Java tutorial
/*
* 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