Prefetch Iterator : Iterator « Collections Data Structure « Java






Prefetch Iterator

    
/* 
 * JGraphT : a free Java graph-theory library
 * 
 *
 * Project Info:  http://jgrapht.sourceforge.net/
 * Project Creator:  Barak Naveh (http://sourceforge.net/users/barak_naveh)
 *
 * (C) Copyright 2003-2007, by Barak Naveh and Contributors.
 *
 * This library is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or
 * (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
 * License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this library; if not, write to the Free Software Foundation,
 * Inc.,
 * 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
 */
/* -----------------
 * PrefetchIterator.java
 * -----------------
 * (C) Copyright 2005-2007, by Assaf Lehr and Contributors.
 *
 * Original Author:  Assaf Lehr
 * Contributor(s):   -
 *
 * $Id: PrefetchIterator.java 568 2007-09-30 00:12:18Z perfecthash $
 *
 * Changes
 * -------
 */

import java.util.*;


/**
 * Utility class to help implement an iterator/enumerator in which the hasNext()
 * method needs to calculate the next elements ahead of time.
 *
 * <p>Many classes which implement an iterator face a common problem: if there
 * is no easy way to calculate hasNext() other than to call getNext(), then they
 * save the result for fetching in the next call to getNext(). This utility
 * helps in doing just that.
 *
 * <p><b>Usage:</b> The new iterator class will hold this class as a member
 * variable and forward the hasNext() and next() to it. When creating an
 * instance of this class, you supply it with a functor that is doing the real
 * job of calculating the next element.
 *
 * <pre><code>
    //This class supllies enumeration of integer till 100.
    public class IteratorExample implements Enumeration{
    private int counter=0;
    private PrefetchIterator nextSupplier;

        IteratorExample()
        {
            nextSupplier = new PrefetchIterator(new PrefetchIterator.NextElementFunctor(){

                public Object nextElement() throws NoSuchElementException {
                    counter++;
                    if (counter>=100)
                        throw new NoSuchElementException();
                    else
                        return new Integer(counter);
                }

            });
        }
        //forwarding to nextSupplier and return its returned value
        public boolean hasMoreElements() {
            return this.nextSupplier.hasMoreElements();
        }
    //  forwarding to nextSupplier and return its returned value
        public Object nextElement() {
            return this.nextSupplier.nextElement();
        }
  }</pre>
 * </code>
 *
 * @author Assaf_Lehr
 */
public class PrefetchIterator<E>
    implements Iterator<E>,
        Enumeration<E>
{
    //~ Instance fields --------------------------------------------------------

    private NextElementFunctor<E> innerEnum;
    private E getNextLastResult;
    private boolean isGetNextLastResultUpToDate = false;
    private boolean endOfEnumerationReached = false;
    private boolean flagIsEnumerationStartedEmpty = true;
    private int innerFunctorUsageCounter = 0;

    //~ Constructors -----------------------------------------------------------

    public PrefetchIterator(NextElementFunctor<E> aEnum)
    {
        innerEnum = aEnum;
    }

    //~ Methods ----------------------------------------------------------------

    /**
     * Serves as one contact place to the functor; all must use it and not
     * directly the NextElementFunctor.
     */
    private E getNextElementFromInnerFunctor()
    {
        innerFunctorUsageCounter++;
        E result = this.innerEnum.nextElement();

        // if we got here , an exception was not thrown, so at least
        // one time a good value returned
        flagIsEnumerationStartedEmpty = false;
        return result;
    }

    /**
     * 1. Retrieves the saved value or calculates it if it does not exist 2.
     * Changes isGetNextLastResultUpToDate to false. (Because it does not save
     * the NEXT element now; it saves the current one!)
     */
    public E nextElement()
    {
        E result = null;
        if (this.isGetNextLastResultUpToDate) {
            result = this.getNextLastResult;
        } else {
            result = getNextElementFromInnerFunctor();
        }

        this.isGetNextLastResultUpToDate = false;
        return result;
    }

    /**
     * If (isGetNextLastResultUpToDate==true) returns true else 1. calculates
     * getNext() and saves it 2. sets isGetNextLastResultUpToDate to true.
     */
    public boolean hasMoreElements()
    {
        if (endOfEnumerationReached) {
            return false;
        }

        if (isGetNextLastResultUpToDate) {
            return true;
        } else {
            try {
                this.getNextLastResult = getNextElementFromInnerFunctor();
                this.isGetNextLastResultUpToDate = true;
                return true;
            } catch (NoSuchElementException noSuchE) {
                endOfEnumerationReached = true;
                return false;
            }
        } // else
    } // method

    /**
     * Tests whether the enumeration started as an empty one. It does not matter
     * if it hasMoreElements() now, only at initialization time. Efficiency: if
     * nextElements(), hasMoreElements() were never used, it activates the
     * hasMoreElements() once. Else it is immediately(O(1))
     */
    public boolean isEnumerationStartedEmpty()
    {
        if (this.innerFunctorUsageCounter == 0) {
            if (hasMoreElements()) {
                return false;
            } else {
                return true;
            }
        } else // it is not the first time , so use the saved value
               // which was initilaizeed during a call to
               // getNextElementFromInnerFunctor
        {
            return flagIsEnumerationStartedEmpty;
        }
    }

    public boolean hasNext()
    {
        return this.hasMoreElements();
    }

    public E next()
    {
        return this.nextElement();
    }

    /**
     * Always throws UnsupportedOperationException.
     */
    public void remove()
        throws UnsupportedOperationException
    {
        throw new UnsupportedOperationException();
    }

