Collections IS311. The Collections Framework. Type Trees for Collections. Java Collections Framework (ต อ)

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1 IS311 Java Collections Framework (ต อ) Collections Collections are holders that let you store and organize objects in useful ways for efficient access. In the package java.util, there are interfaces and classes that provide a generic collection framework. The Collections interfaces: Collection<E>, Set<E>, SortedSet<E>, List<E>, Queue<E>, Map<K,V>, SortedMap<K,V>, Iterator<E>, ListIterator<E>, Iterable<E> Some useful implementations of the interfaces: HashSet<E>, TreeSet<E>, ArrayList<E>, LinkedList<E>, HashMap<K,V>, TreeMap<K,V>, WeakHashMap<K,V> Exceptions: UnsupportedOperationException ClassCastException IllegalArgumentException NoSuchElementException NullPointerException 1 2 Type Trees for Collections The Collections Framework Iterable<E> Collection<E> Set<E> Queue<E> List<E> SortedSet<E> EnumSet<E> PriorityQueue<E> HashSet<E> TreeSet<E> Iterator<E> ListIerator<E > ArrayList<E> LinkedList<E> The Java collection framework is a set of generic types that are used to create collection classes that support various ways to store and manage objects of any kind in memory. A generic type for collection of objects: To get static checking by the compiler for whatever types of objects to want to manage. Generic Types Generic Class/Interface Type The Iterator<T> interface type Description Declares methods for iterating through elements of a collection, one at a time. LinkedHashSet<E> SortedMap<K,V> Map<K,V> EnumMap<K,V> WeakHashMap<K,V> HashMap<E> The Vector<T> type The Stack<T> type The LinkedList<T> type Supports an array-like structure for storing any type of object. The number of objects to be stored increases automatically as necessary. Supports the storage of any type of object in a pushdown stack. Supports the storage of any type of object in a doubly-linked list, which is a list that you can iterate though forwards or backwards. TreeMap<K,V> LinkedHashMap<K,V> The HashMap<K,V> type Supports the storage of an object of type V in a hash table, sometimes called a map. The object is stored using an associated key object of type K. To retrieve an object you just supply its associated key. 3 4

2 Collections of Objects Collections of Objects Three Main Types of Collections Sets Sets Sequences Maps The simple kinds of collection The objects are not ordered in any particular way. The objects are simply added to the set without any control over where they go. Sequences The objects are stored in a linear fashion, not necessarily in any particular order, but in an arbitrary fixed sequence with a beginning and an end. Collections generally have the capability to expand to accommodate as many elements as necessary. The various types of sequence collections Array or Vector LinkedList Stack Queue 5 6 Maps Collections of Objects Each entry in the collection involves a pair of objects. A map is also referred to sometimes as a dictionary. Each object that is stored in a map has an associated key object, and the object and its key are stored together as a name-value pair. Comparable and Comparator The interface java.lang.comparable<t> can be implemented by any class whose objects can be sorted. public int compareto (T other): return a value that is less than, equal to, or greater than zero as this object is less than, equal to, or greater than the other object. If a given class does not implement Comparable or if its natural ordering is wrong for some purpose, java.util.comparator object can be used public int compare(t o1, T o2) boolean equals(object obj) 7 8

3 The Collection Interface Collection Classes The Collection Interface The basis of much of the collection system is the Collection interface. Methods: public int size() public boolean isempty() public boolean contains(object elem) public Iterator<E> iterator() public Object[] toarray() public <T> T[] toarray(t[] dest) public boolean add(e elem) public boolean remove(object elem) public boolean containsall(collection<?> coll) public boolean addall(collection<? extends E> coll) public boolean removeall(collection<?> coll) public boolean retainall(collection<?> coll) public void clear() Classes in Sets: HashSet<T> LinkedHashSet<T> TreeSet<T> EnumSet<T extends Enum<T>> Classes in Lists: To define a collection whose elements have a defined ordereach element exists in a particular position the collection. Vector<T> Stack<T> LinkedList<T> ArrayList<T> Class in Queues: FIFO ordering PriorityQueue<T> Classes in Maps: Does not extend Collection because it has a contract that is different in important ways: do not add an element to a Map(add a key/value pair), and a Map allows looking up. Hashtable<K,V> HashMap<K,V> LinkedHashMap<K,V> WeakHashMap<K,V> IdentityHashMap<K,V> TreeMap<K,V> : keeping its keys sorted in the same way as TreeSet 9 10 Sets The simple kinds of collection The objects are not ordered in any particular way. The objects are simply added to the set without any control over where they go. Contains no methods other than those inherited from Collection Iterator The elements are traversed in no particular order Tree of Sets Iterable<E> Collection<E> Set<E> SortedSet<E> HashSet<E> EnumSet<E> TreeSet<E> LinkedHashSet<E> 11 12

