1. 1 Collections Working with More than One Number or Data Type or Object

2. 2 Collections There are three kinds of collections in Java: Arrays contain data types or objects Ordered Collections contain data types or objects (Vectors, LinkedLists,ArrayList) Dictionaries (or maps- "hash tables" in Java) containing data types or objects All three collections are objects

3. 3 Which Type of Collection Should We Use? Type depends on the nature of the problem Key characteristics to determine which type of collection to use: –Array: easy to create and use, but fixed size. –Sets: Set or SortedSet –Ordered Collection (or Lists -ArrayList): just about as easy to create and use, and can grow and shrink. More flexible. Have Iterators. –Linked Data Structures – true dynamic –Dictionary (or Map - Hashtable): used to store and retrieve values based on a key. Fast and easy for this purpose. Can also enumerate keys and values.

4. 4 Arrays Can hold any data type or object The size is fixed (!!) during declaration For example: –int [ ] daysInMonth; –String monthNames [ ]; To initialize: –daysInMonth = new int [ 12 ]; –monthNames = new String [ 12 ];

5. 5 Arrays Alternate array declaration: For example: –int [ ] daysInMonth; –String monthNames [ ] ; The choice of placing the brackets before or after is a matter of style Garside and James Gosling (author of Java) uses before Patrick Naughton (author of Java) uses after

6. 6 Arrays To populate (beginning at zero): –int daysInMonth [ ] = new int [ 12 ]; –daysInMonth [ 0 ] = 31; –daysInMonth [ 1 ] = 28; And: –String monthNames [ ] = new String [ 12 ]; –monthNames [ 0 ] = “January”; –monthNames [ 1 ] = “February”;

7. 7 Arrays An alternate way to populate: –int [ ] daysInMonth = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 10, 31 }; And: –String [ ] monthNames = { “January”; “February”; “March”, “April”, “May”, “June”, “July”, “August”, “September”, “October”, “November”, “December” };

8. 8 Arrays Other information: Length is obtained from a public class variable as follows: –int size1 = monthNames.length; –int size2 = daysInMonth.length; It is recommended that the programmer always used this class variable rather than a constant to make maintenance easier

9. 9 Using Loops With Arrays int [ ] daysInMonth = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 10, 31 }; int sum = 0; for (int ctr =0; ctr <= 11; ctr++) {sum = sum + daysInMonth[ctr];}

10. 10 Ordered Collections (for general info only) An Ordered Collection is similar to an array but it is more flexible –can grow and shrink –despite its name, an Ordered Collection does not keep its elements in order In Java an Ordered Collection is called a Vector or LinkedList or ArrayList

11. 11 Declaring and Initializing Ordered Collections Declaring a Vector or ArrayList: –Vector orderedCollection; –ArrayList listOfCustomers; And initializing it: –orderedCollection = new Vector ( ); –Could say: Collection myCollection;

12. 12 Declaring and Initializing Ordered Collections (note: this is for Vector or LinkedList) To add to ordered collection: –String thisCarType = new String("Toyota"); –orderedCollection.add(thisCarType); To retrieve from the ordered collection: String car = (String) orderedCollection.get(0); Note Cast – since get(int index) retrieves Object type

13. 13 Ordered Collections To locate in collection: –String string = new String("Toyota"); –if (orderedCollection.contains (string) ) int index = orderedCollection.indexOf ( string); To find the size of a collection use: –int size = orderedCollection.size ( ); Remember a Collection may grow

14. 14 ArrayList Java 2 created a new group of Collections that have many more features than a simple Vector. An ArrayList is one example of these new super classes. An ArrayList has more behavior and is synchronized to permit multiprocessing

15. 15 ArrayList For example to declare and initialize: –java.util.ArrayList cars = new java.util.ArrayList() To add to it: –cars.add ("Toyota"); To get from it: –String string = (String) cars.get(0); To remove from it: –cars.remove(string); To get its size: –int size = cars.size();

16. 16 ArrayList methods are the same for LinkedList Java 2 collections use an iterator pattern instead of subscripts to iterate through a dictionaries values: Iterator iterator = cars.iterator(); while(iterator.hasNext()) { String car = (String)iterator.next(); System.out.println(“Car: " + car ); } Note that subscripts can still be used but seldom are.

