import javax.lang.model.element.UnknownElementException; //导入依赖的package包/类
@Override
public Void visitUnknown(Element e, Void p) {
    show("unknown", e);
    indent(+1);
    try {
        super.visitUnknown(e, p);
    } catch (UnknownElementException ex) {
        System.err.println("caught " + ex);
    }
    indent(-1);
    return null;
}
 

import javax.lang.model.element.UnknownElementException; //导入依赖的package包/类
/**
 * Visits an unknown element.  This method is called if
 * this visitor is used in a version of the language later
 * than 1.6, where new elements have been added.  This
 * implementation always throws a {@link UnknownElementException}
 * but this is not required of subclasses.
 *
 * @param element the element to visit.
 * @param parameter the additional parameter, specific to the visitor.
 * @return the return value specific to the visitor.
 * @throws UnknownElementException by default.
 */
public R visitUnknown(Element element, P parameter)
{
  throw new UnknownElementException(element, parameter);
}