Package org.jgrapht.util
Class PrefetchIterator<E>
java.lang.Object
org.jgrapht.util.PrefetchIterator<E>
- All Implemented Interfaces:
Enumeration<E>
,Iterator<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
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Nested Class Summary
Nested Classes -
Constructor Summary
Constructors -
Method Summary
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Methods inherited from interface java.util.Enumeration
asIterator
Methods inherited from interface java.util.Iterator
forEachRemaining
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Constructor Details
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PrefetchIterator
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Method Details
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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 interfaceEnumeration<E>
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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 interfaceEnumeration<E>
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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() -
next
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remove
Always throws UnsupportedOperationException.- Specified by:
remove
in interfaceIterator<E>
- Throws:
UnsupportedOperationException
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