In this post give some basic on JAVA Stream API which is added in Java 8. It works very well in conjunction with lambda expressions. Pipeline of stream operations can manipulate data by performing operations like search, filter, count, sort, etc. A stream pipeline consists of a source such as an array, a collection, a generator function, an I/O channel. It may have zero or more intermediate operations for transform a stream.

Stream operations are divided into intermediate and terminal operations.

Intermediate operations return a new stream. They are always lazy; executing an intermediate operation does not actually perform any filtering, but instead creates a new stream that, when traversed, contains the elements of the initial stream that match the given predicate. Intermediate operations do not get executed until a terminal operation is invoked as there is a possibility they could be processed together when a terminal operation is executed.
eg: map, filter, flatmap, limit, sorted, distinct, peek

Terminal operations produces a non-stream, result such as primitive value, a collection or no value at all. Terminal operations are typically preceded by intermediate operations which return another Stream which allows operations to be connected in a form of a query.
eg: findAny, allMatch, count, max, min, etc.

Sample in can be found in here

Intermediate operations are further divided into stateless and stateful operations.

Stateless operations, such as filter and map, retain no state from previously seen element when processing a new element, each element can be processed independently of operations on other elements.

Stateful operations, such as distinct and sorted, may incorporate state from previously seen elements when processing new elements. Stateful operations may need to process the entire input before producing a result. For example, one cannot produce any results from sorting a stream until one has seen all elements of the stream.

Sample in can be found in here. You have be care full on this if you use in wrong order it may lead memory issue. As example if we sort() stream first then it will load all in to memory as shown in sample. (test_sorted_notShortCircuiting)

(limit_car_maker_test)

You can see all the elements are loaded in ‘test_sorted_notShortCircuiting’ though it have limit of 2.

Streams in serial or in parallel

You can execute streams in serial or in parallel. When a stream executes in parallel, the Java runtime partitions the stream into multiple sub-streams.

As example - Collection has methods Collection.stream() and Collection.parallelStream(), which produce sequential and parallel streams respectively.

When you do that, the stream is split into multiple chunks, with each chunk processed independently and with the result summarized at the end. In sample implementation of get sum of the longs method you can take advantage of parallelization and utilize all available CPU cores.

Optional

In the sample you will find new class Optional which was introduce with Java 8. It is used to represent a value is present or absent. The main advantage of this new construct is that No more too many null checks and NullPointerException. It avoids any runtime NullPointerExceptions and supports us in developing clean and neat Java APIs or Applications.

When a value is present, the Optional class just wraps it. Conversely, the absence of a value is modeled with an “empty” optional returned by the method Optional.empty. It’s a static factory method that returns a special singleton instance of the Optional class. Dereference a null will invariably cause a NullPointer-Exception, whereas Optional.empty() is a valid, workable object.