Should I Parallelize Java 8 Streams?

[Java] parallel streams

In Java 8, the streams API is easy to iterate over collections, and it's easy to parallelize a stream by calling the parallelStream() method. But should we be using parallelStream() wherever we can? What are the considerations? 

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Look at the following ParallelStreamTester class to generate collections of different sizes for the purpose of testing parallel streams performance against a sequential stream.