Parallel Collections - Parallel Collection Conversions - Scala Documentation

Every sequential collection can be converted to its parallel variant using the par method. Certain sequential collections have a direct parallel counterpart. For these collections the conversion is efficient– it occurs in constant time, since both the sequential and the parallel collection have the same data-structural representation (one exception is mutable hash maps and hash sets which are slightly more expensive to convert the first time par is called, but subsequent invocations of par take constant time). It should be noted that for mutable collections, changes in the sequential collection are visible in its parallel counterpart if they share the underlying data-structure.

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