AbstractOrderedChooser


1   package com.ubermq.kernel.chooser;
2   
3   import com.ubermq.kernel.*;
4   import java.util.*;
5   
6   /**
7    * Chooses an object from the
8    * given ordered Collection. This implementation only
9    * works on a single Collection at a time. If
10   * the <code>choose</code> method is called with
11   * differing collections, an <code>IllegalStateException</code>
12   * will be thrown.
13   */
14  abstract class AbstractOrderedChooser
15      implements Chooser
16  {
17      int size;
18      List myList;
19  
20      AbstractOrderedChooser()
21      {
22          reset();
23      }
24  
25      public void reset()
26      {
27          size = 0;
28          myList = null;
29      }
30  
31      /**
32       * Chooses the next object in
33       * the collection, wrapping to the beginning
34       * of the collection if necessary. The
35       * collection must be an instance of <code>List</code>,
36       * because the sequential nature of this algorithm
37       * requires an ordered collection.<P>
38       *
39       * The initial call may return any of the
40       * items in the Collection, not just the first.<P>
41       *
42       * @param objects a List of objects
43       * @return an Object r, such that <code>objects.contains(r)</code> is true.
44       * @throws IllegalStateException if the chooser can
45       * only operate on a single Collection object, and
46       * the Collection does not match that of the previous
47       * caller.
48       */
49      public Object   choose(Collection objects)
50      {
51          if (!(objects instanceof List))
52              throw new IllegalArgumentException  ("Collection must implement List.");
53  
54          // check our state. set my collection if empty.
55          if (myList == null)
56          {
57              myList = (List)objects;
58              size = myList.size();
59          }
60  
61          // ok return the item.
62          if (myList == objects)
63          {
64              return choose();
65          }
66          else
67          {
68              throw new IllegalStateException  ();
69          }
70      }
71  
72      /**
73       * Chooses an object from the collection.
74       */
75      abstract Object   choose();
76  }
77
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