public void subscribe() {
Provider provider1 = new Provider();
Provider provider2 = new Provider();
Provider baseProvider = new Provider();
HistoricalShockMarketDataProvider shockProvider = new HistoricalShockMarketDataProvider(provider1, provider2, baseProvider);
MarketDataListener listener = mock(MarketDataListener.class);
shockProvider.addListener(listener);
provider1.valueChanged(SPEC1);
verify(listener).valuesChanged(Collections.singleton(SPEC1));
provider1.valueChanged(SPEC2);
verify(listener).valuesChanged(Collections.singleton(SPEC2));