code: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements. See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership. The ASF licenses this file
   * to you 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.
   */
  /*
   * $Id$
   */
  
  package org.apache.xalan.xsltc.compiler.util;
  
  import java.io.DataOutputStream;
  import java.io.IOException;
  
  import org.apache.bcel.Constants;
  import org.apache.bcel.generic.ConstantPoolGen;
  import org.apache.bcel.generic.Instruction;
  import org.apache.bcel.generic.Visitor;
  
  /**
   * A special abstract dummy subclass of
   * {@link org.apache.bcel.generic.Instruction} used to mark locations of
   * interest in an {@link org.apache.bcel.generic.InstructionList}.  It and
   * its subclasses are only used as placeholders, and do not contribute to the
   * actual byte code instruction stream.
   */
  abstract class MarkerInstruction extends Instruction {
      /**
       * Zero-argument constructor.  Sets the opcode to an invalid value and 
       * sets the length to zero, as it will not be written as part of the
       * generated byte code.
       */
      public MarkerInstruction() {
          super(Constants.UNDEFINED, (short) 0);
      }
  
      /**
       * {@link org.apache.bcel.generic.Visitor}s will know nothing about this
       * kind of {@link org.apche.bcel.generic.Instruction}, so this method does
       * nothing.
       */
      public void accept(Visitor v) {
      }
  
      /**
       * The number of JVM stack entries consumed by the instruction.
       * This instruction is just a place holder, so it does not consume any
       * stack entries.
       * @param cpg The {@link org.apache.bcel.generic.ConstantPoolGen} for the
       * current {@link org.apache.bcel.generic.ClassGen}
       * @return <code>0</code> always 
       */
      final public int consumeStack(ConstantPoolGen cpg) {
          return 0;
      }
      /**
       * The number of JVM stack entries produced by the instruction.
       * This instruction is just a place holder, so it does not produce any
       * stack entries.
       * @param cpg The {@link org.apache.bcel.generic.ConstantPoolGen} for the
       * current {@link org.apache.bcel.generic.ClassGen}
       * @return <code>0</code> always 
       */
      final public int produceStack(ConstantPoolGen cpg) {
          return 0;
      }
  
      /**
       * Produce a copy of the instruction.  By default a
       * {@link MarkerInstruction} has no parameters, so the base implementation
       * of {@link #copy()} returns the instruction itself.
       * @return The instruction itself.
       */
      public Instruction copy() {
          return this;
      }
      /**
       * Dump instruction as byte code to stream out.  A {@link MarkerInstruction}
       * has no effect on the generated byte code so it is never emitted to the
       * output stream.
       * @param out Output stream
       */
      final public void dump(DataOutputStream out) throws IOException {
      }
  }
  