Java stream - Simplified part 1 .

Some important points about Java streams

1. Stream is a sequence of elements supporting sequential or parallel aggregations.
2. Stream operations are composed into a stream pipeline.
3. Stream pipeline consists of a :
   • Stream source (array, collection, generator function, I/O channel etc.)
   • Zero or more intermediate operations.
   • Terminal operation.
4. Computations are lazy. Computation on source is performed only when the the terminal operation is inititiaed.
5. Intermediate operations transform a stream to another stream.
6. The terminal operation may produce side-effects (count(), forEach(consumer))
7. Streams are lazy computation of source data is only performed when the the terminal operation is initiated.
8. The source elements are comsumef only as needed.

Stream Vs Collection

They both look similar but have different goals.

Collections- are primmarily concerned about efficient management and access to the elements in a collection.

Streams- deal with source

1. You can’t directly access and manipulate the elements like in collection.
2. It is all about declaratively describing the source and computational operations that will be performed in aggregate on that source.
3. A stream pipeline can be viewed as a query on the stream source.

Stream operations

Most stream operations accept parameters that describe user-defined behaviour. The parameter to stream operation must be :

• non-interfering (should not change the source elements)
• stateless (the result should not depend on anything that might have changed due to stream pipeline operations).

in short the parameter to stream operations are functions. And in java functions are represented as instances of Functional instances such as Function, Consumer, Producer, Predicate.

e.g. filter() operation takes an instance of Predicate.
map() takes on instance of Producer.
forEach() takes an instance of Consumer.
So stream operations can accept either lambda expression or method reference as the target types of those Functional interfaces.