Make ParseTreeVisitor more extensible, updated documentation

runtime/Java/src/org/antlr/v4/runtime/tree/ParseTreeVisitor.java

@@ -1,53 +1,151 @@
 package org.antlr.v4.runtime.tree;
 
-import org.antlr.v4.runtime.ParserRuleContext;
-import org.antlr.v4.runtime.Token;
+import org.antlr.v4.runtime.misc.NotNull;
 
-/** This class defines the basic notion of a parse tree visitor
- *  object. Generated visitors extend this class and implement the XVisitor
- *  interface for grammar X.
+/**
+ * This class defines the basic notion of a parse tree visitor object. Generated
+ * visitors extend this class and implement the {@code XVisitor} interface for
+ * grammar {@code X}.
  *
  * @param <T> The return type of the visit operation. Use {@link Void} for
  * operations with no return type.
  */
 public class ParseTreeVisitor<T> {
-	/** Visit a parse tree, and return a user-defined result of the operation.
+
+	/**
+	 * Visit a parse tree, and return a user-defined result of the operation.
+	 * <p/>
+	 * The default implementation calls {@link ParseTree#accept} on the
+	 * specified tree.
 	 *
 	 * @param tree The {@link ParseTree} to visit.
 	 * @return The result of visiting the parse tree.
 	 */
-	public T visit(ParseTree tree) {
+	public T visit(@NotNull ParseTree tree) {
 		return tree.accept(this);
 	}
 
-	/** Visit all rule, non-leaf children. This returns value returned from last
-	 *  child visited, losing all computations from first n-1 children.  Works
-	 *  fine for contexts with one child then.
-	 *  Handy if you are just walking the tree with a visitor and only
-	 *  care about some nodes.  The {@link ParserRuleContext#accept} method
-	 *  walks all children by default; i.e., calls this method.
+	/**
+	 * Visit the children of a node, and return a user-defined result of the
+	 * operation.
+	 * <p/>
+	 * The default implementation initializes the aggregate result to
+	 * {@link #defaultResult defaultResult}. Before visiting each child, it
+	 * calls {@link #shouldVisitNextChild shouldVisitNextChild}; if the result
+	 * is {@code false} no more children are visited and the current aggregate
+	 * result is returned. After visiting a child, the aggregate result is
+	 * updated by calling {@link #aggregateResult aggregateResult} with the
+	 * previous aggregate result and the result of visiting the child.
+	 *
+	 * @param node The {@link RuleNode} whose children should be visited.
+	 * @return The result of visiting the children of the node.
 	 */
-	public T visitChildren(RuleNode node) {
-		T result = null;
+	public T visitChildren(@NotNull RuleNode node) {
+		T result = defaultResult();
 		int n = node.getChildCount();
 		for (int i = 0; i < n; i++) {
-			ParseTree c = node.getChild(i);
-			result = c.accept(this);
+			if (!shouldVisitNextChild(node, result)) {
+				break;
 			}
+
+			ParseTree c = node.getChild(i);
+			T childResult = c.accept(this);
+			result = aggregateResult(result, childResult);
+		}
+
 		return result;
 	}
 
-	/** Visit a terminal node, and return a user-defined result of the operation.
+	/**
+	 * Visit a terminal node, and return a user-defined result of the operation.
+	 * <p/>
+	 * The default implementation returns the result of
+	 * {@link #defaultResult defaultResult}.
 	 *
 	 * @param node The {@link TerminalNode} to visit.
 	 * @return The result of visiting the node.
 	 */
-	public T visitTerminal(TerminalNode node) { return null; }
+	public T visitTerminal(@NotNull TerminalNode node) {
+		return defaultResult();
+	}
 
-	/** Visit an error node, and return a user-defined result of the operation.
+	/**
+	 * Visit an error node, and return a user-defined result of the operation.
+	 * <p/>
+	 * The default implementation returns the result of
+	 * {@link #defaultResult defaultResult}.
 	 *
 	 * @param node The {@link ErrorNode} to visit.
 	 * @return The result of visiting the node.
 	 */
-	public T visitErrorNode(ErrorNode node) { return null; }
+	public T visitErrorNode(@NotNull ErrorNode node) {
+		return defaultResult();
+	}
+
+	/**
+	 * Gets the default value returned by visitor methods. This value is
+	 * returned by the default implementations of
+	 * {@link #visitTerminal visitTerminal} and
+	 * {@link #visitErrorNode visitErrorNode}. The default implementation of
+	 * {@link #visitChildren visitChildren} initializes its aggregate result to
+	 * this value.
+	 * <p/>
+	 * The default implementation returns {@code null}.
+	 *
+	 * @return The default value returned by visitor methods.
+	 */
+	protected T defaultResult() {
+		return null;
+	}
+
+	/**
+	 * Aggregates the results of visiting multiple children of a node. After
+	 * either all children are visited or {@link #shouldVisitNextChild} returns
+	 * {@code false}, the aggregate value is returned as the result of
+	 * {@link #visitChildren}.
+	 * <p/>
+	 * The default implementation returns {@code nextResult}, meaning
+	 * {@link #visitChildren} will return the result of the last child visited
+	 * (or return the initial value if the node has no children).
+	 *
+	 * @param aggregate The previous aggregate value. In the default
+	 * implementation, the aggregate value is initialized to
+	 * {@link #defaultValue}, which is passed as the {@code aggregate} argument
+	 * to this method after the first child node is visited.
+	 * @param nextResult The result of the immediately preceeding call to visit
+	 * a child node.
+	 *
+	 * @return The updated aggregate result.
+	 */
+	protected T aggregateResult(T aggregate, T nextResult) {
+		return nextResult;
+	}
+
+	/**
+	 * This method is called before visiting each child in
+	 * {@link #visitChildren}. This method is first called before the first
+	 * child is visited; at that point {@code currentResult} will be the initial
+	 * value (in the default implementation, the initial value is returned by a
+	 * call to {@link #defaultValue}.
+	 * <p/>
+	 * The default implementation always returns {@code true}, indicating that
+	 * {@code visitChildren} should only return after all children are visited.
+	 * <p/>
+	 * One reason to override this method is to provide a "short circuit"
+	 * evaluation option for situations where the result of visiting a single
+	 * child has the potential to determine the result of the visit operation as
+	 * a whole.
+	 *
+	 * @param node The {@link RuleNode} whose children are currently being
+	 * visited.
+	 * @param currentResult The current aggregate result of the children visited
+	 * to the current point.
+	 *
+	 * @return {@code true} to continue visiting children. Otherwise return
+	 * {@code false} to stop visiting children and immediately return the
+	 * current aggregate result from {@link #visitChildren}.
+	 */
+	protected boolean shouldVisitNextChild(RuleNode node, T currentResult) {
+		return true;
+	}
 }