class virtual {
  static java.io.File newFile(String path) {
    return new java.io.File(rebase(path));
  }
  
  static java.io.FileInputStream newFileInputStream(String path) {
    return new java.io.FileInputStream(rebase(path));
  }
  
  static java.io.FileOutputStream newFileOutputStream(String path) {
    return new java.io.FileOutputStream(rebase(path));
  }
  
  static String virtual_baseDir = "";
  
  static String rebase(String path) {
    if (new File(path).isAbsolute())
      return path;
    else
      return new File(new File(virtual_baseDir), path).getAbsolutePath();
  }
}

Began life as a copy of #2000350