4 boolean add(e e) interface Set<E> boolean addall(collection<? extends E> c) void clear( ) boolean contains(object o) boolean containsall(collection<?> c) boolean isempty( ) Iterator iterator( ) boolean remove(object o) boolean retainall(collection <?> c) int size( ) Object [] toarray() <T> T[] toarray(t[] a) interface SortedSet<E> SortedSet a Set that maintains its elements in ascending order. Several additional operations are provided to take advantage of the ordering. Sorted sets are used for naturally ordered sets, such as word lists and membership rolls. Iterator The elements are traversed according to the natural ordering (ascending) public interface SortedSet<E> extends Set<E> { // Range-view SortedSet<E> subset(e fromelement, E toelement); SortedSet<E> headset(e toelement); SortedSet<E> tailset(e fromelement); // Endpoints E first(); E last(); interface SortedSet<E> // Comparator access Comparator<? super E> comparator(); Set implementations HashSet implements Set Hash tables as internal data structure (faster) LinkedHashSet extends HashSet Elements are traversed by iterator according to the insertion order TreeSet implements SortedSet red-black tree structure (R-B trees) as internal data structure (computationally expensive) 15 16

5 HashSet LinkedHashSet Implement the interface Set. Implemented using a hash table. อ อบเจ กท ท จะนำมาบรรจ ในเซตชน ดน ได ต องมาจากคลาสท ม กา รอ มพล เม นท เมท อด hashcode() No ordering of elements. add, remove, and contains methods constant time complexity O(c). extend HashSet with linked list implementation support ordering of elements (ตามลำด บท เพ มเข าไป) add, remove, and contains methods linear time complexity O(n), where n is the number of elements in the set TreeSet Implement the interface Set. Implemented using tree structure. Guarantees ordering of elements. add, remove, and contains methods logarithmic time complexity O(log (n)), where n is the number of elements in the set. Maps Map (key, value) binding No duplicate keys Examples identity code (String), person (Person) student ID (Integer), student (Student) 19 20

6 Maps Tree of Maps Each entry in the collection involves a pair of objects. A map is also referred to sometimes as a dictionary. Each object that is stored in a map has an associated key object, and the object and its key are stored together as a name-value pair. Maps do not have an iterator SortedMap<K,V> TreeMap<K,V> Map<K,V> HashMap<E> LinkedHashMap<K,V> EnumMap<K,V> WeakHashMap<K,V> Classes in Maps Does not extend Collection because it has a contract that is different in important ways: do not add an element to a Map(add a key/value pair), and a Map allows looking up. Hashtable<K,V> HashMap<K,V> LinkedHashMap<K,V> WeakHashMap<K,V> IdentityHashMap<K,V> TreeMap<K,V> : keeping its keys sorted in the same way as TreeSet interface Map<K,V> Map an object that maps keys to values. A Map cannot contain duplicate keys; each key can map to at most one value

7 interface Map<K,V> interface SortedMap<K,V> public interface Map<K,V> { // Basic operations V put(k key, V value); V get(object key); V remove(object key); boolean containskey(object key); boolean containsvalue(object value); int size(); boolean isempty(); // Bulk operations void putall(map<? extends K,? extends V> m); void clear(); // Collection Views public Set<K> keyset(); public Collection<V> values(); public Set<Map.Entry<K,V>> entryset(); // Interface for entryset elements public interface Entry { K getkey(); V getvalue(); V setvalue(v value); public interface SortedMap<K, V> extends Map<K, V>{ SortedMap<K, V> submap(k fromkey, K tokey); SortedMap<K, V> headmap(k tokey); SortedMap<K, V> tailmap(k fromkey); K firstkey(); K lastkey(); Comparator<? super K> comparator(); HashMap and TreeMap Classes The HashMap and HashTree classes implement the Map interface. HashMap The implementation is based on a hash table. No ordering on (key, value) pairs. TreeMap The implementation is based on R-B trees structure (key, value) pairs are ordered on the key. Comparable and Comparator The interface java.lang.comparable<t> can be implemented by any class whose objects can be sorted. public int compareto (T other): return a value that is less than, equal to, or greater than zero as this object is less than, equal to, or greater than the other object. If a given class does not implement Comparable or if its natural ordering is wrong for some purpose, java.util.comparator object can be used public int compare(t o1, T o2) boolean equals(object obj) 27 28

