Hey, I have some questions to aggregate functions such as max or min. Take the following example:
//create Stream with event time where Data contains an ID, a timestamp and a temperature value DataStream<Data> oneStream = env.fromElements( new Data(123, new Date(116, 8,8,11,11,11), 5), new Data(124, new Date(116, 8,8,12,10,11), 8), new Data(125, new Date(116, 8,8,12,12,11), 10), new Data(126, new Date(116, 8,8,12,15,11), 2)) .assignTimestampsAndWatermarks(new AscendingTimestampExtractor<Data>() { @Override public long extractAscendingTimestamp(final Data data) { return data.getTimestamp().getTime(); } }); //calluclate max value of temperature per hour DataStream<Data> maxStream = WatermarkStream .keyBy("id") .timeWindow(Time.hours(1)) .max("temp"); maxStream.print(); Here are the questions I ran into: 1.) Why does the resulting stream "maxStream" have to be of type Data? From the documentation and the difference to maxBy I would expect the resulting stream to be of type Integer? 2.) Executing the code as it is above, I would expect the printed result be the data with ID 124 and ID 125. However, the execution prints all 4 data sets. Did I totally get this example wrong? How would the code need to look, to get the expected result? 3.) Can you point me to a good example for using time windows and aggregates? I couldn't find one that explains the above questions. Thanks for your help and best wishes, Claudia