What is the difference between Java Stream and Collection api ?

Java Stream was introduced by Oracle in Java 8 as a new and much efficient way to iterate over collection of data which may use internal parallelism as well without user actually using any thread declarations.

 There are some basic differences between old Collection api and new Streams, which are outlined below:

1.     Usage: Collection API is used for storing data in different kinds of data structures. Stream API is used for computation of data on a large set of Objects. In Other words, Streams are used for manipulating the data for various purposes like converting it into some other format or converting one Collection to other Collection etc. while Collection itself is used to store the data.

2.     Finite: With Collection API we can store a finite number of elements in a data structure. With Stream API, we can handle streams of data that can contain infinite number of elements. Like,  Stream can used for all the odd prime numbers but you cannot use a Collection to do the same as if you try to do that it would be an infinite loop which would never complete, it will keep on computing new odd primes and adding to the collection and never finish.

3.     Eager vs. Lazy: Collection API constructs objects in an eager manner. Stream API creates objects in a lazy manner. In Collection all the elements needs to be stored in the data structure before the user can use it but same is not true for Streams. A real life analogy would be , suppose you want to watch a movie, Collection would be something like a DVD which has all the movie bytes stored in there before you start using it while Stream would be like Online streaming the movie where bytes are transferred on need basis and continuously.

4.     Multiple consumption: Most of the Collection APIs support iteration and consumption of elements multiple times. With Stream API we can consume or iterate elements only once. Once you are done with your operation on the stream it is called to be exhausted and cannot be used further. For new operation you have to create new stream again.

5.     Parallel processing:  With Collection api if you want to go with parallel processing you have to write your own explicit code using java multithreading concepts in most of the cases while in case of Stream you can just use ParallelStream in place of Stream and parallelism would be handled internally by java.

6.     Internal vs External iteration: This is also one of the important differences between Collection and Stream. In Collection, iteration is done through iterator which the developer needs to write by himself in code which is called External iteration while in case of Stream developer does not need to write any Iterator, its java which manages how to iterate based on best possible approach.

           Above mentioned differences are basic and easily understandable ones. Please mention in comments other difference and I would include that in the list.