public <T> CallbackBuilder withOptionalCallback(final Class<T> cls, final Callback<Optional<T>> callback) {
        this.callback = callback;
        return this;
    }
public <T> CallbackBuilder withOptionalCallback(final Class<T> cls, final Callback<Optional<T>> callback) {
        this.callback = callback;
        return this;
    }
public < T > CallbackBuilder withOptionalCallback ( final Class < T > cls , final Callback < Optional < T > > callback ) { this . callback = callback ; return this ; }
Builder method to set callback handler that takes an optional string

@param cls      class
@param callback callback
@param <T>      type of callback
@return this
Builder method to set callback handler that takes an optional string