The Java Collections Package

1
The Java Collections Package

• C. DeJong
• Fall 2001

2
Introduction

• A collection is an object that contains other
  objects
• Ex a poker hand, a phone directory, a mail folder

3
Collection framework features

• Interfaces
• Implementations
• Algorithms
• Polymorphic
• cf. the Standard Template Library (STL) in C

4
Benefits

• Data structures and algorithms for free
• Tested
• A well-known standard
• Software reuse

5
The Collection interface

• Root of the collections hierarchy
• Other interfaces extend Collection
• Java provides implementations of subinterfaces
  (Set, List, etc)
• For passing and manipulating collections
  generically
• Implementations have a constructor that accepts a
  Collection parameter

6
Collection methods (basic)

• int size()
• boolean isEmpty()
• boolean contains(Object element)
• boolean add(Object element)
• boolean remove(Object element)
• Iterator iterator()

7
The Iterator interface

• Used to traverse elements of a Collection
• Can remove elements
• Methods
• boolean hasNext()
• Object next()
• void remove()

8
Using Iterator
// simple method to print out each element in a
collection // using an Iterator public void
printContents(Collection stuff) for(Iterator
istuff.iterator()i.hasNext()) System.out.p
rintln(i.next())
9
Collection methods (bulk ops)

• boolean containsAll(Collection c)
• boolean addAll(Collection c)
• boolean removeAll(Collection c)
• boolean retainAll(Collection c)
• void clear()

10
Collection methods (arrays)

• Object toArray()
• Object toArray(Object a)

11
A note on Collection methods

• All Collection interfaces (Set, List, Map) have
  these methods (why?)
• Not all are supported
• Optional operations not supported throw
  UnsupportedOperationException
• Java uses Exceptions to represent errors at
  runtime (more on this later)

12
The Set interface

• extends Collection interface
• Cannot contain duplicate elements
• Models the mathematical Set abstraction
• Implemented by HashSet and TreeSet classes
• Use HashSet in most cases

13
Set interface methods

• Basic (size, isEmpty, contains, add, remove,
  iterator)
• Bulk (containsAll, addAll, removeAll, retainAll,
  clear)
• Arrays (toArray)

14
Using the Set interface
import java.util. public class FindDups
public static void main(String args)
Set s new HashSet() for (int
i0iltargs.lengthi) if
(!s.add(argsi)) System.out.println("
Duplicate detected " argsi)
System.out.println(s.size() " distinct words
detected " s) java FindDups
i came i saw i left Duplicate detected
i Duplicate detected i 4 distinct words
detected came, left, saw, i
15
The List interface

• extends Collection interface
• Ordered
• Duplicates OK
• Elements have a position and index number
• Similar to arrays
• Implemented by ArrayList, LinkedList and Vector
  (use ArrayList)
• beware of java.awt.List

16
List interface methods

• all methods from Collection interface
• Positional access (get, set, add, remove, addAll)
• Search (indexOf, lastIndexOf)
• Iteration (listIterator)
• Range-view (subList)

17
Using the List interface
List list1 new ArrayList() list1.add("John") l
ist1.add("George") List list2 new
Vector() list2.add("Paul") list2.add("Ringo") l
ist1.addAll(list2) for (int i0iltlist1.size()i
) String name (String)list1.get(i)
System.out.println(name) // John, then George,
etc. System.out.println((String)list1.get(1))
// George
18
The Map Interface

• does not extend Collection interface
• Maps keys to values ("associative array")
• Like List and arrays, but key instead of index
  number
• implemented by HashMap, TreeMap and HashTable
  (use HashMap)

19
Map interface methods

• no methods from Collection interface
• Basic (put, get, remove, containsKey,
  containsValue, size, isEmpty)
• Bulk (putAll, clear)
• Collection views (keySet, values, entrySet)

20
using the Map interface
// sample Map to store state capitols by state
abbreviation private Map capitols new
HashMap() capitols.put("MI","Lansing") capitols.
put("IL","Springfield") capitols.put("TX","Austin
") capitols.put("CA","Sacramento") String
michiganCap (String)capitols.get("MI") System.
out.println("The capitol of Michigan is "
michiganCap) System.out.println("Map has "
capitols.size() " elements") capitols.clear()
// deletes all elements System.out.println("Map
has " capitols.size() " elements")
21
The Collections class

• Not the same as the Collection interface
• Static utility methods to operate on Collection
  objects

22
Collections class methods

• Searching
• Sorting
• Copying
• Min/Max
• Reverse
• Shuffle
• Synchronization

• Unmodifiable
• Singletons
• Fill
• Constants for empty
• List
• Set
• Map
• Etc.

23
using the Collections class
import java.util. public class
CollectionsUse public static void main(String
args) List states new ArrayList() stat
es.add("AL") states.add("IL") states.add("MI
") states.add("CA") System.out.println("fir
st state is " Collections.min(states)) System
.out.println("last state is "
Collections.max(states)) System.out.println("st
ates in original order " states) Collections.
sort(states) System.out.println("states in
sorted order " states) Collections.reverse(st
ates) System.out.println("states in reverse
sorted order " states) Collections.shuffle(st
ates) System.out.println("states in random
order " states)