@Override
    public String toString() {
        StringBuilder s = new StringBuilder(key.toString());
        ConfigState<T> c = config.get();
        s.append(", Current generation: ").append(c.getGeneration())
                .append(", Generation changed: ").append(c.isGenerationChanged())
                .append(", Config changed: ").append(c.isConfigChanged());
        if (exception != null)
            s.append(", Exception: ").append(exception);
        return s.toString();
    }
The class of the subscription's desired {@link ConfigInstance}

@return the config class
config/src/main/java/com/yahoo/config/subscription/impl/ConfigSubscription.java