code: public SingleOutputStreamOperator<T> setParallelism(int parallelism) {
  		Preconditions.checkArgument(canBeParallel() || parallelism == 1,
  				"The parallelism of non parallel operator must be 1.");
  
  		transformation.setParallelism(parallelism);
  
  		return this;
  	}
docs: Sets the parallelism for this operator.
  
  @param parallelism
  The parallelism for this operator.
  @return The operator with set parallelism.