1   /*
    2    * Licensed to the Apache Software Foundation (ASF) under one or more
    3    * contributor license agreements.  See the NOTICE file distributed with
    4    * this work for additional information regarding copyright ownership.
    5    * The ASF licenses this file to You under the Apache License, Version 2.0
    6    * (the "License"); you may not use this file except in compliance with
    7    * the License.  You may obtain a copy of the License at
    8    *
    9    *      http://www.apache.org/licenses/LICENSE-2.0
   10    *
   11    * Unless required by applicable law or agreed to in writing, software
   12    * distributed under the License is distributed on an "AS IS" BASIS,
   13    * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   14    * See the License for the specific language governing permissions and
   15    * limitations under the License.
   16    */
   17   package org.apache.pdfbox.cos;
   18   
   19   
   20   
   21   import java.io.IOException;
   22   import java.io.OutputStream;
   23   
   24   import org.apache.pdfbox.exceptions.COSVisitorException;
   25   
   26   /**
   27    * This class represents a null PDF object.
   28    *
   29    * @author <a href="mailto:ben@benlitchfield.com">Ben Litchfield</a>
   30    * @version $Revision: 1.13 $
   31    */
   32   public class COSNull extends COSBase
   33   {
   34       /**
   35        * The null token.
   36        */
   37       public static final byte[] NULL_BYTES = new byte[] {110, 117, 108, 108}; //"null".getBytes( "ISO-8859-1" );
   38   
   39       /**
   40        * The one null object in the system.
   41        */
   42       public static final COSNull NULL = new COSNull();
   43   
   44       /**
   45        * Constructor.
   46        */
   47       private COSNull()
   48       {
   49           //limit creation to one instance.
   50       }
   51   
   52       /**
   53        * visitor pattern double dispatch method.
   54        *
   55        * @param visitor The object to notify when visiting this object.
   56        * @return any object, depending on the visitor implementation, or null
   57        * @throws COSVisitorException If an error occurs while visiting this object.
   58        */
   59       public Object accept( ICOSVisitor  visitor ) throws COSVisitorException
   60       {
   61           return visitor.visitFromNull( this );
   62       }
   63   
   64       /**
   65        * This will output this string as a PDF object.
   66        *
   67        * @param output The stream to write to.
   68        * @throws IOException If there is an error writing to the stream.
   69        */
   70       public void writePDF( OutputStream output ) throws IOException
   71       {
   72           output.write(NULL_BYTES);
   73       }
   74   }