How to use Lo-level Joins API

[Yuta Morisawa]

Hi

I am trying to use low-level joins.
According to the doc, the way is creating a state and access it from 
both streams, but I can't.

https://ci.apache.org/projects/flink/flink-docs-release-1.8/dev/stream/operators/process_function.html

This is a snippet of my code.
It seems that the processElement1,2 have different ValueStates so that 
v1 in processElement2 is always null.

---
stream1.connect(stream2).keyBy(0,0).process(new MyCPF());

public class MyCPF extends CoProcessFunction{
 ValueState data;

 processElement1(v1){
   data.update(v1);
 }

 processElement2(v2){
   v1 = data.value() // v1 is always null
   out.collect(v1 + v2)
 }

 open(){
   data = getRuntimeContext().getState(descriptor);
 }

}
---

Can you tell me the collect way of the low-level joins and send me a 
sample code if you have?

--
Thank you
Yuta