1. Java Collections CHAPTER 9 @geoffjuma 3-February-2003
07/14/16
Java Collections
CHAPTER 9
@geoffjuma
3-February-2003
Java collection types

2. 07/14/16
Agenda
Lists
Sets
Maps
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Java collection types

3. Readings and References
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Readings and References
References
"Collections", Java tutorial
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Java collection types

4. 07/14/16
Java 2 Collections
A collection is an object that groups multiple elements into a single unit
Very useful
store, retrieve and manipulate data
transmit data from one method to another
data structures and methods written by hotshots in the field
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Java collection types

5. 07/14/16
Problem Example
You want to develop a java application for a client in the matatu industry.
The matatu carries passengers from a fixed location to a fixed destination but has several stops along the way where passengers can board and alight at will;
The matatu can only carry a fixed number of passengers subject to the number of seats available.
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6. Collections Framework
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Collections Framework
Unified architecture for representing and manipulating collections.
A collections framework contains three things
Interfaces
Implementations
Algorithms
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Java collection types

7. Collections Framework Diagram
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Collections Framework Diagram
Interfaces, Implementations, and Algorithms
From Thinking in Java, page 462
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Java collection types

8. Collection Interface Defines fundamental methods
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Collection Interface
Defines fundamental methods
int size();
boolean isEmpty();
boolean contains(Object element);
boolean add(Object element); // Optional
boolean remove(Object element); // Optional
Iterator iterator();
These methods are enough to define the basic behavior of a collection
Provides an Iterator to step through the elements in the Collection
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9. Iterator Interface Defines three fundamental methods
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Iterator Interface
Defines three fundamental methods
Object next()
boolean hasNext()
void remove()
These three methods provide access to the contents of the collection
An Iterator knows position within collection
Each call to next() “reads” an element from the collection
Then you can use it or remove it
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Iterator Position
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11. Example - SimpleCollection
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Example - SimpleCollection
public class SimpleCollection {
public static void main(String[] args) {
Collection c;
c = new ArrayList();
System.out.println(c.getClass().getName());
for (int i=1; i <= 10; i++) {
c.add(i + " * " + i + " = "+i*i); }
Iterator iter = c.iterator();
while (iter.hasNext())
System.out.println(iter.next());
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12. List Interface Context
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List Interface Context
Collection
List
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List Interface
The List interface adds the notion of order to a collection
The user of a list has control over where an element is added in the collection
Lists typically allow duplicate elements
Provides a ListIterator to step through the elements in the list.
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14. ListIterator Interface
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ListIterator Interface
Extends the Iterator interface
Defines three fundamental methods
void add(Object o) - before current position
boolean hasPrevious()
Object previous()
The addition of these three methods defines the basic behavior of an ordered list
A ListIterator knows position within list
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15. Iterator Position - next(), previous()
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Iterator Position - next(), previous()
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16. ArrayList and LinkedList Context
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ArrayList and LinkedList Context
Collection
List
ArrayList
LinkedList
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17. List Implementations ArrayList LinkedList low cost random access
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List Implementations
ArrayList
low cost random access
high cost insert and delete
array that resizes if need be
LinkedList
sequential access
low cost insert and delete
high cost random access
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18. ArrayList overview Constant time positional access (it’s an array)
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ArrayList overview
Constant time positional access (it’s an array)
One tuning parameter, the initial capacity
ArrayList supports dynamic arrays that can grow as needed.
Standard Java arrays are of a fixed length. After arrays are created, they cannot grow or shrink, which means that you must know in advance how many elements an array will hold.
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19. 07/14/16
ArrayList methods
The indexed get and set methods of the List interface are appropriate to use since ArrayLists are backed by an array
Object get(int index)
Object set(int index, Object element)
Indexed add and remove are provided, but can be costly if used frequently
void add(int index, Object element)
Object remove(int index)
May want to resize in one shot if adding many elements
void ensureCapacity(int minCapacity)
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ArrayList
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21. LinkedList overview Stores each element in a node
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LinkedList overview
Stores each element in a node
Each node stores a link to the next and previous nodes
Insertion and removal are inexpensive
just update the links in the surrounding nodes
Linear traversal is inexpensive
Random access is expensive
Start from beginning or end and traverse each node while counting
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22. LinkedList entries 3-February-2003 Java collection types
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LinkedList entries
private static class Entry {
Object element;
Entry next;
Entry previous;
Entry(Object element, Entry next, Entry previous) {
this.element = element;
this.next = next;
this.previous = previous; }
private Entry header = new Entry(null, null, null);
public LinkedList() {
header.next = header.previous = header;
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23. 07/14/16
Linked List Example
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24. LinkedList methods The list is sequential, so access it that way
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LinkedList methods
The list is sequential, so access it that way
ListIterator listIterator()
ListIterator knows about position
use add() from ListIterator to add at a position
use remove() from ListIterator to remove at a position
LinkedList knows a few things too
void addFirst(Object o), void addLast(Object o)
Object getFirst(), Object getLast()
Object removeFirst(), Object removeLast()
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25. Set Interface Context Collection Set 3-February-2003
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Set Interface Context
Collection
Set
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26. Set Interface Java's java.util.Set interface defines the set.
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Set Interface
Java's java.util.Set interface defines the set.
A set can't have any duplicate elements in it. Additionally, the set has no set order. As such, elements can't be found by index.
Same methods as Collection
different contract - no duplicate entries
Defines two fundamental methods
boolean add(Object o) - reject duplicates
Iterator iterator()
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27. Hash Set Hash sets are sets that use hashes to store elements.
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Hash Set
Hash sets are sets that use hashes to store elements.
A hashing algorithm is an algorithm that takes an element and converts it to a chunk of a fixed size called a hash.
For example, let our hashing algorithm be (x mod 10). So the hashes of 232, 217 and 19 are 2,7, and 9 respectively
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28. HashSet Find and add elements very quickly
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HashSet
Find and add elements very quickly
uses hashing implementation in HashMap
Hashing uses an array of linked lists
The hashCode() is used to index into the array
Then equals() is used to determine if element is in the (short) list of elements at that index
No order imposed on elements
The hashCode() method and the equals() method must be compatible
if two objects are equal, they must have the same hashCode() value
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Hash Code
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Buckets
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31. HashSet and TreeSet Context
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HashSet and TreeSet Context
Collection
Set
HashSet
TreeSet
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Tree Set
TreeSet provides an implementation of the Set interface that uses a tree for storage. Objects are stored in sorted, ascending order.
Access and retrieval times are quite fast, which makes TreeSet an excellent choice when storing large amounts of sorted information that must be found quickly.
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33. TreeSet Elements can be inserted in any order
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TreeSet
Elements can be inserted in any order
The TreeSet stores them in order
Red-Black Trees out of Cormen-Leiserson-Rivest
An iterator always presents them in order
Default order is defined by natural order
objects implement the Comparable interface
TreeSet uses compareTo(Object o) to sort
Can use a different Comparator
provide Comparator to the TreeSet constructor
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34. Map Interface Context Map 3-February-2003 Java collection types
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Map Interface Context
Map
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Java collection types

