List Iterators

LinkedList list = . . .;
ListIterator iterator = list.listIterator();
while (iterator.hasNext())
{
   Object current = iterator.next();
   . . .
}

• Why iterators?

Classical List Data Structure

• Traverse links directly
  

  Link currentLink = list.head;
  while (currentLink != null)
  {
     Object current = currentLink.data;
     currentLink = currentLink.next;
  }

• Exposes implementation
• Error-prone

High-Level View of Data Structures

• Queue
  
  
  
  
• Array with random access
  
  
  
  
• List
  ???

List with Cursor

for (list.reset(); list.hasNext(); list.next())
{
   Object x = list.get();
   . . .
}

• Disadvantage: Only one cursor per list
• Iterator is superior concept

The Pattern Concept

• History: Architectural Patterns
• Christopher Alexander
• Each pattern has
• a short name
• a brief description of the context
• a lengthy description of the problem
• a prescription for the solution

Short Passages Pattern

Short Passages Pattern

Context

Problem

a lengthy description of the problem, including

• a depressing picture
• issues of light and furniture
• research about patient anxiety in hospitals
• research that suggests that corridors over 50 ft are considered uncomfortable

Short Passages Pattern

Solution

Iterator Pattern

Context

1. An aggregate object contains element objects
2. Clients need access to the element objects
3. The aggregate object should not expose its internal structure
4. Multiple clients may want independent access

Iterator Pattern

Solution

1. Define an iterator that fetches one element at a time
2. Each iterator object keeps track of the position of the next element
3. If there are several aggregate/iterator variations, it is best if the aggregate and iterator classes realize common interface types.
   

Iterator Pattern

Iterator Pattern

• Names in pattern are examples
• Names differ in each occurrence of pattern

Maintained by John Loomis, last updated 25 February 2007