Java Collection API : Built-in Data Structures for Java

1. Java Collection API :Built-in Data Structures for Java based on original presentation by C.-C. Chen

2. The Java Collection API Interfaces: • Collection  • Set  SortedSet, • List • Map  SortedMap • Iterator  ListIterator • Comparator

3. 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.

4. 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.

5. classes of the java collection API • AbstractCollection (Collection)  • AbstractSet (Set)  HashSet, TreeSet(SortedSet) • AbstractList (List) ArrayList, AbstractSequentialList  LinkedList • AbstractMap (Map)  • HashMap • TreeMap(SortedMap) • WeakHashMap • Arrays • Collections

6. 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.

7. 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

8. 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 ==)

9. 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 help static 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 ! } }

10. 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

11. 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]

12. 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

13. 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();

14. 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); } }

15. Basic Operations • a simple program to generate a frequency table import 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}

16. 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()); }

17. 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 alternativesto the general-purpose implementations for special collections (like singleton sets).

18. General Purpose Implementations

19. 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.

20. 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

21. 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())