Why Join Only can be Operated on Time Window

[zhangsx]

  Hi all, I read Flink Streaming API and found that join only can be operated on time window. Unfortunately, it's unable to meet my requirements. If I want to join two datastream on count window, i.e., join a fixed number of items from each datastream. It's possible to do that for flink. If possible, which classes I should invovle or overwrite. Thanks in advance!