Overview of Java 8 Streams (Part 2) - Vanderbilt …schmidt/cs891f/2018-PDFs/06-Java...2 Learning...

Overview of Java 8 Streams (Part 2)

Douglas C. Schmidtd.schmidt@vanderbilt.edu

www.dre.vanderbilt.edu/~schmidt

Professor of Computer ScienceInstitute for Software Integrated Systems

Vanderbilt University Nashville, Tennessee, USA

Learning Objectives in this Part of the Lesson• Understand the structure & functionality of Java 8 streams, e.g.,

• Fundamentals of streams• Common stream aggregate

operations Intermediate operation (Behavior f)

Input x

Output f(x)

Output g(f(x))

Intermediate operation (Behavior g)

Terminal operation (Behavior h)

Both intermediate & terminal operations

operations• These operations apply to both

sequential & parallel streams

Input x

Output f(x)

Output g(f(x))

Intermediate operation (Behavior f)

Being a good streams programmer makes you a better parallel streams programmer

Input x

Output f(x)

Output g(f(x))

See github.com/douglascraigschmidt/LiveLessons/tree/master/SimpleSearchStream

We’ll use a simple sequential stream example to explain common

Java 8 aggregate operations

map(this::searchForWord)

filter(not(SearchResults::isEmpty))

collect(toList())

45,000+ phrases"do", "re", "mi", "fa","so", "la", "ti", "do"

Search Words

Input String to Search

Overview of SimpleSearchStream Example

• This example finds words in an input stringOverview of SimpleSearchStream Example

We’ll use this example to explain Java 8 aggregate operations

throughout this part of the lesson

Let's start at the very beginning…

collect(toList())

Search Words

stream()

Starting SimpleSearchStreamWord "Re" matched at index [131|141|151|202|212|222|

979|1025|1219|1259|1278|1300|1351|1370|1835|1875|1899|1939|2266|2295]

Word "Ti" matched at index [237|994|1272|1294|1364|1850|1860|1912|1915|1952|1955|2299]

Word "La" matched at index [234|417|658|886|991|1207|1247|1269|1291|1339|1361|1742|1847|1863|1909|1949|2161|2254|2276|2283]...

Ending SimpleSearchSTream

collect(toList())

Search Words

stream()

Let's start at the very beginning…

List<String> …

Input a list of words to find 45,000+ phrases"do", "re", "mi", "fa","so", "la", "ti", "do"

Search Words

stream()

List<String> …

Search Words

stream()

Convert collection to a (sequential) stream

Output a stream of words to find

…Stream<String>

List<String> …

Search Words

stream()

Input a stream of words to find

Stream<String>

List<String> …

Search Words

stream()

Stream<String>

List<String> …

Search Words

stream()

Search for the word in the input string

Output a stream of search results

map(this::searchForWord)Stream<SearchResults>

Stream<String>

List<String> …

Search Words

stream()

SearchResults stores # of times a word appeared in the input string

Input a stream of search results

Stream<SearchResults>

Stream<String>

List<String> …

Search Words

stream()

Stream<String>

List<String> …

Search Words

stream()

Remove empty search results from the stream

Output a stream of non-empty search results

Stream<String>

List<String> …

Search Words

stream()

Input a stream of non-empty search results

collect(toList())

Stream<String>

List<String> …

Search Words

stream()

collect(toList())

Stream<String>

List<String> …

Search Words

stream()

Trigger intermediate operation processing

collect(toList())

Stream<String>

List<SearchResults>

List<String> …

Search Words

stream()

Return a list of search results

• The “physical” processing of a stream differs from the “logical” model• i.e., each element is “pulled” from the source

through each aggregate operation

Overview of SimpleSearchStream Example

collect(toList())

Stream<String>

List<SearchResults>

List<String> …

Search Words

stream()

See www.ibm.com/developerworks/library/j-java-streams-3-brian-goetz

Overview of Common Stream Aggregate

Operations

Input x

Output f(x)

Output g(f(x))

Overview of Common Stream Aggregate Operations• An aggregate operation is a higher-order function that applies a “behavior”

on elements in a stream

Aggregate operation (Behavior f)

Aggregate operation (Behavior g)

Aggregate operation (Behavior h)

See en.wikipedia.org/wiki/Higher-order_function

Output h(g(f(x)))

Input x

Output f(x)

Output g(f(x))

Overview of Common Stream Aggregate Operations• An aggregate operation is a higher-order function that applies a “behavior”

on elements in a stream

The behavior can be a lambda or method reference to a function,

predicate, consumer, supplier, etc.

See en.wikipedia.org/wiki/Higher-order_function

Output h(g(f(x)))

Input x

Output f(x)

Output g(f(x))

Overview of Common Stream Aggregate Operations• Aggregate operations focus on “what” (declarative), not “how” (imperative)

See blog.jooq.org/2015/09/17/comparing-imperative-and-functional-algorithms-in-java-8

Output h(g(f(x)))

Overview of Common Stream Aggregate Operations

Input x

Output f(x)

Output g(f(x))

• There are two types of aggregate operations

• There are two types of aggregate operations• Intermediate operations

• Process elements in their input stream & yield an output stream• e.g., filter(), map(), flatMap(), etc.