17. 17 Linked List A linked list consists of a number of links, each of which has a reference to the next link. Adding and removing elements in the middle of a linked list is efficient. Visiting the elements of a linked list in sequential order is efficient Random access is not efficient

18. 18 Inserting an Element into a Linked List is done by using a ListIterator object for the LinkedList object

19. 19 Java's LinkedList class Easy access to first and last elements with methods o void addFirst(Object obj) o void addLast(Object obj) o Object getFirst() o Object getLast() o Object removeFirst() o Object removeLast()

20. 20 ListIterator ListIterator object gives access to elements inside a LinkedList object ListIterator protects the linked list while giving access ListIterator encapsulates a position anywhere in the linked list

21. 21 A ListIterator object in a LinkedList List Iterator position

22. 22 Conceptual View of the ListIterator

23. 23 List Iterator The listIterator method of the LinkedList class gets a list iterator LinkedList list = new LinkedList();... ListIterator iterator = list.listIterator();

24. 24 List Iterator The next method moves the iterator iterator.next(); next throws a NoSuchElementException if you are already past the end of the list

25. 25 List Iterator hasNext returns true if there is a next element if (iterator.hasNext()) iterator.next();

26. 26 List Iterator hasNext returns true if there is a next element if (iterator.hasNext()) iterator.next();

27. 27 List Iterator The next method returns the object of the link that it is passing while iterator.hasNext() { Object obj = iterator.next(); //do something with the object }

28. 28 List Iterator To move the list position backwards, use: ohasPrevious oprevious

29. 29 Adding and Removing from a LinkedList The add method: o Adds an object after the iterator o Moves the iterator position past the new element iterator.add("Juliet");

30. 30 Adding and Removing from a LinkedList The remove method: o Removes and o Returns the object that was returned by the last call to next or previous

31. 31 Efficiency of Operations for Arrays and List

32. 32 A Queue

33. 33 A Stack of Books A stack can be visualized as a stack of books. You place books on top and remove from the top.

34. 34 Abstract Data Type Stack The Stack class is a concrete implementation of a stack in the Java library The Stack class uses an Object[] to implement a stack …………………………………………… OR you can just use the LinkedList class and only use the addLast() and removeLast() methods.

35. 35 Dictionaries In Java an Dictionary is called a Hashtable For example to declare and initialize: –Hashtable dictionary ; –Hashtable phoneBook; And initialize: –dictionary = new Hashtable ( ); –phoneBook = new Hashtable ( ); Note the parent class of Hashtable is Dictionary

36. 36 Dictionaries To add to a dictionary: SUE is the key –dictionary.put ("Sue", "3249"); To get from dictionary : –String string = (String) dictionary.get ("Sue"); Note 1: what you store is what you retrieve. If you store a car object, then you retrieve a car object, not a String, or an int, or a... Note 2: an object of type Object is returned, so you have to "cast" object to be stored in a String (or into whatever type of value was originally stored).

37. 37 Dictionaries To find the size of a dictionary use: –int size = dictionary.size ( ); To check if key is not there: String string = (String) dictionary.get ("Sue"); if (string = = null) System.out.println ( "Not found: " + string); Remember a Dictionary may grow

38. 38 Dictionaries Sometimes we want to go through all the keys in the dictionary –In the phone book example, this would be the list of people's names Sometimes we want to go through all the values in the dictionary –In the phone book example, this would be a list of all the phone numbers This is called "enumeration", and the list of keys (or values) is also called "an enumeration"

39. 39 Dictionaries To enumerate through a dictionary's keys: Enumeration enumeration; enumeration = dictionary.keys ( ); while(enumeration.hasMoreElements ( ) ) { key = (String) enumeration.nextElement ( ); System.out.println ( "Key is: " + key); }

40. 40 Dictionaries To enumerate through a dictionary's values: Enumeration enumeration; enumeration = dictionary.elements ( ); while(enumeration.hasMoreElements ( ) ) { value = (String) enumeration.nextElement ( ); System.out.println ( "Value is: " + value); } Since you retrieve an object, you need to cast it back to the type of object it really is (e.g., a car) –you then pull out the values from the object using your getters

41. 41 Which Type of Collection Should We Use? Type depends on the nature of the problem Key characteristics to determine which type of collection to use: –Array: easy to create and use, but fixed size. –Ordered Collection (or Vector): just about as easy to create and use, and can grow and shrink. More flexible. –Dictionary (or Hashtable): used to store and retrieve values based on a key. Fast and easy for this purpose. Can also enumerate keys and values.