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

package includes a similar Entry interface, by the way). Other kinds of
compositions include triples, which store three objects, quadruples, which store
four objects, and so on.
Code Fragment 8.1: Java interface for an entry
storing key-value pairs in a priority queue.
Comparators
Another important issue in the priority queue ADT that we need to define is how
to specify the total order relation for comparing keys. We have a number of
design choices concerning how to compare keys that we can make at this point.
One possibility, and the one that is the most concrete, is to implement a different
priority queue for each key type we want to use and each possible way of
comparing keys of such types. The problem with this approach is that it is not
very general and it requires that we create a lot of similar code.
An alternative strategy is to require that keys be able to compare themselves to
one another. This solution allows us to write a general priority queue class that
can store instances of a key class that implements some kind of Comparable
interface and encapsulates all the usual comparison methods. This solution is an
improvement over the specialized approach, for it allows us to write a single
priority queue class that can handle lots of different types of keys. But there are
contexts in which this solution is asking too much of the keys, as keys often do
not "know" how they ought to be compared. Two examples follow.
Example 8.2: Given keys 4 and 11 we have that 4 ≤ 11 if the keys are integer
objects (to be compared in the usual manner), but 11 ≤ 4 if the keys are string
objects (to be compared lexicographically).
Example 8.3: A geometric algorithm may compare points p and q in the
plane, by their x-coordinate (that is, p ≤q ifx(p) ≤ x(q)), to sort them from left to
right, while another algorithm may compare them by their y-coordinate (that is, p
≤ q ify(p) ≤ y(q)), to sort them from bottom to top. In principle, there is nothing
pertaining to the concept of a point that says whether points should be compared
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