What's so special about Java 8 stream API

Java 8 has added functional programming and one of the major addition in terms of API is, stream.

A mechanical analogy is car-manufacturing line where a stream of cars is queued between processing stations. Each take a car, does some modification/operation and then pass it to next station for further processing.

Main benefit of stream API is that, now in Java (8) you can program at higher level of abstraction. So you can transform stream of one type to stream of other type rather than processing each item at a time (using for loop or iterator). With this Java 8 can run a pipeline of stream operations on several CPU cores on different components of the input. This way you are getting parallelism almost free instead of hard work using threads and locks.

Stream focuses on partitioning the data rather than coordinating access to it. 

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Collection is mostly about storing and accessing the data, whereas stream is mostly about describing computation on data.