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

The ability of performing narrowing conversions from interface types to class
types allows us to write general kinds of data structures that only make minimal
assumptions about the elements they store. In Code Fragment 2.14, we sketch
how to build a directory storing pairs of objects implementing the Person
interface. The remove method performs a search on the directory contents and
removes the specified person pair, if it exists, and, like the findOther method,
it uses the equalTo method to do this.
Code Fragment 2.14
PersonPairDirectory.
: Sketch of class
Now, suppose we have filled a directory, myDirectory, with pairs of
Student objects that represent roommate pairs. In order to find the roommate of
a given Student object, smart_one, we may try to do the following (which is
wrong):
Student cute_one = myDirectory.findOther(smart_one);
// wrong!
The statement above causes an "explicit-cast-required" compilation error. The
problem here is that we are trying to perform a narrowing conversion without an
explicit cast. Namely, the value returned by method findOther is of type
Person while the variable cute_one, to which it is assigned, is of the
narrower type Student, a class implementing interface Person. Thus, we use
a cast to convert type Person to type Student, as follows:
Student cute_one = (Student)
myDirectory.findOther(smart_one);
Casting the value of type Person returned by method findOther to type
Student works fine as long as we are sure that the call to
myDirectory.findOther is really giving us a Student object. In general,
interfaces can be a valuable tool for the design of general data structures, which
can then be specialized by other programmers through the use of casting.
2.5.2 Generics
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