The collections api - Data Structures and Problem Solving Using Java - page 272

distinguish between a failed search and a successful search that returns null
for the value. containsKey can be used if null values are known to be in the map.
The Map interface does not provide an iterator method or class. Instead it
returns a Collection that can be used to view the contents of the map.
The keySet method gives a Collection that contains all the keys. Since
duplicate keys are not allowed, the result of keySet is a Set , for which we can
obtain an iterator. If the Map is a SortedMap , the Set is a SortedSet .
Similarly, the values method returns a Collection that contains all the val-
ues. This really is a Collection , since duplicate values are allowed.

Finally, the entrySet method returns a collection of key/value pairs.
Again, this is a Set , because the pairs must have different keys. The objects in
the Set returned by the entrySet are pairs; there must be a type that represents
key/value pairs. This is specified by the Entry interface that is nested in the Map
interface. Thus the type of object that is in the entrySet is Map.Entry .
Figure 6.38 illustrates the use of the Map with a TreeMap . An empty map is
created at line 23 and then populated with a series of put calls at lines 25 to
29. The last call to put simply replaces a value with “unlisted” . Lines 31 and
32 print the result of a call to get , which is used to obtain the value for the key
"Jane Doe" . More interesting is the printMap routine that spans lines 8 to 19.
In printMap , at line 12, we obtain a Set containing Map.Entry pairs. From
the Set , we can use an enhanced for loop to view the Map.Entry s, and we can
obtain the key and value information using getKey and getValue , as shown on
lines 16 and 17.

Map.Entry
abstracts the
notion of a pair in
the map.

Returning to main , we see that keySet returns a set of keys (at line 37) that
can be printed at line 38 by calling printCollection (in Figure 6.11); similarly
at lines 41 and 42, values returns a collection of values that can be printed.
More interesting, the key set and value collection are views of the map, so
changes to the map are immediately reflected in the key set and value collec-
tion, and removals from the key set or value set become removals from the
underlying map. Thus line 44 removes not only the key from the key set but
also the associated entry from the map. Similarly, line 45 removes an entry
from the map. Thus the printing at line 49 reflects a map with two entries
removed.
Views themselves are an interesting concept and we will discuss specifics
of how they are implemented later when we implement the map classes. Some
further examples of views are discussed in Section 6.10.
Figure 6.39 illustrates another use of the map, in a method that returns
items in a list that appear more than once. In this code, a map is being used
internally to group the duplicates together: the key of the map is an item, and
the value is the number of times the item has occurred. Lines 8-12 illustrate
the typical idea seen in building up a map this way. If the item has never been

keySet , values , and
entrySet return
views.