8 Enhanced for loop If a class extends Iterable<E> you can use Java's enhanced for loop of this general form for (E refvar : collection<e> ) { refvar refers to each element in collection<e> example ArrayList<String> list = new ArrayList<String>(); list.add("first"); list.add("second"); for (String s : list) System.out.println(s.toLowerCase()); 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) Algorithms All of the algorithms, provided by the Collections class, take the form of static methods Most of the algorithms operate on List objects, but a couple of them (max and min) operate on arbitrary Collection objects Collections static methods Static library with many useful algorithms Searching int pos = Collections.binarySearch(list, key); Counting int f = Collections.frequency(myColl, item); Sorting... Collections.sort(list); Collections.sort(list, comparator); Max, Min, Shuffling, reversing, performing set operations and much more 31 32

9 Sorting The sort operation uses a slightly optimized merge sort algorithm Fast: This algorithm is guaranteed to run in n log(n) time, and runs substantially faster on nearly sorted lists. Stable: That is to say, it doesn't reorder equal elements. ต วอย างการใช เมท อด sort ของ Collections import java.util.*; public class SortDemo { public static void main( String args[] ) { List <String> l = new ArrayList <String> (); for ( int i = 0; i < args.length; i++ ) l.add( args[ i ] ); Collections.sort( l ); System.out.println( l ); 33 ส งข อม ลให โปรแกรมทาง command line ตอนส งร นโปรแกรม เช น java SortDemo One Two Three Four Five 34 Arrays It is too bad that arrays are not collections You loose all of the power provided by the collection framework The class Arrays contains various methods for manipulating arrays (such as sorting and searching) It also contains methods that allows arrays to be viewed as lists. ต วอย างการใช เมท อด sort ของคลาส Arrays import java.util.*; public class ArraysSortDemo { public static void main( String args[] ) { Arrays.sort( args ); List l = Arrays.asList( args ); System.out.println( l ); 35 ส งข อม ลให โปรแกรมทาง command line ตอนส งร นโปรแกรม เช น java ArraysSortDemo One Two Three Four Five 36

10 Other Algorithms Other algorithms provided by the Collections class include Shuffling Data manipulation reverse() fill() copy() Searching Finding extreme values max() min() What About User Objects? The Collections framework will work with any Java class You need to be sure you have defined equals() hashcode() compareto() Don t use mutable objects for keys in a Map hashcode() hashcode()returns distinct integers for distinct objects. If two objects are equal according to the equals() method, then the hashcode() method on each of the two objects must produce the same integer result. When hashcode() is invoked on the same object more than once, it must return the same integer, provided no information used in equals comparisons has been modified. It is not required that if two objects are unequal according to equals()that hashcode() must return distinct integer values. Interface Comparable This ordering is referred to as the class's natural ordering, and the class's compareto() method is referred to as its natural comparison method. A class's natural ordering is said to be consistent with equals if and only if (e1.compareto((object)e2)==0) has the same boolean value as: e1.equals((object)e2) for every e1 and e2 of class C

11 import java.util.*; ต วอย างคลาส Name ท ใช ก บ collection ได public class Name implements Comparable { private String first; private String last; public Name( String firstname, String lastname ) { first = firstname; last = lastname; public String getfirst() { return first; คลาส Name (ต อ) public boolean equals( Object o ) { boolean retval = false; if (o!=null && o instanceof Name ) { Name n = ( Name )o; retval = n.getfirst().equals( first ) && n.getlast().equals( last ); return retval; public int hashcode() { return first.hashcode() + last.hashcode(); public String getlast() { return last; public String tostring() { return first + " " + last; public int compareto( Object o ) throws ClassCastException { int retval; Name n = ( Name ) o; คลาส Name (ต อ) retval = last.compareto( n.getlast() ); if ( retval == 0 ) retval = first.compareto( n.getfirst() ); ต วอย างการนำคลาส Name ไปใช ก บ collection class SortNameDemo { // run this class to test Name class public static void main( String args[] ) { List <Name> l = new ArrayList <Name> (); l.add( new Name("Sombat", "Maimee")); l.add( new Name("Somsri", "Deejing")); l.add( new Name("Amorn", "Nonnan")); l.add( new Name("Pichai", "Maimee")); Collections.sort( l ); return retval; //Name ต ว การเปร ยบเท ยบอ อบเจกต น ให ความสำค ญก บนามสก ล มากกว า ช อ จ งเปร ยบเท ยบ นามสก ลก อน ถ านามสก ลเด ยวก นจ งจะไปเปร ยบเท ยบช อ ถ าต องการให ความสำค ญก บช อ มากกว า นามสก ล จะแก ไข 43 ต วอย างน อย างไร? System.out.println( l ); ผลล พธ จากการร น จะเร ยงตามนามสก ลก อนแล วจ งเร ยงช อ [Somsri Deejing, Pichai Maimee, Sombat Maimee, Amorn Nonnan] 44

12 Tutorials Java Collections Framework Tutorial ม ล งค อย ในเว บว ชาน ท หน า Link ดาวน โหลดต วอย างโปรแกรมพร อมคำอธ บายได ท 45