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<h2>Combine - CombineFn</h2>
<p>Combine is a Beam transform for combining collections of elements or values in your data.
  When you apply a Combine transform, you must provide the function that contains the logic for
  combining the elements or values. The combining function should be commutative and associative,
  as the function is not necessarily invoked exactly once on all values with a given key. Because
  the input data (including the value collection) may be distributed across multiple workers, the
  combining function might be called multiple times to perform partial combining on subsets of
  the value collection.</p>
<p>Complex combination operations might require you to create a subclass of CombineFn that has an
  accumulation type distinct from the input/output type. You should use CombineFn if the combine
  function requires a more sophisticated accumulator, must perform additional pre- or
  post-processing, might change the output type, or takes the key into account.</p>
<p>In this task, we are going to implement the average of numbers using
  <a href="https://beam.apache.org/releases/javadoc/2.11.0/org/apache/beam/sdk/transforms/Combine.CombineFn.html">
    Combine.CombineFn</a>.
</p>
<br>
<br>
<div class='hint'>Extend the
  <a href="https://beam.apache.org/releases/javadoc/2.11.0/org/apache/beam/sdk/transforms/Combine.CombineFn.html">Combine.CombineFn</a>
  class that counts the average of the number.</div>
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