org.jgrapht.util

Class PrefetchIterator<E>

java.lang.Object

org.jgrapht.util.PrefetchIterator<E>

All Implemented Interfaces:

Enumeration<E>, Iterator<E>

public class PrefetchIterator<E>
extends Object
implements Iterator<E>, Enumeration<E>

Utility class to help implement an iterator/enumerator in which the hasNext() method needs to calculate the next elements ahead of time.

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.

Usage: 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.

    //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();
        }
  }
 

Author:

Assaf_Lehr

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	PrefetchIterator.NextElementFunctor<EE>

Constructor Summary

Constructors

Constructor and Description
PrefetchIterator(PrefetchIterator.NextElementFunctor<E> aEnum)

Method Summary

Methods

Modifier and Type	Method and Description
boolean	hasMoreElements() If (isGetNextLastResultUpToDate==true) returns true else 1.
boolean	hasNext()
boolean	isEnumerationStartedEmpty() Tests whether the enumeration started as an empty one.
E	next()
E	nextElement() 1.
void	remove() Always throws UnsupportedOperationException.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

PrefetchIterator

public PrefetchIterator(PrefetchIterator.NextElementFunctor<E> aEnum)

Method Detail

nextElement

public E nextElement()

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!)

Specified by:

nextElement in interface Enumeration<E>

hasMoreElements

public boolean hasMoreElements()

If (isGetNextLastResultUpToDate==true) returns true else 1. calculates getNext() and saves it 2. sets isGetNextLastResultUpToDate to true.

Specified by:

hasMoreElements in interface Enumeration<E>

isEnumerationStartedEmpty

public boolean isEnumerationStartedEmpty()

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))

hasNext

public boolean hasNext()

Specified by:

hasNext in interface Iterator<E>

next

public E next()

Specified by:

next in interface Iterator<E>

remove

public void remove()
            throws UnsupportedOperationException

Always throws UnsupportedOperationException.

Specified by:

remove in interface Iterator<E>

Throws:

UnsupportedOperationException