package org.neo4j.helpers.collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* Abstract class for how you usually implement iterators when you don't know
* how many objects there are (which is pretty much every time)
*
* Basically the {@link #hasNext()} method will look up the next object and
* cache it with {@link #setPrefetchedNext(Object)}. The cached object is
* then set to {@code null} in {@link #next()}. So you only have to implement
* one method, {@code fetchNextOrNull} which returns {@code null} when the
* iteration has reached the end, and you're done.
*/
public abstract class PrefetchingIterator<T> implements Iterator<T>
{
private boolean hasFetchedNext;
private T nextObject;
/**
* Tries to fetch the next item and caches it so that consecutive calls
* (w/o an intermediate call to {@link #next()} will remember it and won't
* try to fetch it again.
*
* @return {@code true} if there was a next item to return in the next
* call to {@link #next()}.
*/
public boolean hasNext()
{
if ( hasFetchedNext )
{
return getPrefetchedNextOrNull() != null;
}
T nextOrNull = fetchNextOrNull();
hasFetchedNext = true;
if ( nextOrNull != null )
{
setPrefetchedNext( nextOrNull );
}
return nextOrNull != null;
}
/**
* Uses {@link #hasNext()} to try to fetch the next item and returns it
* if found, otherwise it throws a {@link NoSuchElementException}.
*
* @return the next item in the iteration, or throws
* {@link NoSuchElementException} if there's no more items to return.
*/
public T next()
{
if ( !hasNext() )
{
throw new NoSuchElementException();
}
T result = getPrefetchedNextOrNull();
setPrefetchedNext( null );
hasFetchedNext = false;
return result;
}
protected abstract T fetchNextOrNull();
protected void setPrefetchedNext( T nextOrNull )
{
this.nextObject = nextOrNull;
}
protected T getPrefetchedNextOrNull()
{
return nextObject;
}
public void remove()
{
throw new UnsupportedOperationException();
}
}