Collections - The Java Programming Language - page 904

public void remove() {
throw new UnsupportedOperationException();
}
}
The class ShortStrings is a type of iterator that will read String objects
from another iterator, returning only those that are no longer than a
specified length. The constructor takes the iterator that will provide the
strings and the maximum length, storing those in the object's fields.
The field nextShort will hold the next short string, or null when there
isn't one. If nextShort is null , the hasNext method searches for the next
short string, remembering it in nextShort . If hasNext reaches the end of
its source iteration without finding a short string it returns false .
The method next checks to see if there is a next short string, either re-
turning it if there is one or throwing NoSuchElementException if there are
none to return. Notice that hasNext does all the real work of finding the
short strings, and next just returns the results, setting nextShort to null
to indicate that the next short string, if any, is as yet undiscovered.
Finally, remove is not supported by this iterator implementation, so remove
throws UnsupportedOperationException .
A few things to notice. First, hasNext is carefully written so that it will
work if invoked multiple times before a next . This is requiredthe calling
code may invoke hasNext as many times as it wants between invocations
of next . Second, next is carefully written so that it works even if pro-
grammer using it has never invoked hasNext . Although generally a poor
practice, you could never invoke hasNext and simply loop invoking next
until an exception is generated.
Third, remove is not allowed because it cannot work correctly. Imagine,
for example, if remove invoked remove on the underlying iterator. The fol-
lowing legal (although odd) code can cause incorrect behavior: