1

Java's Collection Framework

By Rick Mercer with help from

The Java Tutorial, The Collections Trail, by Joshua Block

2

Overview

Framework (in software): a general system of components and architectural solutions that provides development tools to programmers for use with a relatively wide range of applications.

Collection: (hmm...) any collection of elements

3

Collection Framework

A collections framework is a unified architecture for representing and manipulating collections. It has: – Interfaces: abstract data types representing collections

– Implementations: concrete implementations of the collection interfaces

– Algorithms: methods that perform useful computations, such as searching and sorting• These algorithms are said to be polymorphic: the same method

can be used on different implementations

4

Interfaces

An interface describes a set of methods: – no constructors or instance variables

Interfaces must be implemented by classes– 646 java classes implement >= 1 interfaces (‘02)

2 or more classes implement an interface– Classes guaranteed to have the same methods– Objects can be treated as the same type– Can use different algorithms / instance variables

5

Collection interfaces

Queuesince Java 5

Click here for a Annotated Outline of Collections Framework

6

Implementations

A collection class– implements an ADT as a Java class– implements all methods of the interface– selects appropriate instance variables– can be instantiated

Java implements interfaces with– List: ArrayList, LinkedList, Vector– Map: HashMap, TreeMap– Set: TreeSet, HashSet

7

Algorithms

Java has polymorphic algorithms to provide functionality for different types of collections– Sorting (e.g. sort)– Shuffling (e.g. shuffle)– Routine Data Manipulation (e.g. reverse, addAll)– Searching (e.g. binarySearch)– Composition (e.g. frequency)– Finding Extreme Values (e.g. max)

8

Two Useful ADTs

List: a collection with a first element, a last element, distinct predecessors and successors– duplicates that "equals" each other are allowed

Map: maps keys to values – Maps cannot contain duplicate keys

– Each key maps at most one value

9

List

A list is– a collection of elements (numbers, strings,

accounts, pictures,…) – ordered (a sequence): there is a first, and a last

element• lists can be empty – no elements

– elements with a unique predecessor and successor– also known as a sequence

Peruse the Java 5 List Interface

10

ArrayList A Java Collection Class that Implements List

ArrayList – stores a collection of any type of object

– can be all the same type, or different types

– similar functionality to an array, but does not use subscripts

– is an indexed collection

The next slide is part of Sun's documentation

11

ArrayList Java

Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null. In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list.

The size, isEmpty, get, set, iterator, and listIterator operations run in constant time. The add operation runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.

12

List implemented by 3 classes

// Interface name: List// Three classes that implement the List interface: List<String> bigList = new ArrayList<String>();List<String> littleList = new LinkedList<String>();List<String> sharedList = new Vector<String>();

// All three have an add methodbigList.add("in array list");littleList.add("in linked list");sharedList.add("in vector"); // All three have a get methodassertEquals("in array list", bigList.get(0));assertEquals("in linked list", littleList.get(0));assertEquals("in vector", sharedList.get(0));

13

Java 5 Generics

A List can be made to store only one type

List<BankAccount> accountList = new ArrayList <BankAccount>();

accountList.add(new BankAccount("One", 111.11)); accountList.add(new BankAccount("Two", 222.22)); accountList.add(new BankAccount("Three", 333.33)); accountList.add(new BankAccount("Four", 444.44)); System.out.println(accountList.toString());

Output

[One $111.11, Two $222.22, Three $333.33, Four $444.44]

14

Iterators

Iterators provide a general way to traverse all elements in a collection

ArrayList<String> list = new ArrayList<String>(); list.add("1-FiRsT"); list.add("2-SeCoND"); list.add("3-ThIrD"); Iterator<String> itr = list.iterator(); while (itr.hasNext()) { System.out.println(itr.next().toLowerCase()); }

Output1-first2-second3-third

15

Map and SortedMap

The Map interface defines methods – get, put, contains, keySet, values, entrySet

TreeMap implements Map– put, get, remove: O(log n)

HashMap implements Map– put, get, remove: O(1)

16

Set and SortedSet

Some Map methods return SetThe Set interface

– add, addAll, remove, size, but no get!

Some implementations– TreeSet: values stored in order, O(log n)– HashSet: values in a hash table, no order, O(1)

17

Let's play

Set up a small collection of mappings Map<String, BankAccount> tm = new TreeMap<String, BankAccount>();

tm.put("M", new BankAccount("Michel", 111.11)); tm.put("G", new BankAccount("Georgie", 222.22)); tm.put("S", new BankAccount("Sam", 333.33)); tm.put("P", new BankAccount("Pei", 444.44)); tm.put("Z", new BankAccount("Zac", 555.55));

// Coming up: Some Map methods

18

keySet()

// Get the set of keys in the Map. // With TreeMap, the keys are ordered. Set<String> keys = tm.keySet(); assertEquals("[G, M, P, S, Z]", keys.toString());

19

values()

// The valuesCollection<BankAccount> accountCollection = tm.values();

Iterator<BankAccount> itr = accountCollection.iterator();

double allTheMoney = 0.0;while (itr.hasNext()) { allTheMoney += itr.next().getBalance();}assertEquals(1666.65, allTheMoney, 0.01);

20

entrySet()

// The mappingsCollection<Entry<String, BankAccount>> mapping = tm.entrySet();

Iterator<Entry<String, BankAccount>> itr2 = mapping.iterator();

while (itr2.hasNext()) { System.out.print("<" +itr2.next().toString() + "> ");}

<G=Georgie $222.22> <M=Michel $111.11> <P=Pei $444.44> <S=Sam $333.33> <Z=Zac $555.55>

21

Enhanced for Loop

for can iterate over collections for (Type element : collection) { element is the next thing visited for each iteration }ArrayList<String> names = new ArrayList<String>();names.add("Aaron");names.add("Moksha");names.add("Drew");names.add("Qiyam");for (String name : names) System.out.print(name + " "); Output Aaron Moksha Drew Qiyam

22

Use for over iterator

Collection<BankAccount> accountCollection = tm.values();double allTheMoney = 0.0;

for (BankAccount nextAccount : accountCollection) allTheMoney += nextAccount.getBalance();

assertEquals(1666.65, allTheMoney, 0.01);

23

Algorithms

See the Java API for the Collections class– play with sort, binarySearch, reverse, shuffle

• your choice