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Transparency No. 1 Java Collection API : Built-in Data Structures for Java.

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Presentation on theme: "Transparency No. 1 Java Collection API : Built-in Data Structures for Java."— Presentation transcript:

1 Transparency No. 1 Java Collection API : Built-in Data Structures for Java

2 Java Collection Transparency No. 2 The Java Collection API Interfaces: Collection Set SortedSet, List Map SortedMap Iterator ListIterator Comparator

3 Java Collection Transparency No. 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 Java Collection Transparency No. 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 Java Collection Transparency No. 5 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

6 Java Collection Transparency No. 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 Java Collection Transparency No. 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 Java Collection Transparency No. 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 Java Collection Transparency No. 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.remove(); // is removed from c i.remove(); // exception raised, cannot remove more than once ! } }

10 Java Collection Transparency No. 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 Java Collection Transparency No. 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 4 distinct words detected: [came, left, saw, i]

12 Java Collection Transparency No. 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 Java Collection Transparency No. 13 The Map interface public interface Map { // Map does not extend Collection // Basic Operations // put or replace, return replace object // NOTE: different from Weisss 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 Java Collection Transparency No. 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 Java Collection Transparency No. 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 Java Collection Transparency No. 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(; 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); System.out.println(e.getKey() + ": " + e.getValue()); }

17 Java Collection Transparency No. 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 alternatives to the general-purpose implementations for special collections (like singleton sets).

18 Java Collection Transparency No. 18 General Purpose Implementations Hash Table Resizable array balanced tree linked list SetHashSet TreeSet (sortedSet) List ArrayList Vector LinkedList Map HashMap Hashtable TreeMap (sortedMap)

19 Java Collection Transparency No. 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 Java Collection Transparency No. 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 Java Collection Transparency No. 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())

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