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Java Collection API :

Built-in Data Structures for Java

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The Java Collection API

Interfaces: Collection

Set SortedSet, List

Map SortedMap Iterator ListIterator Comparator

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Summary of all interfaces in the java Collection API

Collection Interfaces : The primary means by which collections are manipulated.

Collection Set, List A group of objects. May or may not be ordered; May or may not contain duplicates.

Set SortedSet The familiar set abstraction. No duplicates; May or may not be ordered.

SortedSet elements automatically sorted, either in their natural ordering (see the

Comparable interface), or by a Comparator object provided when a SortedSet instance is created.

List Ordered collection, also known as a sequence. Duplicates permitted; Allows positional access.

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Map SortedMap A mapping from keys to values. Each key can map to at most one value (function).

SortedMap A map whose mappings are automatically sorted by key, either in the

keys' natural ordering or by a comparator provided when a SortedMap instance is created.

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classes of the java collection API

1. AbstractCollection (Collection) AbstractSet (Set) HashSet, TreeSet(SortedSet) AbstractList (List) ArrayList, AbstractSequentialList LinkedList

AbstractMap (Map) HashMap TreeMap (SortedMap) WeakHashMap

Arrays Collections

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General-Purpose Implementation classes

The primary implementations of the collection interfaces. HashSet : Hash table implementation of the Set interface. TreeSet : Red-black tree implementation of the SortedSet interface. ArrayList : Resizable-array implementation of the List interface.

(Essentially an unsynchronized Vector.) The best all-around implementation of the List interface.

LinkedList : Doubly-linked list implementation of the List interface. May provide better performance than the ArrayList implementation if elements are

frequently inserted or deleted within the list. Useful for queues and double-ended queues (deques).

HashMap : Hash table implementation of the Map interface. (Essentially an unsynchronized Hashtable that supports null keys and values.) The

best all-around implementation of the Map interface.

TreeMap : Red-black tree implementation of the SortedMap interface.

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The java.util.Collection interface

represents a group of objects, known as its elements. no direct implementation The primary use : pass around collections of objects where

maximum generality is desired.

Ex: List l = new ArrayList( c ) ; // c is a Collection object

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The definition

public interface Collection {

// Basic properties

int size();

boolean isEmpty();

boolean contains(Object element); // use equals() for comparison

boolean equal(Object);

int hashCode(); // new equals() requires new hashCode()

// basic operations

boolean add(Object); // Optional; return true if this changed

boolean remove(Object); // Optional; use equals() (not ==)

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Why using Iterators instead of Enumerations Iterator allows the caller to remove elements from the underlying

collection during the iteration with well-defined semantics. Method names have been improved. public interface Iterator { boolean hasNext(); // cf: hasMoreElements() Object next(); // cf: nextElement() void remove(); // Optional }// a simple Collection filter using iterator that Enumeration could not helpstatic void filter(Collection c) { for (Iterator i = c.iterator(); i.hasNext(); ) { if ( no-good( i.next() ) ) i.remove(); // i.next() is removed from c i.remove(); // exception raised, cannot remove more than once ! } }

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The Set Interface is a Collection that cannot contain duplicate elements. models the mathematical set abstraction. contains no methods other than those inherited from Collection.

same signatures but different semantics ( meaning ) Collection c = new LinkedList(); Set s = new HashSet(); String o = “a”; c.add(o); c.add(o) ; // both return true; c.size() == 2 s.add(o); s.add(o) ; // 2nd add() returns false; s.size() == 1

It adds the restriction that duplicate elements are prohibited. Collection noDups = new HashSet(c); // a simple way to eliminate

duplicate from c Two Set objects are equal if they contain the same elements. Two direct implementations:

HashSet TreeSet

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Basic Operations A simple program to detect duplicates using set:import java.util.*;public class FindDups { public static void main(String args[]) { Set s = new HashSet(); // or new TreeSet(), another implementation of Set // following code uses Set methods only for (int i=0; i<args.length; i++) if (!s.add(args[i])) System.out.println("Duplicate detected: “ + args[i]); 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]

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The Map Interface

A Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value. Three implementations:

HashMap, which stores its entries in a hash table, is the best-performing implementation.

TreeMap, which stores its entries in a red-black tree, guarantees the order of iteration.

Hashtable has also been retrofitted to implement Map.

