The collections api - Data Structures and Problem Solving Using Java - page 268

Figure 6.35 illustrates a SimpleStudent class in which two SimpleStudent s
are equal if they have the same name (and are both SimpleStudent s). This could
be overridden using the techniques in Figure 6.34 as needed, or this method
could be declared final . If it was declared final , then the test that is present
allows only two identically typed SimpleStudent s to be declared equal. If, with
a final equals , we replace the test at line 40 with an instanceof test, then any
two objects in the hierarchy can be declared equal if their names match.
The hashCode method at lines 47 and 48 simply uses the hashCode of the
name field. Thus if two SimpleStudent objects have the same name (as declared
by equals ) they will have the same hashCode , since, presumably, the implemen-
tors of String honored the contract for hashCode .
The accompanying test program is part of a larger test that illustrates all
the basic containers. Observe that if hashCode is unimplemented, all three
SimpleStudent objects will be added to the HashSet because the duplicate will
not be detected.
It turns out that on average, the HashSet operations can be performed
in constant time. This seems like an astounding result because it means that
the cost of a single HashSet operation does not depend on whether the HashSet
contains 10 items or 10,000 items. The theory behind the HashSet is fascinating
and is described in Chapter 20.

maps

6.8

A Map is used to store a collection of entries that consists of keys and their val-
ues . The Map maps keys to values. Keys must be unique, but several keys can
map to the same value. Thus, values need not be unique. There is a SortedMap
interface that maintains the map logically in key-sorted order.
Not surprisingly, there are two implementations: the HashMap and TreeMap .
The HashMap does not keep keys in sorted order, whereas the TreeMap does. For
simplicity, we do not implement the SortedMap interface but we do implement
HashMap and TreeMap .
The Map can be implemented as a Set instantiated with a pair (see
Section 3.9), whose comparator or equals / hashCode implementation refers
only to the key. The Map interface does not extend Collection ; instead it
exists on its own. A sample interface that contains the most important
methods is shown in Figures 6.36 and 6.37.
Most of the methods have intuitive semantics. put is used to add a key/
value pair, remove is used to remove a key/value pair (only the key is speci-
fied), and get returns the value associated with a key. null values are allowed,
which complicates issues for get , because the return value from get will not

A Map is used to
store a collection
of entries that
consists of keys
and their values .
The Map maps keys
to values.