### Question:
DefaultHandlerHolder implements HandlerHolder,
        ScrapingAware { @Override public void register(final EventHandler<?> handler) { if (handler == null) { throw new IllegalArgumentException("Handler is required"); } handlers.add(handler); } @Inject  DefaultHandlerHolder(final Registry<Harvester, EventBus> registry,
            final Provider<EventBus> provider); @Override void register(final EventHandler<?> handler); @Override void onBeforeScraping(final Harvester harvester); @Override void onAfterScraping(final Harvester harvester);  }### Answer:
@Test(expectedExceptions=IllegalArgumentException.class) public void testRegisterWithoutListener() { holder.register(null); }