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

emove(k): Remove from M the entry with key equal to k, and return
its value; if M has no such entry, then return null.
keys(): Return an iterable collection containing all the keys stored in
M (so keys().iterator() returns an iterator of keys).
values(): Return an iterable collection containing all the values as
sociated with keys stored in M (so values().iterator() re turns an iterator
of values).
entries(): Return an iterable collection containing all the key-value
entries in M (so entries().iterator() returns an iterator of entries).
When operations get(k), put(k,v) and remove(k) are performed on a map M that
has no entry with key equal to k, we use the convention of returning null. A special
value such as this is known as a sentinel. (See also Section 3.3.) The disadvantage
with using null as such a sentinel is that this choice can create ambiguity should we
every want to have an entry (k, null) with value null in the map. Another choice, of
course, would be to throw an exception when someone requests a key that is not in
our map. This would probably not be an appropriate use of an exception, however,
since it is normal to ask for something that might not be in our map. Moreover,
throwing and catching an exception is typically slower than a test against a sentinel;
hence, using a sentinel is more efficient (and, in this case, conceptually more
appropriate). So we use null as a sentinel for a value associated with a missing key.
Example 9.1: In the following, we show the effect of a series of operations on
an initially empty map storing entries with integer keys and single-character values.
Operation
Output
Map
isEmpty()
true
φ
put(5,A)
null
{(5,A)}
put(7,B)
518