Java 8 Streams

An Introduction to Streams
Manvendra Singh
@Manvendra_SK
Java 8

() -> Agenda
What is a Stream?
Creating Streams.
Optional Values.
Operations on Streams.
Collecting data using Collectors.

Get code at...
git clone
git@github.com:manvendrasinghkadam/ja
va8streams.git

1. What is a Stream?
An aggregate operation computes a single
value from a collection of values. Result may be
primitive value, void or an object which can be
a Person, List or Map!
A stream is a sequence of data elements
supporting sequential and parallel aggregate
operations.
Steams are not collections. Streams focus on
aggregate computations on data elements from
a data source that is typically, but not
necessarily, collections!

... No storage
A collection is an in-memory data structure that
stores all its elements. Whereas stream has no
storage; it does not store elements.
A stream pulls elements from a data source on-
demand and passes them to a pipeline of
operations for processing.

... Infinite Streams
A collection cannot represent a group of infinite
elements whereas a stream can.
A stream pulls its elements from a data source
that can be a collection, a function that
generates data, an I/O channel, etc.
Because a function can generate an infinite
number of elements, it is possible to have a
stream representing a sequence of infinite data
elements!

... Imperative vs. Functional
Collections support imperative programming
whereas streams support declarative
programming.
When we use collections, we need to know
what we want and how to get it; this is the
feature of imperative programming. When we
use streams, we specify only what we want in
terms of stream operations; the how part is
taken care by the Streams API.
When we use Streams, we specify what
operations we want to perform on its elements
using the built-in methods provided by the
Streams API, typically by passing a lambda

... Stream Operations
A Stream support 2 types of operations.
Intermediate operations (Lazy operations).
Terminal operations (Eager operations).
Operations are known as lazy and eager
based on the way they pull the data
elements from the data source.
A lazy operation on a stream does not
process the elements of the stream until
another eager operation is called on the
stream.
Example is here...

... Stream Operations (cont...)
1, 2,
3, 4,
5
filter map reduce1, 2, 3, 4, 5 1, 3, 5 1, 9, 25 35
numbers.stream().filter(x -> x % 2 == 1).map(x -> x * x).reduce(0, (x, y) -> x + y);

... Streams are not reusable
Unlike collections, Streams are not reusable. They
are one-shot objects.
A Stream cannot be reused after calling a terminal
operation on it.
A Stream implementation may throw an
IllegalStateException if it detects that the
stream is being reused.
Example is here...

2. Creating Streams
There are many ways to create streams. Many
existing classes in the Java libraries have received
new methods that return a stream. Based on the
data source, stream creation can be categorized as
follows:
Streams from values
Empty streams
Streams from Functions
Streams from Arrays
Streams from Collections
Streams from Files

... Streams from values
The Stream interface contains two of() overloaded
methods to create a sequential Stream from a
single value and multiple values.
Example is here...

... Empty Streams
An empty stream is a stream with no elements.
The Stream interface contains an empty() static
method to create an empty sequential stream.
Example is here...

... Streams from Functions
An infinite stream is a stream with a data source
capable of generating infinite number of elements.
The Stream interface contains the two static
methods to generate an infinite stream:
generate: Generates sequential unordered stream.
iterate: Generates sequential ordered stream.
Example is here...

... Streams from Arrays
The Arrays class in the java.util package contains an
overloaded stream() static method to create
sequential streams from arrays.
Example is here...

... Streams from Collections
The Collection interface contains the stream()
method that create sequential stream.
Example is here...

... Streams from Files
Java 8 has added many methods to the classes in
the java.io and java.nio.file packages to support
I/O operations using Streams.
Example is here...

3. Optional Values
null is used to represent nothing or an empty result.
Most often, a method returns null if it does not have a
result to return. This has been a source of frequent
NullPointerException in Java programs.
Java 8 has introduced an Optional<T> class in
the java.util package to deal with
NullPointerException gracefully. Methods that
may return nothing should return an Optional
instead of null.
Actually there is no solution for this
NullPointerException. As Optional is the
wrapper around null, it throws a
NoSuchElementException if the value it
contains is null.

4. Operations on Streams
Here is a list of commonly used Stream operations.
distinct – Intermediate
filter – Intermediate
limit – Intermediate
map – Intermediate
peek – Intermediate
skip – Intermediate
sorted – Intermediate

4. Operations on Streams (cont...)
List is continued here...
allMatch – Terminal
anyMatch – Terminal
findAny – Terminal
findFirst – Terminal
noneMatch – Terminal
forEach – Terminal
reduce – Terminal

... Debugging Stream Pipeline
Sometimes we may want to inspect individual
elements that we get after certain Stream
operation. To do this we Streams API provides us
with the peek operation.
Example is here...

... Iteration using Streams
We can iterate the stream using the forEach
operation.
Example is here...

... Mapping the Stream elements
A map operation applies a function to each
element of the input stream to produce another
output stream. The number of elements in the
input and output streams is the same. The
operation does not modify the elements of the
input stream.
Example is here...

... Filtering the Stream elements
The filter operation is applied on an input stream
to produce another stream, which is known as the
filtered stream. The returned stream by filter
operation is a subset of the input stream.
Example is here...

... Reducing the Stream elements
The reduce operation combines all elements of a
stream to produce a single value by applying a
combining function repeatedly. It is also called
reduction operation or a fold.
Computing the sum, maximum, average, count,
etc. of elements of a stream of integers are
examples of the reduce operation.
Streams API Provides us some specialized methods
for reduce operation on Integers, Doubles and
Floats..
Example is here...

5. Collecting data using Collectors.
There are several cases in which we want to
collect the results of executing a stream
pipeline into a collection such as a List, a Set,
a Map, etc. Sometimes we may want to apply
complex logic to summarize the stream’s data.
This is possible using the collect method.
Example is here...

Java 8 Streams

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