This calls the convert() method of a MapReduce object, which converts a KeyValue object into a KeyMultiValue object. It does this by finding duplicate keys (stored only by this processor) and concatenating their values into a list of values which it associates with the key in the KeyMultiValue object. The method returns the total number of key/value pairs in the KeyMultiValue object, which will be the number of unique keys in the KeyValue object.
This operation creates a hash table to find duplicate keys efficiently. More details are given in the Technical Details section.
This method is an on-processor operation, requiring no communication. When run in parallel, each processor converts only the key/value pairs it owns into key/multi-value pairs. Thus, this operation is typically performed only after the aggregate() method has collected all duplicate keys to the same processor. The collate() method performs an aggregate() followed by a convert().
Related methods: collate()