In this assignment, you implement a hash table using chained hashing. The hash table size is determined at initialization time and never changes. Each hash table entry is a linked list, so each hash table entry can hold any number of values. You may use a linked list from the Java API, or one from one of your previous assignments.

Your hash table must be designed to map keys to values. None of the keys or values are allowed to be null. Keys are compared using the .equals method.

Your hash table should use the hashCode() method to compute the hash for a given string. Use Math.abs to turn that into a positive value, and modulo to make it into an array index (this description is not quite enough, so you will have to figure out exactly how to make this work).

Your hash table class (called Hash211<K,V>) must provide the following methods:

  Hash211<K,V>(int capacity, boolean printTimes);   // constructor
  V put(K key, V value);              // add or replace a Key,Value
  V get(Object key);                  // return a value for the given key

Each of these functions is described in more detail here.

The second parameter to the constructor specifies whether your code should print out the time (in nanoseconds) for each call to put and get. To do this, you may re-use your code from Assignment 9. Since executing any print statements substantially increases the runtime, when printTimes is false, your code may run considerably faster.

Analysis and measurement

You should analyze and measure the runtime of your hash table put and get operations in two cases:

1. When the hash table size s is much larger (at least 10 times larger) than the number n of elements added.
2. When the hash table size s is much less (at least 10 times less) than the number n of elements added.

Your analysis should match your measured results. Write up your analysis and make enough measurements to be clear and convincing.

This part of the assignment is worth 20% of the grade.

Test Code

Anthony Christe (who TA'd this course in the past) has provided a test program, HashTableStressTest.java, to make it easy for you to add a large number of strings to your hash table. This is a command line program that takes two arguments and an optional third argument.

java HashTableStressTest dictionary_file capacity [-pt --print-times]

where dictionary_file is the location of the dictionary file which is a text file with a single word per line, and capacity is the initial capacity of the hashtable.

The final option will pass either true or false to printTimes in the Hash211 constructor. If the option (either the short or long form) is not specified then printTimes will be false. If the option is specified, then the times will be printed.

Anthony has also provided two dictionary files, dict.txt (234,937 words) and dict-small.txt (99,171 words).

Turning in the Assignment