Iterators - Data Structures and Abstractions with Java - page 400

C HAPTER S UMMARY

The interface Iterator specifies three methods: hasNext , next , and remove . An iterator that implements
this interface need not provide a remove operation. Instead, the method remove would throw the exception
UnsupportedOperationException .

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The interface ListIterator specifies nine methods, including the three methods that Iterator specifies. They are
hasNext , next , hasPrevious , previous , nextIndex , previousIndex , add , remove , and set . The methods add ,
remove , and set are optional in the sense that they can throw the exception UnsupportedOperationException
instead of affecting the list.

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You can implement each of the interfaces Iterator and ListIterator as its own class. This class could be an
inner class of the class that implements the ADT in question, or it could be public and separate from the ADT's class.

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An inner class iterator enables you to have several independent iterators that traverse a collection. It also
allows the iterator direct access to the underlying data structure, so its implementation can be efficient.

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A separate class iterator also allows multiple and distinct iterations to exist simultaneously. However, since the
iterator can access the list's data fields only indirectly via ADT operations, the iteration takes more time than
one performed by an inner class iterator. On the other hand, the implementation is usually straightforward.

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Certain ADTs do not provide sufficient public access to their data to make a separate class iterator possible.
However, to provide an iterator for an ADT's implementation that exists and cannot be altered, you might
need to define a separate class iterator.

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P ROGRAMMING T IPS

All of the exceptions mentioned in the interfaces Iterator and ListIterator are run-time exceptions, so no
throws clause is necessary in any of the methods' headers. In addition, you do not have to write try and catch
blocks when you invoke these methods. However, you will need to import NoSuchElementException from the
package java.util . The other exceptions are in java.lang , so no import statement is necessary for them.

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A class that defines an inner class iterator should implement the interface Iterable . A client of the class can
then use a for-each loop to traverse the objects in an instance of the class.

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E XERCISES

1.

Suppose that nameList is a list that contains the following strings: Kyle, Cathy, Sam, Austin, Sara. What output is
produced by the following sequence of statements?

Iterator<String> nameIterator = nameList.getIterator();
System.out.println(nameIterator.next());
nameIterator.next();
System.out.println(nameIterator.next());
nameIterator.remove();
System.out.println(nameIterator.next());
displayList(nameList);

2.

Repeat Exercise 1, but instead use the following statements:

Iterator<String> nameIterator = nameList.getIterator();
nameIterator.next();
nameIterator.remove();
nameIterator.next();