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  [The "BSD license"]
  Copyright (c) 2005-2009 Terence Parr
  All rights reserved.
  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions
  are met:
  1. Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
 2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the distribution.
 3. The name of the author may not be used to endorse or promote products
     derived from this software without specific prior written permission.
package org.antlr.runtime.tree;

Do a depth first walk of a tree, applying pre() and post() actions as we discover and finish nodes.
public class TreeVisitor {
    protected TreeAdaptor adaptor;
    public TreeVisitor(TreeAdaptor adaptor) {
        this. = adaptor;
    public TreeVisitor() { this(new CommonTreeAdaptor()); }
Visit every node in tree t and trigger an action for each node before/after having visited all of its children. Execute both actions even if t has no children. If a child visit yields a new child, it can update its parent's child list or just return the new child. The child update code works even if the child visit alters its parent and returns the new tree. Return result of applying post action to this node.
    public Object visit(Object tTreeVisitorAction action) {
        // System.out.println("visit "+((Tree)t).toStringTree());
        boolean isNil = .isNil(t);
        if ( action!=null && !isNil ) {
            t = action.pre(t); // if rewritten, walk children of new t
        for (int i=0; i<.getChildCount(t); i++) {
            Object child = .getChild(ti);
            Object visitResult = visit(childaction);
            Object childAfterVisit = .getChild(ti);
            if ( visitResult !=  childAfterVisit ) { // result & child differ?
        if ( action!=null && !isNil ) t =;
        return t;
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