Coder issues with Apache Beam and CombineFn

The probable reason you are seeing java.lang.Object is because Beam is trying to infer a coder for an unresolved type variable, which will be resolved to Object. This may be a bug in how coder inference is done within Combine.

Separately, I would expect the Accum class to also cause a failure of coder inference. You can override getAccumulatorCoder in your CombineFn to provide one quite directly.

Did you check if adding Serializable to your Accumulator works directly?

So add"implements Serializable" to Accum class ...

public static class Accum implements Serializable {
            EventListWrapper eventListWrapper = new EventListWrapper();
        }