All implementation must provide two constructors: (like Collections) Assume M is your implementation M() // empty map M(Map m) // a copy of map from m

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The Map interface

public interface Map { // Map does not extend Collection

// Basic Operations

// put or replace, return replace object

// NOTE: different from Weiss’s insert(Hashable x)

Object put(Object key, Object value); // optional

Object get(Object key);

Object remove(Object key);

boolean containsKey(Object key);

boolean containsValue(Object value);

int size();

boolean isEmpty();

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The Map interface

// Bulk Operations

void putAll(Map t); //optional

void clear(); // optional

// Collection Views;

// backed by the Map, change on either will be reflected on the other.

public Set keySet(); // cannot duplicate by definition!!

public Collection values(); // can duplicate

public Set entrySet(); // no equivalent in Dictionary

// nested Interface for entrySet elements

public interface Entry {

Object getKey();

Object getValue();

Object setValue(Object value); } }

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Basic Operations a simple program to generate a frequency tableimport java.util.*;public class Freq { private static final Integer ONE = new Integer(1); public static void main(String args[]) { Map m = new HashMap(); // Initialize frequency table from command line for (int i=0; i<args.length; i++) { Integer freq = (Integer) m.get(args[i]); // key is a string m.put(args[i], (freq==null ? ONE : new Integer( freq.intValue() + 1))); } // value is Integer System.out.println( m.size()+" distinct words detected:"); System.out.println(m); } } > java Freq if it is to be it is up to me to delegate 8 distinct words detected: {to=3, me=1, delegate=1, it=2, is=2, if=1, be=1, up=1}

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Collection Views methods allow a Map to be viewed as a Collection in three ways:

keySet: the Set of keys contained in the Map. values: The Collection of values contained in the Map. This Collection is not a Set, as

multiple keys can map to the same value. entrySet: 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. the standard idiom for iterating over the keys in a Map: for (Iterator i = m.keySet().iterator(); i.hasNext(); ) {

System.out.println(i.next());

if(no-good(…)) i.remove() ; } // support removal from the back Map Iterating over key-value pairs

for (Iterator i=m.entrySet().iterator(); i.hasNext(); ) {

Map.Entry e = (Map.Entry) i.next();

System.out.println(e.getKey() + ": " + e.getValue()); }

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Actual Collection and Map Implementations

Implementations are the actual data objects used to store collections (and Maps).

Three kinds of implementations: General-purpose Implementations

the public classes that provide the primary implementations of the core interfaces.

Wrapper Implementations used in combination with other implementations (often the general-

purpose implementations) to provide added functionality. Convenience Implementations

Convenience implementations are mini-implementations, typically made available via static factory methods that provide convenient, efficient alternatives to the general-purpose implementations for special collections (like singleton sets).

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General Purpose Implementations

Hash TableResizable

arraybalanced

treelinked list

Set HashSetTreeSet

(sortedSet)

ListArrayList

VectorLinkedList

MapHashMap

Hashtable

TreeMap

(sortedMap)

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Properties of the implementations

consistent names as well as consistent behavior. full implementations [of all the optional operations]. All permit null elements, keys and values. unsynchronized.

remedy the deficiency of Hashtable and Vector can become synchronized through the synchronization wrappers

All have fail-fast iterators, which detect illegal concurrent modification during iteration and fail quickly and cleanly.

All are Serializable, all support a public clone() method. should be thinking about the interfaces rather than the

implementations. The choice of implementation affects only performance.

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HashSet vs treeSet (and HashMap vs TreeMap)

HashSet/ HashMap is much faster (constant time vs. log time for most operations), but offers no ordering guarantees.

always use HashSet/HashMap unless you need to use the operations in the SortedSet, or in-order iteration.

choose an appropriate initial capacity of your HashSet if iteration performance is important. The default initial capacity is 101, and that's often more than you need. can be specified using the int constructor. Set s= new HashSet(17); // set bucket size to 17

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The Collections class

static int binarySearch(List list, Object key [, Comparator c]) static void copy(List dest, List src) // foreach i d.set(i, s.get(i));static Enumeration enumeration(Collection c)

Returns an enumeration over the specified collection.

static void fill(List list, Object o) static Object max(Collection coll [, Comparator comp] ) static Object min(Collection coll [, Comparator comp] ) static List nCopies(int n, Object o) static void reverse(List l) static Set singleton(Object o) static Comparator reverseOrder() // assume a is of type String[] // usage: Arrays.sort(a, Collections.reverseOrder());static void shuffle(List list [, Random rnd]) static void swap(List, int, int)static void rotate(List, int d) ;// obj at i moved to ( i - d mod size())