100_lecture26 - Introduction to Computation and Problem...

Introduction to Computation and Problem
Solving
Prof. Steven R. Lerman
and
Dr. V. Judson Harward
Class 26:
Class 26:
Linked Lists
Linked Lists
2
The Java Collection Classes
• The java.util package contains
implementations of many data structures that we
are also going to discuss and implement in a
simpler way in class.
• You are welcome to use the standard Java
implementations in problem sets. They are more
elaborate and abstract than the implementations
we will develop.
• Learning how to implement a few data structures
makes you more sophisticated in how you use all
data structures.
• We will cover the Java implementations in a later
lecture.
1

3
Goals
•
data structure (a Linked List) using linking
rather than arrays.
•
data structure is as much as its interface
(set of methods).
We will examine and build a new kind of
The most important theme, however, is
that implementation affects how useful a
4
A list is a collection of elements that has a
–
–
–
order an element at a time.
Lists as an Abstract Data Type
particular order.
It can have arbitrary length.
You should be able to insert or delete an
element anywhere.
You should be able to go through the list in
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5
A List Interface
public interface List<E> {
public boolean isEmpty();
public void addFirst( E e );
public void addLast( E e );
public boolean contains( E e );
public boolean remove( E e );
public E removeFirst()
throws NoSuchElementException;
public void clear();
public int size();
}
6
• There are certain obvious things we would
like to do with lists that we can't do using
–
–
end?
• One approach is to number or index the
positions in the list.
Lists and Ordinal Position
only this interface. Two examples:
How would you access the elements of the list
if you did not know them in advance?
How would you insert an item into the list at
any other position than the beginning and the
3

7
Indexed Lists
• We could then add three methods
– public E get( int n )
throws IndexOutOfBoundsException;
– public E remove( int n )
throws IndexOutOfBoundsException;
– public void add( E e, int n )
throws IndexOutOfBoundsException;
•
for ( int i = 0; i < myList.size(); i++ )
{
E e = myList.get( i );
. . .
}
The following code block will traverse an indexed
list, myList:
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Indexed Lists, 2
• ArrayList )
•
–
–
•
Lists implemented using arrays (e.g., Java’s
often provide such methods because they are easy to
implement.
The idea of using an index to access list members can lead
to problems, however.
Since the index depends on ordinal position, it can
change every time an item is added to or deleted from
the list.
If the list is not implemented on top of an indexed data
structure like an array, accessing an indexed element is
slow.
When we use large lists like a telephone directory in
everyday life, we don't consider the index of an entry, only
its relative position.