JoinBolt didn't deal with buffer==null

[zqhxuyuan]

I see the logic of cacheWindow(buffersForRule) and window(buffer) is different between AggregatorBolt, SortBolt and JoinBolt. In JoinBolt ,there are non processing in situation① below (Line 176):

  if (buffersForRule != null) {
        buffer = buffersForRule.getIfPresent(flowInfo.getPartition());
        if (buffer != null) {    // if we have a buffer already, process it
              if(op.getEvictionPolicy() == Policy.TIME)
                    buffer.timeEvict(op.getEvictionThreshold());
        }
       //Here no else ....  ①
 } else {
       //new buffersForRule and buildWindow, then put to buffer and buffersForRule...
 }

but in Agg and Sort there both has this logic:

                if (cacheWindow != null) { 
                    window = cacheWindow.getIfPresent(flowInfo.getPartition());
                    if (window != null) {    // if we have a buffer already, process it
                        if (op.getEvictionPolicy() == Policy.TIME)
                            window.timeEvict(op.getEvictionThreshold());
                    } else {
                        //....
                    }
                } else {
                    //....
                }

Can u explain why here does't need?
As I understand, different partition should given different window/buffer.