FilteringIterator


1   /*
2    * FilteringIterator.java
3    */
4   
5   package polyglot.util;
6   
7   import java.util.Iterator  ;
8   import java.util.Collection  ;
9   
10  /**
11   * FilteringIterator
12   *
13   * Overview:
14   *     This iterator wraps another iterator, and returns only those elements
15   *     for which a given predicate is true.  
16   *
17   *     Does not support Remove.
18   **/
19  public final class FilteringIterator implements Iterator   {
20    /**
21     * Constructs a new FilteringIterator which returns only those elements of
22     * <coll> which have <pred> true.
23     **/
24    public FilteringIterator(Collection   coll, Predicate pred) {
25      this(coll.iterator(), pred);
26    }
27  
28    /**
29     * Constructs a new FilteringIterator which returns all the elements
30     * of <iter>, in order, only when they have <pred> true.
31     **/
32    public FilteringIterator(Iterator   iter, Predicate pred) {
33      backing_iterator = iter;
34      predicate = pred;
35      findNextItem();
36    }
37  
38    public Object   next() {
39      Object   res = next_item;
40      if (res == null)
41        throw new java.util.NoSuchElementException  ();
42      findNextItem();
43      return res;
44    }
45  
46    public boolean hasNext() {
47      return next_item != null;
48    }
49    
50    public void remove() {
51      throw new UnsupportedOperationException  ("FilteringIterator.remove");
52    }
53  
54    // Advances the internal iterator.
55    private void findNextItem() {
56      while (backing_iterator.hasNext()) {
57        Object   o = backing_iterator.next();
58        if (predicate.isTrue(o)) {
59      next_item = o;
60      return;
61        }
62      }
63      next_item = null;
64    }
65    
66    // AF:  if next_item==null, this iterator has no more elts to yield.
67    //      otherwise, this iterator will yield next_item, followed by
68    //      those elements e of backing_iterator such that predicate.isTrue(e).
69    Object   next_item;
70    Iterator   backing_iterator;
71    Predicate predicate;
72  }
73  
74  
75
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