Collections

1
Collections

• James Brucker

2
Collection

• A Collection is a group or set of objects. Also
  called a container.
• Set
• an unordered collection
• no duplicates.
• List
• ordered collection, duplicates are allowed
• can insert/delete elements anywhere
• Queue and Stack
• special rules for LIFO, FIFO disciplines

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Collections in Programming

• Collections are essential to many applications.
• Example read some words from the console and
  save in a list. We don't know how many words.

ListltStringgt wordlist new ArrayListltStringgt( )
while ( console.hasNext( ) ) String word
console.next( ) wordlist.add( word ) // how
many words are there? int number wordlist.size(
) // sort the words Collections.sort( wordlist
) // does it contain the word "dog" if (
wordlist.contains( "dog" ) ) ...
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Java Collections
Collection a group of objects
Map mapping of a set to another
Set unordered, no duplicates
List ordered, duplicates ok
Queue FIFO discipline
HashMap map using a hash table
Sorted Set Set with total ordering
ArrayList
Priority Queue
LinkedList
ArrayDeque
HashSet
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What can a Collection do?
Collection a group of objects
add( Element ) clear( ) contains( Element
) isEmpty( ) iterator( ) remove( Element ) size(
) toArray( )
creates
Iterator hasNext( ) bool next( )
Element remove( )
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What can a Set do?
Collection a group of objects
add( Element ) clear( ) contains( Element
) isEmpty( ) iterator( ) remove( Element ) size(
) toArray( )
Set unordered, no duplicates
Sorted Set Set with total ordering
A set provides the same behavior as a Collection,
but with some extra guarantees, such as "no
duplicates".
HashSet
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Set Example
Get students in my class, and kick out any hacker.
// The getClass( ) method returns students as a
Set SetltStudentgt myclass Registrar.getClass(
"214531" ) // Is the student "hacker" in the
set? if ( myclass.contains( hacker ) )
myclass.remove( hacker )
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What can a List do?
Collection a group of objects
add( Element ) clear( ) contains( Element
) isEmpty( ) iterator( ) remove( Element ) size(
) toArray( )
List ordered, duplicates ok
ArrayList
add( index, Element ) get( index ) indexOf(
Element ) remove( index )
LinkedList
A list adds ability to access any element by its
position
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List Example
// getClassList returns a List of
students ListltStudentgt myclass new
ArrayListltStudentgt( ) myclass
Registrar.getClassList( "214531" ) // sort the
students Collections.sort( myclass ) // print
all the students in the class for ( Student st
myclass ) System.out.println( st.toString()
)
A "for-each" loop words with any collection
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What can a Map do?
Map keys to values key ? value
Map mapping of a set to another
HashMap map using a hash table
clear( ) get( Key ) Value keySet( ) get a Set
of all keys put( Key, Value ) remove( Key ) size(
) values( ) a Collection of values
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Map Example
Create a map between Coupon colors and money
values.
MapltString,Integergt map new HashMapltString,Integ
ergt( ) map.put( "red", 100 ) // red is worth
100 Baht map.put( "blue", 50 ) // blue is worth
50 Baht map.put( "green", 10 ) int x map.get(
"blue") /// returns x 50 // now process a
coupon String color console.next( ) // get
the coupon's color if ( map.containsKey( color )
) value map.get( color ) total total
value
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The Java Collections Framework
ltltinterfacegtgt Collection
ltltinterfacegtgt Map
extends
ltltinterfacegtgt Set
ltltinterfacegtgt List
ltltinterfacegtgt Queue
HashMap map using a hash table
implements
Sorted Set Set with total ordering
ArrayList
Priority Queue
LinkedList
ArrayDeque
HashSet
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Collections all use a Type parameter

• This is for safer and easier programming.
• You don't need to use "cast" to convert to data
  type.
• Student student (Student)list.get(k)

No
ListltStudentgt myclass new ArrayListltStudentgt(
) Student who myclass.get( 0 )
myclass can only contain elements of type
"Student". myclass.get( k ) always returns a
Student object.
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All Collections are Iterable
ltltinterfacegtgt Iterable iterator() Iterator
This means you can use a "foreach" loop to
transverse the elements of any collection. ListltSt
udentgt myclass myclass regis.getClass(
) for(Student s myclass ) ...
ltltinterfacegtgt Collection
ltltinterfacegtgt Set
ltltinterfacegtgt List
ltltinterfacegtgt Queue
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How to Iterate over a Collection (1)
Collection collection ... Iterator it
collection.iterator( ) while ( it.hasNext( ) )
Object obj it.next( ) // do something
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A "for each" loop

• for-each x in myAddressbook
• call x and invite him to party
• end

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How to Iterate over a Collection (2)
Java "for - each" loop for( Type x Iterable )
... for( Type x array ) ...
ListltCoingt coins new ArrayListltCoingt( ) for(
Coin x coins ) x.getValue( )
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Parts of the Collections Framework

• Interfaces - define behavior of the collection
  types
• Implementations - the concrete collection classes
• Polymorphic Algorithms - the same operation is
  implemented by many different types of
  collection.
• and
• the algorithms apply to any data type in the
  collection.

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Example of Polymorphism
any implementation is OK
CollectionltStudentgt coll new LinkedListltStudentgt
( ) coll .add( new Student( id1, name1) ) coll
.add( new Student( id2, name3) ) int n coll
.size( ) Collections.sort( coll ) for( Student
student coll ) System.out.println( student
)
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Example of Polymorphism (2)
CollectionltBirdgt coll new LinkedListltBirdgt( )
coll .add( new Bird( id1, name1) ) coll .add(
new Bird( id2, name3) ) int n coll .size(
) Collections.sort( coll ) for( Object student
coll ) System.out.println( student )
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Array Operations

• All Collections have a toArray( ) method.
• This lets you convert any collection to an array.

CollectionltStudentgt myclass Registrar.getClass(
"214531" ) // create an array to hold the
students Student array new Student
myclass.size( ) // copy the List to
array myclass.toArray( array )
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Exercise

• For the KU Coupons application, write a Store
  class that records all the Transactions in an
  ArrayList.
• Write a getTotalSales( ) method that sums the
  sales amount in all Transactions.
• Write a printSalesReport( ) to print the sales.

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Resource

• "Collections Trail" in the Java Tutorial
• http//java.sun.com/docs/books/tutorial/index.htm
  l