Collections

1
Collections

• Robert Stumpf, Professor
• Computer Information Systems
• California State Polytechnic University, Pomona

2
Collections

• There are three kinds of collections in Java
• Arrays containing data types or objects
• Ordered Collections containing objects only
• Dictionaries containing objects only
• All three collections are objects

3
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

4
Arrays

• Alternate array declaration
• For example
• int daysInMonth
• String monthNames
• The choice of placing the brackets before or
  after is a matter of style
• James Gosling (author) uses before
• Patrick Naughton (author) uses after

5
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

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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

7
Arrays

• Other information
• Length is obtained from a public static 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

8
Ordered Collections

• In Java an Ordered Collection is called a Vector
• For example to declare and initialize
• Vector orderedCollection
• And initialize
• orderedCollection new Vector ( )

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Ordered Collections

• To add to collection
• String string new String("Toyota")
• orderedCollection.add(string)
• To get from collection
• String string (String) orderedCollection.
  get(0)
• Remember an Vector can only hold objects

10
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 Vector may grow

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Ordered Collections

• To iterate through a vectors values
• Iterator iterator
• iterator orderedCollection.iterator()
• while (iterator.hasNext())
• value (String) iterator.next()
• System.out.println ( "Value is " value)

12
Dictionaries

• In Java an Dictionary is called a Hashtable
• For example to declare and initialize
• Hashtable dictionary
• And initialize
• dictionary new Hashtable ( )
• Note the parent class of Hashtable is Dictionary

13
Dictionaries

• To add to a dictionary
• dictionary.put ("Sue", "3249")
• To get from dictionary
• String string (String) dictionary.get
  ("Sue")
• Remember a Dictionary can only hold objects
• Remember a Dictionarys key must be an object

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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

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Dictionaries

• To iterate through a dictionaries keys
• iterator dictionary.keySet().iterator()
• while (iterator.hasNext())
• key (String) iterator.next()
• System.out.println("Key is " key)

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Dictionaries

• To iterate through a dictionaries values
• iterator dictionary.values().iterator()
• while (iterator.hasNext())
• value (String) iterator.next()
• System.out.println("Value is " value)

17
Summary

• Remember arrays in Java are objects
• An Ordered Collection is called a Vector
• A Dictionary is called a Hash Table
• Arrays are fixed in size while Ordered
  Collections and Dictionaries may grow
• Arrays can hold data types while Ordered
  Collections and Dictionaries hold objects

18
Thank You

• Collections are the workhorses of Java
• Any questions should be directed to Professor
  Robert Stumpf
• Email rvstumpf_at_csupomona.edu
• Web Sitehttp//www.csupomona.edu/rvstumpf