Data Structures & Algorithms Lecture Notes

15 October 2010 • Java Collections

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Outline

• The Java Collection Framework.
• Interfaces
  • Collections and iterators.
• Implementations
  • Abstract and concrete.

Objectives

• Gather and organize several related and common data structures.
  • Gather and organize along object-oriented (polymorphic) lines.
• Provide a structure for developing other, more specialized data structures.
• Provide a mechanism for decoupling system components via consistent abstractions.
• Design and implementation leverage.

Collections

• The Java Collection Framework (JCF) contains
  • Interfaces describing collection behavior.
  • Implementation classes providing data structures.
  • Abstract classes for other implementations.
• Also other odds and ends: backwards compatibility, adapters, and such like.

Example

• A part of the Queue hierarchy showing the three components.

  

Collection Interface Hierarchy

Collection Interface

• A group of elements (objects).
  • Elements are ordered or not, duplicated or not.
  • The interface is generic in the element type.
• The Collection interface extends the Iterable interface.
  • Collections respond to the for-each statement.

The Collection Interface

interface Collection<E> 
implements Iterable<E> {
   boolean     add(E e)*
   boolean     addAll(Collection<? extends E> c)*
   void        clear()*
   boolean     contains(Object o)
   boolean     containsAll(Collection<?> c)
   boolean     equals(Object o)
   int         hashCode()
   boolean     isEmpty()
   Iterator<E> iterator()
   boolean     remove(Object o)*
   boolean     removeAll(Collection<?> c)*
   boolean     retainAll(Collection<?> c)*
   int         size()
   Object[]    toArray()
   <T> T[]     toArray(T[] a)
  }

* Optional

Optional Methods

• The element adding and removing methods are optional.
  • Some collections are immutable.
• Collection implementations should throw a UnsupportedOperationException when inapplicable optional methods are called.
  • All methods are implemented.
  • Some of the don't do much (just throw an exception).

Group Methods

• Some Collection methods accept a collection of elements and operate over the whole data set.

  boolean containsAll(Collection<?> c)

  s.containsAll(c) returns true if everything in c is in s.

  boolean removeAll(Collection<?> c)

  s.removeAll(c) removes from s everything in c.

  boolean retainAll(Collection<?> c)

  s.retainAll(c) removes from s everything not in c.

Group Method Examples

• Let s be [ 1 2 3 ].

  s.containAll([ 1 3 ])	returns true, leaves s unchanged.
  s.containAll([ 3 4 ])	returns false, leaves s unchanged.
  s.removeAll([ 1 3 ])	returns true, s will be [ 2 ].
  s.removeAll([ 3 4 ])	returns true, s will be [ 1 2 ].
  s.removeAll([ 4 ])	returns false, leaves s unchanged.
  s.retainAll([ 1 3 ])	returns true, s will be [ 1 3 ].
  s.retainAll([ 3 4 ])	returns true, s will be [ 3 ].
  s.retainAll([ 4 ])	returns true, s will be [ ].

Group-Method Implementations

• containsAll() can be implemented using Collection interface methods:

  boolean containsAll(Collection<?> c)
    for (Object o: c)
      if (!contains(o))
        return false
    return true
  

• What about removeAll() and retainAll()? Can they too be implemented using Collection interface methods?

Collection Interface Hierarchy

List Interface

• The List interface tries to be general over sequences.
  • In particular, it wants to cover indexible as well as non-indexible sequences.
• The List interface also tries to be implementation independent.
• As a result, the List interface is somewhat complex to navigate and understand.

The List Interface

interface List<E>
extends Collection<E> {
  boolean         add(E e)
  void            add(int index, E element)
  boolean         addAll(Collection<? extends E> c)
  boolean         addAll(int index, Collection<? extends E> c)
  void            clear()
  boolean         contains(Object o)
  boolean         containsAll(Collection<?> c)
  boolean         equals(Object o)
  E               get(int index)
  int             hashCode()
  int             indexOf(Object o)
  boolean         isEmpty()
  Iterator<E>     iterator()
  int             lastIndexOf(Object o)
  ListIterator<E> listIterator()
  ListIterator<E> listIterator(int index)
  E               remove(int index)
  boolean         remove(Object o)
  boolean         removeAll(Collection<?> c)
  boolean         retainAll(Collection<?> c)
  E               set(int index, E element)
  int             size()
  List<E>         subList(int fromIndex, int toIndex)
  Object[]        toArray()
  <T> T[]         toArray(T[] a)
  }

Indexible List Methods

• What happens if the underlying implementation doesn't support indexing, or provides expensive support?
  • Optional methods (set(), for example) can be skipped.
  • Required methods (get(), for example) have expensive (linear) implementations.

Collection Interface Hierarchy

The Queue Interface

interface Queue<E>
extends Collection<E> {
  boolean         add(E e)
  boolean         addAll(Collection<? extends E> c)
  void            clear()
  boolean         contains(Object o)
  boolean         containsAll(Collection<?> c)
  E               element()
  boolean         equals(Object o)
  int             hashCode()
  boolean         isEmpty()
  Iterator<E>     iterator()
  boolean         offer(E e) 
  E               peek() 
  E               poll() 
  E               remove() 
  boolean         remove(Object o)
  boolean         removeAll(Collection<?> c)
  boolean         retainAll(Collection<?> c)
  int             size()
  Object[]        toArray()
  <T> T[]         toArray(T[] a)
  }

The Queue Interface

• The Queue interface provides two sets of methods differing in their error behavior.