35. Map Interface Stores key/value pairs Maps from the key to the value
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Map Interface
Stores key/value pairs
Maps from the key to the value
Keys are unique
a single key only appears once in the Map
a key can map to only one value
Values do not have to be unique
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36. Map methods Object put(Object key, Object value)
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Map methods
Object put(Object key, Object value)
Object get(Object key)
Object remove(Object key)
boolean containsKey(Object key)
boolean containsValue(Object value)
int size()
boolean isEmpty()
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37. Map views A means of iterating over the keys and values in a Map
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Map views
A means of iterating over the keys and values in a Map
Set keySet()
returns the Set of keys contained in the Map
Collection values()
returns the Collection of values contained in the Map. This Collection is not a Set, as multiple keys can map to the same value.
Set entrySet()
returns the Set of key-value pairs contained in the Map. The Map interface provides a small nested interface called Map.Entry that is the type of the elements in this Set.
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38. HashMap and TreeMap Context
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HashMap and TreeMap Context
Map
HashMap
TreeMap
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39. HashMap and TreeMap HashMap TreeMap
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HashMap and TreeMap
HashMap
The keys are a set - unique, unordered
Fast
TreeMap
The keys are a set - unique, ordered
Same options for ordering as a TreeSet
Natural order (Comparable, compareTo(Object))
Special order (Comparator, compare(Object, Object))
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Bulk Operations
In addition to the basic operations, a Collection may provide “bulk” operations
boolean containsAll(Collection c);
boolean addAll(Collection c); // Optional
boolean removeAll(Collection c); // Optional
boolean retainAll(Collection c); // Optional
void clear(); // Optional
Object[] toArray();
Object[] toArray(Object a[]);
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Utilities Context
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Utilities
The Collections class provides a number of static methods for fundamental algorithms
Most operate on Lists, some on all Collections
Sort, Search, Shuffle
Reverse, fill, copy
Min, max
Wrappers
synchronized Collections, Lists, Sets, etc
unmodifiable Collections, Lists, Sets, etc
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43. Legacy classes Still available Don’t use for new development
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Legacy classes
Still available
Don’t use for new development
unless you have to, eg, J2ME, J2EE in some cases
Retrofitted into Collections framework
Hashtable
use HashMap
Enumeration
use Collections and Iterators
if needed, can get an Enumeration with Collections.enumeration(Collection c)
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44. More Legacy classes Vector Stack BitSet Properties use ArrayList
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More Legacy classes
Vector
use ArrayList
Stack
use LinkedList
BitSet
use ArrayList of boolean, unless you can’t stand the thought of the wasted space
Properties
legacies are sometimes hard to walk away from …
see next few pages
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45. Properties class Located in java.util package
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Properties class
Located in java.util package
Special case of Hashtable
Keys and values are Strings
Tables can be saved to/loaded from file
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System properties
Java VM maintains set of properties that define system environment
Set when VM is initialized
Includes information about current user, VM version, Java environment, and OS configuration
Properties prop = System.getProperties();
Enumeration e = prop.propertyNames();
while (e.hasMoreElements()) {
String key = (String) e.nextElement();
System.out.println(key + " value is " +
prop.getProperty(key)); }
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