Made the parse tree visitor walk ParseTree objects not ParserRuleContext objects. This necessitated pulling up the accept() and toStringTree(Parser) methods into ParseTree.

runtime/Java/src/org/antlr/v4/runtime/ParserRuleContext.java

@@ -34,7 +34,6 @@ import org.antlr.v4.runtime.misc.NotNull;
 import org.antlr.v4.runtime.misc.Nullable;
 import org.antlr.v4.runtime.tree.ParseTree;
 import org.antlr.v4.runtime.tree.ParseTreeListener;
-import org.antlr.v4.runtime.tree.ParseTreeVisitor;
 
 import java.util.ArrayList;
 import java.util.Collections;
@@ -132,7 +131,7 @@ public class ParserRuleContext<Symbol extends Token> extends RuleContext {
 		this(parent, parent!=null ? parent.s : -1 /* invoking state */, stateNumber);
 	}
 
-	// Double dispatch methods for listeners and visitors
+	// Double dispatch methods for listeners
 
 	// parse listener
 	public void enterRule(ParseListener<Symbol> listener) { }
@@ -142,10 +141,6 @@ public class ParserRuleContext<Symbol extends Token> extends RuleContext {
 	public void enterRule(ParseTreeListener<Symbol> listener) { }
 	public void exitRule(ParseTreeListener<Symbol> listener) { }
 
-	// visitor
-	public <T> T accept(ParseTreeVisitor<? extends T> visitor) { return visitor.visitChildren(this); }
-
-
 	/** Does not set parent link; other add methods do */
 	public void addChild(TerminalNode<Symbol> t) {
 		if ( children==null ) children = new ArrayList<ParseTree>();

runtime/Java/src/org/antlr/v4/runtime/RuleContext.java

@@ -28,8 +28,11 @@
  */
 package org.antlr.v4.runtime;
 
-import org.antlr.v4.runtime.misc.*;
+import org.antlr.v4.runtime.misc.Interval;
-import org.antlr.v4.runtime.tree.*;
+import org.antlr.v4.runtime.misc.Nullable;
+import org.antlr.v4.runtime.tree.ParseTree;
+import org.antlr.v4.runtime.tree.ParseTreeVisitor;
+import org.antlr.v4.runtime.tree.Trees;
 import org.antlr.v4.runtime.tree.gui.TreeViewer;
 
 import javax.print.PrintException;
@@ -236,6 +239,9 @@ public class RuleContext implements ParseTree.RuleNode {
 		return new Interval(start, stop);
 	}
 
+	@Override
+	public <T> T accept(ParseTreeVisitor<? extends T> visitor) { return visitor.visitChildren(this); }
+
 	public void inspect(Parser parser) {
 		TreeViewer viewer = new TreeViewer(parser, this);
 		viewer.open();

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

@@ -29,6 +29,7 @@
 
 package org.antlr.v4.runtime.tree;
 
+import org.antlr.v4.runtime.Parser;
 import org.antlr.v4.runtime.RuleContext;
 import org.antlr.v4.runtime.Token;
 import org.antlr.v4.runtime.misc.Interval;
@@ -78,6 +79,14 @@ public interface ParseTree extends SyntaxTree {
 		@Override
 		public int getChildCount() { return 0; }
 
+		@Override
+		public <T> T accept(ParseTreeVisitor<? extends T> visitor) { return null; }
+
+		@Override
+		public String toStringTree(Parser parser) {
+			return toString();
+		}
+
 		public boolean isErrorNode() { return this instanceof ErrorNodeImpl; }
 
 		@Override
@@ -115,4 +124,12 @@ public interface ParseTree extends SyntaxTree {
 	ParseTree getParent();
 	@Override
 	ParseTree getChild(int i);
+
+	/** The ParseTreeVisitor needs a double dispatch method */
+	public <T> T accept(ParseTreeVisitor<? extends T> visitor);
+
+	/** Specialize toStringTree so that it can print out more information
+	 * 	based upon the parser.
+	 */
+	public String toStringTree(Parser parser);
 }

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

@@ -5,7 +5,7 @@ import org.antlr.v4.runtime.Token;
 
 /** T is return type of visit methods. Use T=Void for no return type. */
 public class ParseTreeVisitor<T> {
-	public T visit(ParserRuleContext<?> ctx) {
+	public T visit(ParseTree ctx) {
 		return ctx.accept(this);
 	}
 
@@ -17,9 +17,11 @@ public class ParseTreeVisitor<T> {
 	 *  care about some nodes.  The ParserRuleContext.accept() method
 	 *  walks all children by default; i.e., calls this method.
 	 */
-	public T visitChildren(ParserRuleContext<? extends Token> ctx) {
+	public T visitChildren(ParseTree ctx) {
 		T result = null;
-		for (ParseTree c : ctx.children) {
+		int n = ctx.getChildCount();
+		for (int i=0; i<n; i++) {
+			ParseTree c = ctx.getChild(i);
 			if ( c instanceof ParseTree.RuleNode ) {
 				ParseTree.RuleNode r = (ParseTree.RuleNode)c;
 				ParserRuleContext<?> rctx = (ParserRuleContext<? extends Token>)r.getRuleContext();