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Explain Java 8 Streams ?

Java 8 Streams lets you process the data in a declarative way and streams can leverage multi-core architectures without you having to write a single line of multithread code.
Stream does not store elements. It simply conveys elements from a source such as a data structure, an array, or an I/O channel, through a pipeline of operations.
Operations performed on a stream does not modify it's source. For example, filtering a Stream obtained from a collection produces a new Stream with the filtered elements.
Stream is lazy and evaluates code only when required.
Streams are more declarative style or more expressive.
Stream encourages less mutability i.e not using multiple variables assignment during computation eventually reduces the side effect.
There are different types of Streams Sequential , Parallel.
The elements of a stream are only visited ONCE during the life of a stream. Like an Iterator, a new stream must be re-generated to revisit the same elements of the source.

https://github.com/net-vinothkumar/java8streams

This example-driven tutorial gives an in-depth overview about Java 8 streams. When I first read about the Stream API, I was confused about the name since it sounds similar to InputStream and OutputStream from Java I/O. But Java 8 streams are a completely different thing. Streams are Monads, thus playing a big part in bringing functional programming to Java:

In functional programming, a monad is a structure that represents computations defined as sequences of steps. A type with a monad structure defines what it means to chain operations, or nest functions of that type together.

This guide teaches you how to work with Java 8 streams and how to use the different kind of available stream operations. You'll learn about the processing order and how the ordering of stream operations affect runtime performance. The more powerful stream operations reduce, collect and flatMap are covered in detail. The tutorial ends with an in-depth look at parallel streams.

If you're not yet familiar with Java 8 lambda expressions, functional interfaces and method references, you probably want to read my Java 8 Tutorial first before starting with this tutorial.

How streams work#
A stream represents a sequence of elements and supports different kind of operations to perform computations upon those elements:

Stream operations are either intermediate or terminal. Intermediate operations return a stream so we can chain multiple intermediate operations without using semicolons. Terminal operations are either void or return a non-stream result. In the above example filter, map and sorted are intermediate operations whereas forEach is a terminal operation. For a full list of all available stream operations see the Stream Javadoc. Such a chain of stream operations as seen in the example above is also known as operation pipeline.

Most stream operations accept some kind of lambda expression parameter, a functional interface specifying the exact behavior of the operation. Most of those operations must be both non-interfering and stateless. What does that mean?

A function is non-interfering when it does not modify the underlying data source of the stream, e.g. in the above example no lambda expression does modify myList by adding or removing elements from the collection.

A function is stateless when the execution of the operation is deterministic, e.g. in the above example no lambda expression depends on any mutable variables or states from the outer scope which might change during execution.

STREAM VS PARALLELSTREAM
There are two types of streams in Java. On the one hand we have the classic streams and on the other hand we have the parallel streams. Both streams can be created from any collection, list or set. We will see exactly what the differences are with an example.

STREAM
In the context of Java 8, a stream is a sequence of objects on which certain methods can be executed. We'll get to know exactly which method below.

The “classic” streams in Java 8, which are generated with the .stream () method. As mentioned above, these can be created from a collection, list or set. The elements of the “classic” streams are processed one after the other. Basically, it can be compared to an iteration. The only difference is that you do not implement in the iteration step what should happen to the current element.

PARALLEL STREAM
In addition to the “classic” streams, Java 8 also offers the parallel streams. As the name suggests, parallel streams are streams that are processed in parallel. This has the great advantage that you don't have to worry about multithreading yourself. The Stream API does that for us.