58  *          return 1;
  59  *      }
  60  *      {@literal @}Override
  61  *      public Integer reduce(Integer r1, Integer r2) {
  62  *          return (r1 == null ? 0 : r1) + (r2 == null ? 0 : r2);
  63  *      }
  64  *   }
  65  * </pre>
  66  *
  67  * @param <R> the return type of this visitor's methods.  Use {@link
  68  *            Void} for visitors that do not need to return results.
  69  * @param <P> the type of the additional parameter to this visitor's
  70  *            methods.  Use {@code Void} for visitors that do not need an
  71  *            additional parameter.
  72  *
  73  * @author Peter von der Ah&eacute;
  74  * @author Jonathan Gibbons
  75  * @since 1.6
  76  */
  77 public class TreeScanner<R,P> implements TreeVisitor<R,P> {




  78 
  79     /**
  80      * Scans a single node.
  81      * @param tree the node to be scanned
  82      * @param p a parameter value passed to the visit method
  83      * @return the result value from the visit method
  84      */
  85     public R scan(Tree tree, P p) {
  86         return (tree == null) ? null : tree.accept(this, p);
  87     }
  88 
  89     private R scanAndReduce(Tree node, P p, R r) {
  90         return reduce(scan(node, p), r);
  91     }
  92 
  93     /**
  94      * Scans a sequence of nodes.
  95      * @param nodes the nodes to be scanned
  96      * @param p a parameter value to be passed to the visit method for each node
  97      * @return the combined return value from the visit methods.

  58  *          return 1;
  59  *      }
  60  *      {@literal @}Override
  61  *      public Integer reduce(Integer r1, Integer r2) {
  62  *          return (r1 == null ? 0 : r1) + (r2 == null ? 0 : r2);
  63  *      }
  64  *   }
  65  * </pre>
  66  *
  67  * @param <R> the return type of this visitor's methods.  Use {@link
  68  *            Void} for visitors that do not need to return results.
  69  * @param <P> the type of the additional parameter to this visitor's
  70  *            methods.  Use {@code Void} for visitors that do not need an
  71  *            additional parameter.
  72  *
  73  * @author Peter von der Ah&eacute;
  74  * @author Jonathan Gibbons
  75  * @since 1.6
  76  */
  77 public class TreeScanner<R,P> implements TreeVisitor<R,P> {
  78     /**
  79      * Constructor for subclasses to call.
  80      */
  81     public TreeScanner() {}
  82 
  83     /**
  84      * Scans a single node.
  85      * @param tree the node to be scanned
  86      * @param p a parameter value passed to the visit method
  87      * @return the result value from the visit method
  88      */
  89     public R scan(Tree tree, P p) {
  90         return (tree == null) ? null : tree.accept(this, p);
  91     }
  92 
  93     private R scanAndReduce(Tree node, P p, R r) {
  94         return reduce(scan(node, p), r);
  95     }
  96 
  97     /**
  98      * Scans a sequence of nodes.
  99      * @param nodes the nodes to be scanned
 100      * @param p a parameter value to be passed to the visit method for each node
 101      * @return the combined return value from the visit methods.