qbit/core/src/main/java/io/advantageous/qbit/reactive/CallbackBuilder.java
public <T> CallbackBuilder setCallback(final Class<T> returnType, final Callback<T> callback) {
        return withCallback(returnType, callback);
    }
public <T> CallbackBuilder setCallback(final Class<T> returnType, final Callback<T> callback) {
        return withCallback(returnType, callback);
    }
Builder method to add a callback handler.

@param returnType returnType
@param callback   callback
@param <T>        T
@return this