Streams in Java8 -Post1

 Stream :- 

• Stream is a new addtion to Collection API in Java8 for building  complex operations on collections using functional approach.These are different from input and output streams

• Stream takes Collections ,Arrays or I/O operations as input build up internal sequence of elements and perform required operations on them and gives the final result

• Streams allow us  to write collections-processing code at a higher level of abstraction. The Stream interface contains a series of functions that will be used to perform common operations on collection. For example  iterating over a collections, filtering out a list of values based on some criteria      

• No storage. A stream is not a data structure that stores elements; instead, it conveys elements from a source such as a data structure, an array, a generator function, or an I/O channel, through a pipeline of computational operations.

• Streams as said are  functional in nature . Streams doesn't modify directly  the  collection/source its working  it just produces the final result  

• Laziness-seeking :- Many functions[filter,map etc] that are provided by stream perform lazy evaluation, Methods that build up a Stream recipe but donot force a new value to be generated at the end are referred to as lazy.

•  Intermediate operations  return a new stream which meets the condition of predicate passed, they are always lazily executed.Traversal of pipeline source  will not be started until Terminal operation is called. 

• Terminal operations are executed eagerly. all the  intermediate operations are executed when they reach a terminal operation.
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•       In below example only filter is used on employees list, as this is a lazy function actual stream result did not build

Lazy operation. out put not printed

           

          Here count method is called which will build  required Stream result . Methods that use the 

      stream recipe generated and produce a final result are called Eager methods

Eager operation count

• Its easy to find a function Lazy or Eager. If a function  returns a stream then its lazy,if it returns  a value other than stream  or void then its eager function       

       There are two variants of streams available in Java8

1. Stream :-  Returns a sequential stream
2. ParllelStream :- Stream executes in parallel i.e Java runtime  divides the stream into multiple sub streams . Aggregate operations iterate over and process the substream in parallel and then combine the result