Some Other Collections: Bags, Sets, Queues and Maps COMP T2 Lecture 4 School of Engineering and Computer Science, Victoria University of Wellington Marcus Frean

RECAP  We met the almighty ArrayList and saw it was an implementation of the abstract data type List TODAY  A note on “Autoboxing”  Some other collection types: Bags, Sets, Queues, Maps Announcements:  Assignment#1 is out today (pick it up here at end of lecture)  Tutorials started today (9am!)  class reps: can I have your forms / details thanks 2 RECAP and TODAY

3 What can you put in a Collection ?  The type parameter of a collection can be any object type.  private List creatures = new ArrayList ();  public void processStrings(Set strings){…  Set > allJobs = new HashSet > ();  What about collections of numbers, or booleans, or chars…  private List myNums = new ArrayList ();  public int computeScore(List answers){….  Must use “wrapper classes”:  Integer, Boolean, Double, Char, etc  private List myNums = new ArrayList ();  public int computeScore (List answers) {…. Problem: these are “primitive types”, not Objects

4 Collection of Primitive Types  You could “wrap” a primitive type inside a wrapper object: List myNums = new ArrayList (); Integer num = new Integer(15); myNums.add(num);  And could write methods that “unwrap” again: int sum = 0; for (Integer num : myNums){ int n = num.intValue(); sum = sum + n; }  But this is just a pain!

 Good news: Java will do it automatically for you private List myNumbers = new ArrayList (); myNumbers.add(15); int sum = 0; for (Integer num : myNumbers) { sum = sum + num; } Or for (int num : myNumbers){ sum = sum + num; }  Java Auto-boxes a primitive type if a wrapper type is expected  and Auto-unboxes a wrapper type when primitive type expected. Auto-boxing 5

Sets  Set is a collection with:  no structure or order maintained  no access constraints (access any item any time)  Only property is that duplicates are excluded  Operations: (same as Bag, but different behaviour)  add(value) → true iff value was added (eg, not duplicate)  remove(value) → true iff value removed (was in set)  contains(value) → true iff value is in the set  findElement(value) → matching item, iff in value in the set ……  Sets are as common as Lists 6

Using Sets  Checking vocabulary of children’s books: private Set words = new HashSet (); public void readBook(Scanner sc){ while (sc.hasNext ()) words.add(sc.next()); } public void checkWord(String wd){ if (words.contains(wd)) UI.println(“Yes, ”+wd+“ is in the book”); else UI.println(“No, ”+wd+“ is not in the book”); } make an empty set add words to it check if a particular word is in the set 7

Interfaces:  Collection = Bag (most general)  List = ordered collection  Set = unordered, no duplicates  [Stack (not an interface!) ordered collection, limited access (add/remove at one end) ]  Map = key-value pairs (or mapping)  Queue ordered collection, limited access (add at end, remove from front) Collections library Classes:  List classes: ArrayList, LinkedList, Vector  Set classes: HashSet, TreeSet, EnumSet, LinkedHashSet,…  Stack classes: Stack  Map classes: EnumMap, HashMap, TreeMap, LinkedHashMap, WeakHashMap, …  … 8

Bags  A Bag is a collection with  no structure or order maintained  no access constraints (access any item any time)  duplicates allowed  examples ? A collection of current logged-on users. Bingo cards. The books in a book collection …  But not all that common! Typically we don’t really have duplicate items.  There are no standard implementations of Bag!! 9

Bags Java could have a Bag interface, like this: public interface Bag extends Collection {... But we might as well use the “Collection” interface itself. So if java had an implementation that used an array, then public class ArrayBag implements Collection {...  Minimal Operations:  add(value) → returns true iff a collection was changed  remove(value) → returns true iff a collection was changed  contains(value) → returns true iff value is in bag, uses equal to test.  findElement(value) → returns a matching item, iff in bag  Plus.... size(), isEmpty(), iterator(), clear(), addAll(collection), … 10

Queues  Queues embody the principle of “First In, First Out” (FIFO)  Collection of values with an order  Constrained access: Only remove from the front  Two varieties: Ordinary queues: only add at the back Priority queues: add with a given priority 11

Queues Some uses:  Operating Systems, Network Applications, multi-user systems Handling requests/events/jobs that must be done in order (often called a “buffer” in this context)  Simulation programs Representing queues in the real world (traffic, customers, deliveries, ….) Managing the events that must happen in the future  Search Algorithms Artificial Intelligence, Computer Games Java provides :  a Queue interface  several classes (ie. implementations) : LinkedList, PriorityQueue,... 12

Queue Operations  offer(value) ⇒ boolean  add a value to the queue  (sometimes called “enqueue”, like “push” )  poll() ⇒ value  remove and return value at front/head of queue or null if the queue is empty  (sometimes called “dequeue”, like “pop”)  peek() ⇒ value  return value at head of queue, or null if queue is empty (doesn’t remove from queue)  remove() and element()  like poll() and peek(), but throw exception if queue is empty.  A question: Why use Queue instead of List ?? 13

Maps 14 Key Value Not this kind of MAP Something that MAPs a key to a value NameAge Bob25 Rita34 Sam15 Adam 6 : : :

Maps  Collection of data, but not of single values:  Map = a set of pairs of keys and values  Constrained access: get values via keys.  No duplicate keys  Lots of implementations, most common is HashMap. “Sharon” ⇒ 228 “Marcus” ⇒ 227 “Becky” ⇒ 353 “Pondy” ⇒ 336 get (“Becky”) put (“Sharon”, 336) 15 remove(“Marcus”) put (“Pondy”, 336)

Maps  When declaring and constructing, must specify two types:  Type of the key, and type of the value private Map phoneBook; : phoneBook = new HashMap ();  Operations:  get(key) → returns value associated with key (or null)  put(key, value) → sets the value associated with key (and returns the old value, if any)  remove(key) → removes the key and associated value (and returns the old value, if any)  containsKey(key) → boolean  size() 16