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Google XML Pages (GXP) is a templating system used to generate XML/SGML markup (most often HTML).

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/*
 * Copyright (C) 2008 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.gxp.compiler.parser;

import com.google.gxp.compiler.alerts.AlertSink;
import com.google.gxp.compiler.alerts.SourcePosition;

import java.util.*;

/**
 * The Namespace of things that don't have a namespace. This is essentially an
 * instance of the "Null Object Design Pattern".
 */
public class NullNamespace implements Namespace {
  private NullNamespace(){}

  public String getUri() {
    return null;
  }

  public static final Namespace INSTANCE = new NullNamespace();

  public ParsedElement createElement(AlertSink alertSink,
                                     SourcePosition sourcePosition,
                                     String displayName,
                                     String tagName,
                                     List attrs,
                                     List children) {
    alertSink.add(new UnknownElementError(sourcePosition, this, displayName));
    return null;
  }

  public  T acceptVisitor(NamespaceVisitor visitor) {
    return visitor.visitNullNamespace(this);
  }
}