Stream Collection types

Java is inheritably sequential and there is no direct mean to introduce parallel processing at library level, Stream API is going to fill that gap. By using this, you can filter elements of collection on given criterion e.g. if you have a List of employees, you can filter all employees based upon their name, department and so on.

Stream API allows you to process Collection both sequentially and parallel. This is also useful for bulk data operation. You can create a sequential and parallel stream as follows :

List employees = getListOfEmployees();
 
// sequential version
Stream  stream = employees.stream();
 
//parallel version
Stream  parallelStream = employees.parallelStream();

Collection and Stream

Collection interface is enhanced to provide stream support. It now has a stream() method which returns a sequential Stream with this collection as its source. Once you get the reference of stream, you can perform bulk data operations with this collection.

One of the important things to note is that Stream do not modify the original source. For every operation, a new Stream is created and original collection remains unmodified. Similarly, you cannot reuse Stream either. Reusing a closed stream will throw IllegalStateException as shown below:

Exception in thread "main" java.lang.IllegalStateException: stream has already been operated upon or closed
    at java.util.stream.AbstractPipeline.(AbstractPipeline.java:203)
    at java.util.stream.ReferencePipeline.(ReferencePipeline.java:94)
    at java.util.stream.ReferencePipeline$StatelessOp.(ReferencePipeline.java:618)
    at java.util.stream.ReferencePipeline$2.(ReferencePipeline.java:163)
    at java.util.stream.ReferencePipeline.filter(ReferencePipeline.java:162)
    at Test.main(Test.java:33)

It supports functional programming idioms such as map and reduce along with parallel processing capability, support for filtering elements from Collection.

Stream operations 

Stream operations are mainly divided into two categories : intermediate and terminal operations. Intermediate operations such as filter() or map() returns a new Stream, while terminal operations such as Stream.forEach() produce a result or side effect. After the terminal operation, the stream pipeline is considered consumed, and can no longer be used.

Intermediate operations are also of two types stateless and stateful. As name suggests, stateless operations doesn't retain any state from previously processed element. For example : filter() and map(). On the other hand distinct() and sorted() are example of stateful operations, which can have state from previously processed elements, while processing new elements.

Important methods in java.util.stream

Return Type	Method Name	Short Description
Long	count()	Returns the count of elements in this stream
Stream	Distinct()	Returns a stream consisting of the distinct elements (according to Object.equals(Object)) of this stream.
static Stream	Empty()	Returns an empty sequential Stream.
Stream	Filter(Predicate predicate)	Returns a stream consisting of the elements of this stream that match the given predicate.
Stream	flatMap(Function> mapper)	Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.
void	forEach(Consumer action)	Performs an action for each element of this stream.
static Stream	generate(Supplier s)	Returns an infinite sequential unordered stream where each element is generated by the provided Supplier.
static Stream	iterate(T seed, UnaryOperator f)	Returns an infinite sequential ordered Stream produced by iterative application of a function f to an initial element seed, producing a Stream consisting of seed, f(seed), f(f(seed)), etc.
Stream	Limit(long maxSize)	Returns a stream consisting of the elements of this stream, truncated to be no longer than maxSize in length.
Stream	map(Function mapper)	Returns a stream consisting of the results of applying the given function to the elements of this stream.
Optional	max(Comparator comparator)	Returns the maximum element of this stream according to the provided Comparator.
Optional	min(Comparator comparator)	Returns the minimum element of this stream according to the provided Comparator.
Stream	skip(long n)	Returns a stream consisting of the remaining elements of this stream after discarding the first n elements of the stream.
Stream	sorted()	Returns a stream consisting of the elements of this stream, sorted according to natural order.
Stream	Sorted (Comparator comparator)	Returns a stream consisting of the elements of this stream, sorted according to the provided Comparator.
Object[]	toArray()	Returns an array containing the elements of this stream.

Reference : http://javarevisited.blogspot.com/2014/03/2-examples-of-streams-with-Java8-collections.html#ixzz3jEqs26XQ