  		Throws		Returns
  		exception		special value
  Insert		add(e)		offer(e)
  Remove		remove()		poll()
  Examine		element()		peek()

• For example,

  emptyq.element() throws an exception
  emptyq.peek() return null.

The Deque Interface

• The Deque interface extends the Queue interface.
  • It adds front and back versions of the Queue operations.

    addFirst(e), removeFirst(), getFirst()
    offerFirst(e), pollFirst(), peekFirst()
    addLast(e), removeLast(), getLast()
    offerLast(e), pollLast(), peekLast()
    

Collection Interface Hierarchy

The Iterator Interface

public interface Iterator<E> {
  boolean hasNext()
  E       next()
  void    remove()*
  }

* Optional

• Example use:

  final Iterator i = c.iterator()
  while (i.hasNext())
    f(i.next())
  

List Navigation

• The next() and previous() methods
  • move the iterator appropriately and
  • return the element moved over.

  

List Iterators

• The Iterator interface sacrifices features for generality.
  • It doesn't do much, but it applies widely.
• The ListIterator interface extends the Iterator interface to provide more features.
  • Bi-directional navigation (next and previous).
  • In-traversal list modifications.
  • More control over list positioning (indexing).

The List Iterator Interface

public interface ListIterator<E>
extends Iterator<E> {
  void    add(E e)*
  boolean hasNext()
  boolean hasPrevious()
  E       next()
  int     nextIndex()
  E       previous()
  int     previousIndex()
  void    remove()*
  void    set(E e)*
  }

* Optional

List Iterator Indexing

• ListIterator (or cursor) positions lie between adjacent list elements.
  • Or before the first element or after the last element.

  

List Manipulations

• A new element is added before the next or after the previous element.

  

• Adding to an empty list is easy.

  

Removing Elements

• remove() removes the element returned by next() or previous().

  

• next() or previous() must be called before each call to remove().

Better Iteration Code

• Iterating over values is such a common activity that having to write

  final Iterator i = c.iterator()
  while (i.hasNext())
    f(i.next())
  

  (or similar) every time is poor design.

• Package iterations in a more abstract for-each loop form:

  for (T e: c)
    f(e)
  

The Iterable Interface

• The Iterable interface indicates a class that can iterated over using for-each loops.

  public interface Iterable<E> {
    Iterator<E> iterator()
    }
  

  • Essentially: here's the iterator you need, without all the details.
• Classes not implementing the Iterable interface cannot partake in for-each loops.

Interface Declarations

• Remember: for greatest flexibility, use the highest (most abstract) interfaces possible as variable types.
• From high to low:

  Collection<Integer> pending = 
    new ArrayDeque<Integer>();
  
  Queue<Integer> pending = 
    new ArrayDeque<Integer>();
  
  Deque<Integer> pending = 
    new ArrayDeque<Integer>();
  

Implementation Hierarchy

Abstract Implementations

• The more practical collection interfaces contain many method definitions.
  • Implementing classes have to implement them all, if only to fail when called.
• The abstract implementation classes provide a skeleton implementation for further development.
  • The concrete class is a child of the abstract implementation class.

Example

• The AbstractQueue class implements the Queue interface.

  public abstract class AbstractQueue<E>
  extends AbstractCollection<E>
  implements Queue<E>

  • It contains around 40 methods.
• The PriorityQueue extends AbstractQueue.

  public class PriorityQueue<E>
  extends AbstractQueue<E>
  implements Serializable

Linked Lists

• The LinkedList class extends the AbstractSequentialList class.
  • And implements the Queue and Deque interfaces.
• The interfaces specify a doubly-linked list.
• The AbstractSequentialList class assumes a dynamic link implementation.
  • That is, indexing is expensive.

Array Lists

• The ArrayList class implements a linked list assuming indexing is cheap.
  • That is, an implementation based on dynamic arrays.
• ArrayLists are more efficient than are Vectors.
  • But ArrayLists aren’t thread safe and Vectors are.

Random-Access Iterators

• The RandomAccess interface indicates efficient indexing.
  • Faster:

    for (int i = 0; i < lst.size(); i++)
      f(lst.get(i))

  • Slower:

    for (Iterator i = lst.iterator(); i.hasNext(); )
      f(i.next())

• RandomAccess contains no methods; it just flags classes with efficient indexing.

Priority Queues

• The PriorityQueue class stores elements in increasing priority order (a min-priority queue).
• The queue elements can implement the Comparable interface.
• A queue can be created with a Comparator implementing, overriding the element comparable, if any.

Where’s Stacks?

• Stacks were added to Java before the Collection Framework (v. 1.0 vs v. 1.2).

Summary

• The Java Collection Framework designs and implements a group of common data structures.
• Declare variables with the most abstract interface type possible.
• Keep in mind that retreiving a value through a list iterator also moves the iterator.

References

• Collections (Chapter 13), Core Java vol. 1 eighth ed. by Cay Horstmann and Gary Cornell, Prentice Hall, 2008.
• The Java Collections Trail by Josh Bloch, Sun Microsystems.

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This page last modified on 15 October 2010.