    //~ Inner Interfaces -------------------------------------------------------

    public interface NextElementFunctor<EE>
    {
        /**
         * You must implement that NoSuchElementException is thrown on
         * nextElement() if it is out of bound.
         */
        public EE nextElement()
            throws NoSuchElementException;
    }
}

// End PrefetchIterator.java
/////////////////////////
/* 
 * JGraphT : a free Java graph-theory library
 * 
 *
 * Project Info:  http://jgrapht.sourceforge.net/
 * Project Creator:  Barak Naveh (http://sourceforge.net/users/barak_naveh)
 *
 * (C) Copyright 2003-2007, by Barak Naveh and Contributors.
 *
 * This library is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or
 * (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
 * License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this library; if not, write to the Free Software Foundation,
 * Inc.,
 * 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
 */
/* -----------------
 * PrefetchIteratorTest.java
 * -----------------
 * (C) Copyright 2005-2007, by Assaf Lehr and Contributors.
 *
 * Original Author:  Assaf Lehr
 * Contributor(s):   -
 *
 * $Id: PrefetchIteratorTest.java 568 2007-09-30 00:12:18Z perfecthash $
 *
 * Changes
 * -------
 */
package org.jgrapht.util;

import java.util.*;

import junit.framework.*;


public class PrefetchIteratorTest
    extends TestCase
{
    //~ Methods ----------------------------------------------------------------

    public void testIteratorInterface()
    {
        Iterator iterator = new IterateFrom1To99();
        for (int i = 1; i < 100; i++) {
            assertEquals(true, iterator.hasNext());
            assertEquals(i, iterator.next());
        }
        assertEquals(false, iterator.hasNext());
        Exception exceptionThrown = null;
        try {
            iterator.next();
        } catch (Exception e) {
            exceptionThrown = e;
        }
        assertTrue(exceptionThrown instanceof NoSuchElementException);
    }

    public void testEnumInterface()
    {
        Enumeration enumuration = new IterateFrom1To99();
        for (int i = 1; i < 100; i++) {
            assertEquals(true, enumuration.hasMoreElements());
            assertEquals(i, enumuration.nextElement());
        }
        assertEquals(false, enumuration.hasMoreElements());
        Exception exceptionThrown = null;
        try {
            enumuration.nextElement();
        } catch (Exception e) {
            exceptionThrown = e;
        }
        assertTrue(exceptionThrown instanceof NoSuchElementException);
    }

    //~ Inner Classes ----------------------------------------------------------

    // This test class supplies enumeration of integer from 1 till 100.
    public static class IterateFrom1To99
        implements Enumeration,
            Iterator
    {
        private int counter = 0;
        private PrefetchIterator nextSupplier;

        public IterateFrom1To99()
        {
            nextSupplier =
                new PrefetchIterator<Integer>(
                    new PrefetchIterator.NextElementFunctor<Integer>() {
                        public Integer nextElement()
                            throws NoSuchElementException
                        {
                            counter++;
                            if (counter >= 100) {
                                throw new NoSuchElementException();
                            } else {
                                return new Integer(counter);
                            }
                        }
                    });
        }

        // forwarding to nextSupplier and return its returned value
        public boolean hasMoreElements()
        {
            return this.nextSupplier.hasMoreElements();
        }

        // forwarding to nextSupplier and return its returned value
        public Object nextElement()
        {
            return this.nextSupplier.nextElement();
        }

        public Object next()
        {
            return this.nextSupplier.next();
        }

        public boolean hasNext()
        {
            return this.nextSupplier.hasNext();
        }

        public void remove()
        {
            this.nextSupplier.remove();
        }
    }
}

// End PrefetchIteratorTest.java

   
    
    
    
  








Related examples in the same category

1.Listing the Elements of a Collection
2.De-mystify the Iterator interface, showing how to write a simple Iterator for an Array of Objects
3.Iterate over Set
4.Demonstrate iterators.
5.Use the for-each for loop to cycle through a collection.
6.List IteratorList Iterator
7.Iterate a Collection and remove an item (Exception, wrong version)
8.Use an Iterator and remove the item with Iterator.remove()
9.An Iterator wrapper for an Enumeration.
10.EmptyIterator is an iterator which is empty.
11.Implements an java.util.Iterator over any array
12.Treat an Iterator as an Iterable
13.Iterator class for sparse values in an array.
14.Iterator class for values contained in an array range.
15.Array Iterator
16.Cyclic Iteration
17.Create singleton Iterator
18.Empty Iterator
19.An Iterator that wraps a number of Iterators
20.An Iterator to iterate over the elements of an array
21.Sorted Iterator
22.Iterator Union of Iterators
23.Iterator Utils
24.Linked Iterator
25.Protects an given iterator by preventing calls to remove().
26.An Iterator that returns the elements of a specified array, or other iterators etc.
27.An Iterator wrapper for an Object[], allow us to deal with all array like structures in a consistent manner.
28.An array iterator
29.Adapt iterator to iterable
30.Static utility methods, classes, and abstract classes for iteration.
31.Iterator Collection
32.Convert Iterable to List
33.A singleton null object Iterator implementation.