Example: LinkedSet<T> LinkedSet<T> Class LinkedSet Attributes/Constructor Linked Set Methods LinkedSet iterator method LinkedIterator<T> class Reading: L&C 3rd: 15.4 2nd: 4.1-4.5 1

LinkedSet<T> Class The LinkedSet<T> class implements the same SetADT<T> interface as ArraySet<T> using a singly linked list instead of an array Externally, your code is not much different when using the LinkedSet<T> class instead of the ArraySet<T> class: SetADT<String> mySet = new ArraySet<String>(); OR = new LinkedSet<String>(); Internally, method code of the LinkedSet<T> class is very different due to the difference in the underlying data structure being used. 2

LinkedSet<T> Class Again: We assume that SetADT<T> interface extends Iterable. 3

LinkedSet Attributes/Constructor The class definition starts with: public class LinkedSet<T> implements SetADT<T> { private static Random rand = new Random(); private int count; private LinearNode<T> contents; public LinkedSet() // default constructor count = 0; contents = null; // NOTE: not an array } 4

LinkedSet Attributes/Constructor We don’t need to define a default capacity or pass an initial capacity to the constructor We don’t need to instantiate an array object or any LinearNode objects in the constructor – just set contents to null 5

LinkedSet Methods Note that because we are not using a fixed size data structure such as an array, we don’t need a private expandCapacity() method. 6

LinkedSet Methods add – O(n) public void add (T element) { if (!contains(element)) LinearNode<T> node = new LinearNode<T>(element); node.setNext(contents); contents = node; count++; } 7

LinkedSet Methods contains – O(n) public boolean contains (T target) { LinearNode<T> next = contents; while (next != null) { if (next.getElement().equals(target) return true; next = next.getNext(); } return false; 8

LinkedSet Methods removeRandom – O(n) not O(1) like ArraySet public T removeRandom() throws EmptySetException { LinearNode<T> current; T result = null; if (isEmpty) throw new EmptySetException(); int choice = rand.nextInt(count) + 1; 9

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 < choice; skip++) current = current.getNext(); result = current.getNext().getElement(); current.setNext(current.getNext().getNext()); count--; return result; } // bypassed LinearNode becomes garbage 10

LinkedSet iterator Method iterator – O(1) public Iterator<T> iterator { return new LinkedIterator<T>(contents) } We will study the LinkedIterator class to understand how it is implemented 11

LinkedIterator<T> 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 12

LinkedIterator<T> 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 13

LinkedIterator<T> Class Class / Attribute Definitions and Constructor public class LinkedIterator<T> implements Iterator<T> { private LinearNode<T> current; // current position public LinkedIterator(LinearNode<T> current) this.current = current; } 14

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?) 15

LinkedIterator Methods remove – O(1) We don’t 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 don’t implement it, we indicate that the code is not implemented by throwing an exception public void remove() throws UnsupportedOperationException { throw new UnsupportedOperationException(); } 16

LinkedIterator Methods Again, if we implement the remove method, notice that we don’t 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 17

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) 18

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? 19

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) 20