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

for the keys, and we provide additional methods that refer to this ordering (see
Section 9.5.2). In an unordered dictionary, however, no order relation is assumed on
the keys; hence, only equality testing between keys is used.
As an ADT, an (unordered) dictionary D supports the following methods:
size(): Return the number of entries in D.
isEmpty(): Test whether D is empty.
find(k): If D contains an entry with key equal to k, then return such an
entry, else return null.
findAll(k): Return an iterable collection containing all entries with key
equal to k.
insert(k,v): Insert an entry with key k and value v into D, returning the
entry created.
remove(e): Remove from D an entry e, returning the removed entry or
null if e was not in D.
entries(): Return an iterable collection of the key-value entries in D.
Notice that our dictionary operations use entries, which are the key-value pairs stored
in the dictionary. We assume each entry comes equipped with getKey() and
getValue() methods to access its key and value components respectively.
When the method find(k) is unsuccessful (that is, there is no entry with key equal to
k), we use the convention of returning a sentinel null. Another choice, of course,
would be to throw an exception for an unsuccessful find(k), but that would probably
not be an appropriate use of an exception, since it is normal to ask for a key that
might not be in our dictionary. Moreover, throwing and catching an exception is
typically slower than a test against a sentinel; hence, using a sentinel is more efficient.
Note that, as we have defined it, a dictionary D can contain different entries with
equal keys. In this case, operation find(k) returns an arbitrary entry (k,v), whose
key is equal to k. We mention, in passing, that our dictionary ADT should not be
confused with the abstract class java.util. Dictionary, which actually
corresponds to the map ADT given above and is now considered obsolete.
Example 9.2: In the following, we show a series of operations on an initially
empty dictionary storing entries with integer keys and character values.
Operation
Output
545