| 1 | package spiffy.core.util; |
| 2 | |
| 3 | import java.util.Iterator; |
| 4 | |
| 5 | /** |
| 6 | * The PushBackIterator enables you to iterate and push back elements should you find that "you have come to far". You |
| 7 | * can push elements back in the iterator which then will be re-visited upon the subsequent call to <tt>next()</tt> |
| 8 | * |
| 9 | * @author Kasper B. Graversen |
| 10 | */ |
| 11 | public class PushBackIterator<T> implements Iterator<T> { |
| 12 | /** the iterator we wrap */ |
| 13 | private final Iterator<T> iterator; |
| 14 | |
| 15 | /** when pushing back, fill this cache */ |
| 16 | private T pushBackCache = null; |
| 17 | |
| 18 | /** record last fetched element such that we know what to push back */ |
| 19 | private T lastFetchedElement = null; |
| 20 | |
| 21 | /** |
| 22 | * @param iterator |
| 23 | * the iterator to wrap in order to get the push back functionality. |
| 24 | */ |
| 25 | public PushBackIterator(final Iterator<T> iterator) { |
| 26 | this.iterator = iterator; |
| 27 | } |
| 28 | |
| 29 | /** |
| 30 | * Returns true if the iteration has more elements. (In other words, returns true if next would return an element rather |
| 31 | * than throwing an exception.) |
| 32 | * |
| 33 | * @return true if the iterator has more elements. |
| 34 | */ |
| 35 | public boolean hasNext() { |
| 36 | return iterator.hasNext() || pushBackCache != null; |
| 37 | } |
| 38 | |
| 39 | /** |
| 40 | * Returns the next element in the iteration. Calling this method repeatedly until the hasNext() method returns false |
| 41 | * will return each element in the underlying collection exactly once. |
| 42 | * |
| 43 | * @return the next element in the iteration. |
| 44 | */ |
| 45 | public T next() { |
| 46 | // if we have something in the cache.. use that |
| 47 | if( pushBackCache != null ) { |
| 48 | lastFetchedElement = pushBackCache; |
| 49 | pushBackCache = null; |
| 50 | } else { |
| 51 | lastFetchedElement = iterator.next(); |
| 52 | } |
| 53 | return lastFetchedElement; |
| 54 | } |
| 55 | |
| 56 | /** |
| 57 | * Push back the last fetched element |
| 58 | */ |
| 59 | public void pushBack() { |
| 60 | if( lastFetchedElement == null ) |
| 61 | throw new IllegalStateException( |
| 62 | "next() must be called before pushBack(). Cannot push back non-existing element..."); |
| 63 | if( pushBackCache != null ) |
| 64 | throw new IllegalStateException("Cannot push back more than one object!"); |
| 65 | pushBackCache = lastFetchedElement; |
| 66 | } |
| 67 | |
| 68 | /** |
| 69 | * Operation currently not supported |
| 70 | */ |
| 71 | public void remove() { |
| 72 | throw new RuntimeException("Operation not supported yet..."); |
| 73 | } |
| 74 | } |