Iterators

1
Iterators

• Chapter 7

2
Chapter Contents

• What is an Iterator?
• A Basic Iterator
• Iterator Methods that Modify the ADT
• Implementing an Internal Iterator
• Implementing an Iterator as Its Own Class
• An External Iterator
• An Inner Class

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What Is an Iterator?

• A program component
• Enables you to step through, traverse a
  collection of data
• Can tell whether next entry exists
• Iterator may be manipulated
• Asked to advance to next entry
• Give a reference to current entry
• Modify the list as you traverse it

4
A Basic Iterator
import java.util.NoSuchElementException public
interface BasicIteratorInterface /
Have we gone past last? / public boolean
hasCurrent() / Advance position by one
/ public boolean advance() / Retrieve
current entry / public Object getCurrent()
throws NoSuchElementException / Restart at
beginning / public void reset() // end
BasicIteratorInterface
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A Basic Iterator
Fig. 7-1 The effect of iterator methods on a list.
6
Iterator Methods That Modify the ADT
import java.util.NoSuchElementException public
interface IteratorInterface extends
BasicIteratorInterface public boolean
hasCurrent() public boolean advance() public
Object getCurrent() throws NoSuchElementException
public void reset() / Add a new entry
after current / public void addAfterCurrent(Obje
ct newEntry) throws NoSuchElementException /
remove current advance to next / public void
removeCurrent() throws NoSuchElementException /
if hasCurrent(), replace stay where we are.
/ public void replaceCurrent(Object
newEntry) throws NoSuchElementException //
end IteratorInterface
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Iterator Methods That Modify the ADT
Fig. 7-2 The effect of iterator methods on a
list.
8
Implementing an Internal Iterator

• Including IteratorInterface methods

import java.util.NoSuchElementExceptionpublic
class LinkedListWithInternalIterator
implements ListInterface, IteratorInterface
private Node firstNode private int
length private Node currentNode // current
node in iteration private Node priorNode //
node before the current node public
LinkedListWithInternalIterator() clear()
// end default constructor public final void
clear() firstNode null length
0 currentNode null priorNode null
// end clear lt Other implementations go
heregt // end LinkedListWithInternalIterator
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Implementing an Internal Iterator
Fig. 7-3 Before and after removing current entry
when its node is first in the chain (bottom of
p.159)
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Implementing an Internal Iterator
Fig. 7-4 Before and after removing current entry
when its node is not first in the chain (top of
160)
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Implementing an Internal Iterator
Fig. 7-5 Before and after adding an entry after
the current entry (bottom of p. 161)
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Implementing an Iterator as Its Own Class
Fig. 7-6 Counting the number of times that Jane
appears in a list of names.
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Implementing an Iterator as Its Own Class

• Internal iterator easy to understand, use
• But only one iteration of a list can occur at any
  one time
• May need multiple iterators for a single list
• Create an external iterator
• Allows multiple instances of an iterator for a
  list

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An External Iterator
Fig. 7-7 An external iterator with a reference to
an ADT, an indicator of its position within the
iteration, and no knowledge of the ADT's
implementation
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An External Iterator

• An external iterator must access an ADT's data by
  using public methods of the ADT
• Therefore slower in performing its operations
• At the same time its implementation is
  straightforward
• Possible to have multiple instances of
  independent external iterators
• To provide a new type of iterator for an existing
  implementation of an ADT that cannot be altered,
  one must use an external iterator (p. 166 note).

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Inner Class Iterators

• Have direct access to an ADT's data
• Thus is as efficient as an internal iterator
• Has similar implementation effort
• Advantage
• Can have several iterator objects at same time
• Each can traverse list independently of one
  another

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Inner Class Iterators
Fig. 7-8 An inner class iterator with direct
access to the linked chain that implements the
ADT.