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consumer.subscribe(Arrays.asList("foo", "bar"))
System.out.println(consumer.assignment()); // prints []

consumer.poll(0)
  --> onPartitionsRevoked([]) 
  --> onPartitionsAssigned([(foo, 0), (bar,0)])
System.out.println(consumer.assignment()); // prints [(foo, 0), (bar, 0)]
 
consumer.subscribe(Arrays.asList("foo"))
System.out.println(consumer.assignment()); // prints [(foo, 0), (bar, 0)] # notice the change

consumer.poll(0)
  --> onPartitionsRevoked([(foo, 0), (bar, 0)]) // # notice the change
  --> onPartitionsAssigned([(foo, 0)])
System.out.println(consumer.assignment()); // prints [(foo, 0)] 

consumer.subscribe(Pattern.compile("..."))
System.out.println(consumer.assignment()); // prints []

consumer.poll(0)
  --> onPartitionsRevoked([]) 
  --> onPartitionsAssigned([(foo, 0), (bar,0)])
System.out.println(consumer.assignment()); // prints [(foo, 0), (bar, 0)]
 
consumer.subscribe(Pattern.compile("f.."))
System.out.println(consumer.assignment()); // prints [(foo, 0), (bar, 0)]

consumer.poll(0)
  --> onPartitionsRevoked([(foo, 0), (bar, 0)]) // # notice the change
  --> onPartitionsAssigned([(foo, 0)])
System.out.println(consumer.assignment()); // prints [(foo, 0)]