Collections

1
Collections

• Working with More than One Number or Data Type

2
Collections

• There are three kinds of collections in Java
• Arrays contain
• Can contain data types or objects
• Ordered Collections
• Called Vectors in Java
• Contain objects
• Dictionaries
• Called Hashtables in Java
• Both keys and related values must be objects
• All three collections are themselves objects

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.
• 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.

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
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
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
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
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
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 lt 11 ctr)
• sum sum daysInMonthctr

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

11
Declaring and Initializing Ordered Collections

• Declaring a Vector
• Vector orderedCollection
• Vector listOfCustomers
• And initializing it
• orderedCollection new Vector ( )

12
Declaring and Initializing Ordered Collections

• To add to ordered collection
• String thisCarType new String("Toyota")
• orderedCollection.addElement(thisCarType)
• To retrieve from the ordered collection
• String car (String) orderedCollection.elementAt
  (0)
• Note how you have to cast the Object returned by
  the method elementAt(int) back to the specific
  type of Object, in this case a String, which was
  actually stored there

13
Ordered Collections

• To locate in collection
• String carName new String("Toyota")
• if (orderedCollection.contains (carName) ) int
  index orderedCollection.indexOf (carName)
• To find the size of a collection use
• int size orderedCollection.size ( )
• Remember a Vector may grow
• Now might be a good time to look at the API for
  the Vector class

14
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

15
Dictionaries

• To add to a dictionary
• 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 Person object, then you retrieve a
  Person object, not a String which is the name of
  the person, or an int which is their age, or
• Note 2 an object of type Object is returned, so
  you have to "cast" that Object into a Person
  object (or into whatever type of object was
  originally stored).

16
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

17
Dictionaries

• Sometimes we want to go through all the keys in
  the dictionary
• In the phone book example, this is 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"

18
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)

19
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 (for
  example a Car or Person object)
• you then pull out the values from the object
  using your getters

20
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.

21
Summary

• Remember Arrays in Java are objects
• All general purpose computer languages have
  arrays, and you need to be able to program using
  them
• An Ordered Collection is called a Vector
• A Dictionary is called a Hashtable
• Arrays are fixed in size
• Arrays can not contain a mix of things
• Arrays can be specified to contain data types or
  Objects
• Vectors and Dictionaries may grow in size
• Vectors and Dictionaries can not store data
  types, only Objects