title: KStream leftJoin KStream with the same Key
body: <p>I have the problem taht i try to leftjoin 2 Streams. mergedKey has 100+ 
  listings with the same key and DataStream has just 1 listing with the same key as mergedKey. I want in the endStream Value from mergedKey Merged with DataStream.</p>
  
  <pre><code>      //get DataStream 
          final KStream&lt;String, GenericRecord&gt; DataStream = builder.stream("Datastreams");
          // Transform merged to Equals Keys to DataStream.Iot
          final KStream&lt;String, GenericRecord&gt; mergedKey = mergedFoIObs
                  .map((key, value) -&gt; KeyValue.pair(value.get("Datastream").toString(), value)); 
          // Join the DataStream with MergedStream
  
  
          final KStream&lt;String, String&gt; mergedFoIObsData = mergedKey.leftJoin(
                  DataStream,
              (value, data) -&gt; {
                  try {
                      if(data != null{
                          value.put("Datastream", data.toString());
                          JSONObject jo = (JSONObject) new JSONParser().parse(value.toString());
                          return jo.toJSONString();}
                        return null
  
  
  
                  } catch (ParseException e) {
                      // TODO Auto-generated catch block
                      e.printStackTrace();
                  }
                  return null;
  
              }, JoinWindows.of(10000));
  </code></pre>
  
  <p>But My problem is that in the Endstream I get just 1 listing with the right values and the other listings has null as values.</p>
  
  <p>Taht means that data is null after the first "round".</p>
  
  <p>When i transfrom DataStream to a KTable I have the problem taht i get right listing but just 37 listings so 60 are Misssing.</p>
  
  <p>I hope you can help me out.</p>