Interfaces - The Java Programming Language - page 186

ation is that the programmer makes such changes and performs such
sharing in a manner that makes sense to the application. If the pro-
grammer is not to be trusted in this, then methods should specify that
they make defensive copies of parameters, and/or return values, such
that no harmful changes can occur.
The attributes are accessed through an Iterator object returned from
the attrs method. Iterator is a generic interface defined in java.util for
collection classes to use to provide access to their contents (see " Itera-
tion " on page 571 ). In effect, the Attributed interface defines a collection
typea set of attributesso we use the normal mechanism for accessing
the contents of a collection, namely, the Iterator type. Using Iterator
has another benefit: It is easy to implement Attributed with a standard
collection class (such as HashMap ) that uses Iterator , as you'll soon see.
Many classes that provide an Iterator declare that they implement the
Iterable interface, which defines the single method iterator to return an
Iterator instance. Although an Attributed object does provide an Iterat-
or , it would be wrong to have Attributed extend Iterable or to rename
attrs as iterator , because that would restrict the ability of the class im-
plementing Attributed to control its own iteration behavior. For example,
it would mean that an Attributed collection class would not be able to
provide an iterator for its elements rather than its attributes.
4.4.1. Implementing Interfaces
Interfaces describe contracts in a pure, abstract form, but an interface
is interesting only if a class implements it.
Some interfaces are purely abstractthey do not have any useful general
implementation but must be implemented afresh for each new class.
Most interfaces, however, may have several useful implementations. In
the case of our Attributed interface, we can imagine several possible im-
plementations that use various strategies to store a set of attributes.
One strategy might be simple and fast when only a few attributes are in
a set; another one might be optimized for attribute sets that are queried