Data Structures and Algorithms in Java[1].pdf - Fulvio Frisone

9.2.6 A Java Hash Table Implementation
In Code Fragments 9.3–9.5, we show class, HashTableMap, which implements
the map ADT using a hash table with linear probing to resolve collisions. These
code fragments include the entire implementation of the map ADT, except for the
methods values() and entries(), which we leave as an Exercise (R-9.10).
The main design elements of the Java class HashTableMap are as follows:
• We maintain, in instance variables, the size, n, of the map, the bucket
array, A, and the capacity, N, of A.
• We use method hash Value to compute the hash function of a key by
means of the built-in hashCode method and the multiply-add-and-divide (MAD)
compression function.
• We define a sentinel, AVAILABLE, as a marker for deactivated entries.
• We provide an optional constructor that allows us to specify the initial
capac ity of the bucket array.
• If the current bucket array is full and one tries to insert a new entry, we
rehash the entire contents into a new array that is twice the size as the old version.
• The following (protected) auxiliary methods are used:
checkKey(k), which checks if the key k is valid. This method currently
just checks that k is not null, but a class that extends HashTableMap can
override this method with a more elaborate test.
rehash(), which computes a new MAD hash function with random pa
rameters and rehashes the entries into a new array with double capacity.
findEntry(k), which looks for an entry with key equal to k, starting at
the index A[h(k)] and going through the array in a circular fashion. If the method
finds a cell with such an entry, then it returns the index i of this cell. Otherwise, it
returns -i-1, where i is the index of the last empty or available cell encountered.
Code Fragment 9.3: Class HashTableMap
implementing the map ADT using a hash table with
linear probing. (Continues in Code Fragment 9.4.)
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