code: //==============================================================================
  //	
  //	Copyright (c) 2002-
  //	Authors:
  //	* Vojtech Forejt <vojtech.forejt@cs.ox.ac.uk> (University of Oxford)
  //	
  //------------------------------------------------------------------------------
  //	
  //	This file is part of PRISM.
  //	
  //	PRISM is free software; you can redistribute it and/or modify
  //	it under the terms of the GNU General Public License as published by
  //	the Free Software Foundation; either version 2 of the License, or
  //	(at your option) any later version.
  //	
  //	PRISM is distributed in the hope that it will be useful,
  //	but WITHOUT ANY WARRANTY; without even the implied warranty of
  //	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  //	GNU General Public License for more details.
  //	
  //	You should have received a copy of the GNU General Public License
  //	along with PRISM; if not, write to the Free Software Foundation,
  //	Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  //	
  //==============================================================================
  
  package parser.type;
  
  import prism.PrismLangException;
  
  public class TypeVoid extends Type 
  {
  	private static TypeVoid singleton;
  	
  	static
  	{
  		singleton = new TypeVoid();
  	}
  	
  	private TypeVoid()
  	{		
  	}	
  	
  	public boolean equals(Object o)
  	{
  		return (o instanceof TypeVoid);
  	}
  	
  	@Override
  	public String getTypeString()
  	{
  		return "void";
  	}
  	
  	@Override
  	public Object defaultValue()
  	{
  		return null;
  	}
  	
  	public static TypeVoid getInstance()
  	{
  		return singleton;
  	}
  	
  	@Override
  	public boolean canAssign(Type type)
  	{
  		return false;
  	}
  	
  	@Override
  	public Object castValueTo(Object value) throws PrismLangException
  	{
  		throw new PrismLangException("Can't convert " + value.getClass() + " to type " + getTypeString());
  	}
  }