you implement the map as a linked list? What will your node class look like? You may make your node class either as a stand-alone class or as a nested class.

4. The SortedListMap class

Now, open SortedListMap and implement it. This includes making a Node class. Notice that the constructor of SortedListMap takes a comparator. Use the comparator to determine how to add new associations to the map/list.

5. The comparator

When you are ready to test your SortedListMap, you'll also need to write a comparator. In this case, we want the key/value associations stored in alphabetical order of the keys. You will write a comparator for strings that performs that comparison. (The comparison itself is very easy, because String already has a compareTo() method which does exactly that comparison.)

However, instead of writing a plain old class that implements Comparator, practice nested classes by writing an anonymous inner class. Change the line in SLMDriver that reads

Map<String, Integer> tally = null; //new SortedListMap<String, Integer>(...your comparator...);

to

Map<String, Integer> tally = new SortedListMap<String, Integer>(...your comparator...);

except write your anonymous inner class in place of "...your comparator...".

6. To turn in

Cat and demonstrate your program in a type script, and turn in a hard copy.