C SC 205 Lecture 11: Doubly-Linked Lists
major resources: Data Structures and the Java Collections Framework Second Edition, William Collins, McGraw-Hill, 2005
Introduction to Programming and OO Design using Java, Niño and Hosch, Wiley & Sons, 2002

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Wednesday October 24 (week 7)

Doubly-Linked Lists

• In general, each list entry has references to both the entry that follows it on the list and the one that precedes it.
• This introduces additional space overhead, two references per entry.
• This should result in some time savings -- what can be done faster?
• There are multiple implementation strategies
• Appreciate the separation of specification and implementation

The LinkedList class

• Member of the Java Collection Framework
• Extends AbstractSequentialList and implements several interfaces
• Specification includes two kinds of iterators:
• Iterator, which we've already studied. One-way traversal
• ListIterator, which provides two-way traversals and thus additional methods such as previous() and hasPrevious()
• Implemented using doubly-linked list. More on this later.

Data Structures for Doubly-Linked Lists

• Simple two-way structure
• Each entry has three references: element, previous entry, next entry
• Head is reference to first entry
• First entry has null for previous
• Last entry has null for next
• Consider code to add a new entry in front of an arbitrary entry: what are the special cases?
• Consider code to remove an arbitrary entry: what are the special cases?
• Simple two-way circular structure
• Each entry has three references: element, previous entry, next entry
• Head is reference to first entry
• First entry links to last entry as its previous
• Last entry links to first entry as its next
• Consider code to add a new entry in front of an arbitrary entry: what are the special cases?
• Consider code to remove an arbitrary entry: what are the special cases?
• Any advantages over the simple two-way structure?
• Two-way circular structure with dummy header entry
• Each entry has three references: element, previous entry, next entry
• Head is reference to a dummy entry
• dummy entry has null element and is not a member of the list per se
• dummy entry links to last entry as its previous
• dummy entry links to first entry as its next
• empty list consists of dummy entry linking to itself as both previous and next
• First entry links to dummy entry as its previous
• Last entry links to dummy entry as its next
• Consider code to add a new entry in front of an arbitrary entry: any special cases?
• Consider code to remove an arbitrary entry: any special cases?
• Any advantages over the other two structures? Oh, yeah!

Implementation of LinkedList class

• Implemented in java.util package
• Data structure is two-way circular with dummy header entry
• Even though implementation is circular, specification is linear (there is a first and a last entry)
• Maintains a size variable for constant time size()
• has iterator() for one-way traversal
• has listIterator() for two-way traversal