Example: LinkedSet<T>

1
Example LinkedSetltTgt

• LinkedSet UML Class Diagram
• LinkedSetltTgt Class
• LinkedSet Attributes/Constructor
• Linked Set Methods
• LinkedSet iterator method
• LinkedIteratorltTgt class
• Project 1 Q/A
• Reading LC 15.4

2
LinkedSet UML Class Diagram
ltltinterfacegtgt SetADTltTgt
LinkedSetltTgt

• - rand Random
• - count int
• contents LinearNode
• LinkedSet()

add( ) void addAll( ) void removeRandom(
) T remove( ) union( ) contains( )
equals( ) isEmpty( ) size( ) iterator( )
toString( )
LinearNodeltTgt
- next LinearNode - element T
LinearNode(void) LinearNode
(element T) setNext(node ) void
getNext(void) LinearNode
getElement( ) T set Element (element
T) void
LinkedIteratorltTgt
ltltinterfacegtgt IteratorltTgt
ltltinterfacegtgt IterableltTgt
Self referential or Recursive Assoc.
3
LinkedSetltTgt Class

• The LinkedSetltTgt class implements the same
  SetADTltTgt interface as ArraySetltTgt using a singly
  linked list instead of an array
• Externally, your code is not much different when
  using the LinkedSetltTgt class instead of the
  ArraySetltTgt class
• SetADTltStringgt mySet new ArraySetltStringgt()
• OR new LinkedSetltStringgt()
• Internally, method code of the LinkedSetltTgt class
  is very different due to the difference in the
  underlying data structure being used

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LinkedSetltTgt Class

• Again We assume that SetADTltTgt interface extends
  Iterable
• If not, we would need to add Iterable after
  SetADTltTgt in the implements clause of the
  LinkedSetltTgt class header

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LinkedSet Attributes/Constructor

• The class definition starts with
• public class LinkedSetltTgt implements SetADTltTgt
• private static Random rand new Random()
• private int count
• private LinearNodeltTgt contents
• public LinkedSet() // default constructor
• count 0
• contents null // NOTE not an array

6
LinkedSet Attributes/Constructor

• Variable contents is an object reference
  variable not an array
• We dont need to define a default capacity or
  pass an initial capacity to the constructor
• We dont need to instantiate an array object or
  any LinearNode objects in the constructor just
  set contents to null

7
LinkedSet Methods

• Note that because we are not using a fixed size
  data structure such as an array, we dont need a
  private expandCapacity() method
• Again, these lecture notes will do some code
  differently from the textbook so that you can see
  more than one way of doing it

8
LinkedSet Methods

• add O(n)
• public void add (T element)
• if (!contains(element))
• LinearNodeltTgt node new LinearNodeltTgt(element)
  
• node.setNext(contents)
• contents node
• count

9
LinkedSet Methods

• contains O(n)
• public boolean contains (T target)
• LinearNodeltTgt next contents
• while (next ! null)
• if (next.getElement().equals(target)
• return true
• next next.getNext()
• return false

10
LinkedSet Methods

• removeRandom O(n) not O(1) like ArraySet
• public T removeRandom() throws EmptySetException
• LinearNodeltTgt current
• T result null
• if (isEmpty)
• throw new EmptySetException()
• int choice rand.nextInt(count) 1

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LinkedSet Methods

• removeRandom (Continued)
• if (choice 1) // remove from beginning
• result contents.getElement()
• contents contents.getNext()
• else // remove from middle or end
• current contents
• for (int skip 2 skip lt choice skip)
• current current.getNext()
• result current.getNext().getElement()
• current.setNext(current.getNext().getNext())
• count--
• return result
• // bypassed LinearNode becomes garbage

12
LinkedSet iterator Method

• iterator O(1)
• public IteratorltTgt iterator
• return new LinkedIteratorltTgt(contents)
• We will study the LinkedIterator class to
  understand how it is implemented

13
LinkedIteratorltTgt Class

• We may have several collection classes like the
  LinkedSet class that use an underlying singly
  linked linear data structure
• Again, we would like to reuse one Iterator class
  for all of these collection classes
• So we write a general purpose Iterator class for
  use with linked data structures

14
LinkedIteratorltTgt Class

• The iterator method of the LinkedSet class
  instantiates and returns a reference to a
  LinkedIterator object to its caller
• The LinkedIterator constructor needs to get a
  reference to the first LinearNode object in the
  specific linked structure to be iterated

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LinkedIteratorltTgt Class

• Class / Attribute Definitions and Constructor
• public class LinkedIteratorltTgt
• implements IteratorltTgt
• private LinearNodeltTgt current // current
  position
• public LinkedIterator(LinearNodeltTgt current)
• this.current current

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LinkedIterator Methods

• hasNext O(1)
• public boolean hasNext()
• return current ! null
• next O(1)
• public T next()
• if (!hasNext())
• throw new NoSuchElementException()
• T result current.getElement()
• current current.getNext()
• return result
• // old LinearNode does not become garbage
  (Why?)

17
LinkedIterator Methods

• remove O(1)
• We dont need to implement real code for the
  remove method, but there is no return value that
  we can use to indicate that it is not implemented
• If we dont implement it, we indicate that the
  code is not implemented by throwing an exception
• public void remove() throws
  UnsupportedOperationException
• throw new UnsupportedOperationException()

18
LinkedIterator Methods

• Again, if we implement the remove method, notice
  that we dont specify the element that is to be
  removed and we do not return a reference to the
  element being removed
• It is assumed that the calling code has been
  iterating on condition hasNext() and calling
  next() and already has a reference
• The last element returned by next() is the
  element that will be removed

19
LinkedIterator Method Analysis

• All three LinkedIterator methods are O(1)
• However, they are usually called inside an
  external while loop or for-each loop
• Hence, the process of iterating through a
  collection using an Iterator is O(n)

20
LinkedIterator Class in Text

• There is an anomaly in the definition of the
  LinkedIterator class in the textbook
• The class has a count attribute and the
  constructor has a parameter to set count
• But that attribute is not used anywhere in the
  code of the other methods
• The code can and does always identify the end of
  the list by the null terminator value
• Why was count included?

21
LinkedSet Alternative

• If the count attribute was of no use in the
  LinkedIterator class, is it also of no use in the
  LinkedSet class?
• It is possible to implement the code of the
  LinkedSet class without a count attribute
• What is adversely affected by removing it?
• Size method would become O(n) instead of O(1)
• RemoveRandom method would need to use size method
  to calculate its choice of node to remove, but it
  is already O(n)

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Project 1 Q/A

• Project 1 is due before next class
• Any questions about Project 1?