Input x

Output f(x)

Output g(f(x))

• There are two types of aggregate operations• Intermediate operations • Terminal operations

• Trigger intermediate operations &produce a non-stream result• e.g., forEach(), reduce(), collect(),

Input x

Output f(x)

Output g(f(x))

• Overview of the map() intermediate operation Overview of Common Stream Aggregate Operations

Stream map(Function<…> mapper)

Stream filter(Predicate<…> pred)

R collect(Collector<…> collector)

See docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#map

Input x

Output f(x)

Output g(f(x))

Applies the mapper function to every element of the input stream & returns

an output stream consisting of the results

The # of output stream elements matches the # of input stream elements

Input x

Output f(x)

Output g(f(x))

Applies the mapper function to every element of the input stream & returns

an output stream consisting of the results

For each word to find determine the indices (if any) where the word matches the input string

Stream<String>

List<String> …

Search Words

stream()

Stream<String>

List<String> …

Search Words

stream()

map() may transform the type of elements it processes

Search Words

stream()List<SearchResults> results = wordsToFind.stream().map(this::searchForWord).filter(not

(SearchResults::isEmpty)).collect(toList());

• Overview of the filter() intermediate operation Overview of Common Stream Aggregate Operations

See docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#filter

Tests the given predicate against each element of the input stream & returns an output stream consisting only of

the elements that match the predicate

Input x

Output f(x)

Output g(f(x))

Tests the given predicate against each element of the input stream & returns an output stream consisting only of

the elements that match the predicate

The # of output stream elements may be less than the # of input stream elements

Input x

Output f(x)

Output g(f(x))

Filter out empty SearchResults

Stream<String>

List<String> …

Search Words

stream()

The filter() aggregate operation can’t change the type of elements it processes

Filter out empty SearchResults

Search Words

stream()List<SearchResults> results = wordsToFind.stream().map(this::searchForWord).filter(not

• Intermediate operations are “lazy” & don’t run until a terminal operator is reached

See www.logicbig.com/tutorials/core-java-tutorial/java-util-stream/lazy-evaluation

Input x

Output f(x)

Output g(f(x))

Output h(g(f(x)))

• A terminal operation triggers intermediate operation processingOverview of Common Stream Aggregate Operations

Input x

Output f(x)

Output g(f(x))

Output h(g(f(x)))

• Overview of the collect() terminal operation Overview of Common Stream Aggregate Operations

See docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#collect

This terminal operation uses a collector to perform a reduction on the elements of its input stream & returns the results of the reduction

Input x

Output f(x)

Output g(f(x))

Output h(g(f(x)))

collect(toList())

Triggers intermediate operation processing

Stream<String>

List<SearchResults>

List<String> …

Search Words

stream()

List<SearchResults> results = wordsToFind.stream().map(this::searchForWord).filter(not

collect(toList())

Create a list of SearchResults

Search Words

stream()

• An aggregate operation may process all elements in a stream, e.g.• map() processes all of the elements

in its input stream

Stream limit(long maxSize)

Optional findFirst()

Input x

Output f(x)

Output g(f(x))

• An aggregate operation may process all elements in a stream, e.g.• map() processes all of the elements

in its input stream• limit() & findFirst() are “short-circuit”

operations that halt further processingafter their condition is reached

Stream limit(long maxSize)

Optional findFirst()

Input x

Output f(x)

Output g(f(x))

See www.logicbig.com/tutorials/core-java-tutorial/java-util-stream/short-circuiting

End of Overview of Java 8 Streams (Part 2)

Overview of Java 8 Streams (Part 2) - Vanderbilt …schmidt/cs891f/2018-PDFs/06-Java...2 Learning...

Documents

Java 8 CompletableFutures ImageStreamGang Example (Part 3)schmidt/cs891f/2018-PDFs/16-Java... · 2018-11-17 · set of futures before continuing with the program collect() also triggers

Introduction to Java 8 Parallelism Frameworksschmidt/cs891f/2018-PDFs/03-Java-8-concurrency-and-parallelism...•Java 8 adds two new parallelism frameworks related to functional programming

Java 8 Functional Interfaces - Vanderbilt Universityschmidt/cs891f/2019-PDFs/java-func-inf-pt… · Learning Objectives in This Lesson •Recognize foundational functional programming

Java 8 Parallel ImageStreamGang Example (Part 3)schmidt/cs891f/2018-PDFs/18-Java... · 2018-10-24 · 8 Implementing a Parallel Stream in ImageStreamGang •We focus on processStream()

Overview of Basic Java 8 CompletableFuture Features (Part 2)schmidt/cs891f/2018-PDFs/...8 Applying Basic Completable Future Features •We show how to apply basic completable future

Overview of Java 8 Parallel Streams (Part 2)schmidt/cs891f/2018-PDFs/... · 6 •A Java 8 parallel stream implements a “map/reduce” variant optimized for multi-core processors

Overview of Java 8 Streams (Part 1) - Vanderbilt Universityschmidt/cs891f/2018-PDFs/... · •Java 8 streams are a framework in the Java class library that provides several key benefits

Java 8 SearchWithParallelStreams Example (Part 2)schmidt/cs891f/2018-PDFs/... · applied in SearchWithParallelStreams ... RXJAVA_INPUTS executed in 866 msecs SEQUENTIAL_LOOPS executed

Java 8 Parallel Stream Internals (Part 5)schmidt/cs891f/2018-PDFs/...4 •After the common fork-join pool finishes processing chunks their partial results are combined into a final

Overview of Java 8 Foundationsschmidt/cs891f/2018-PDFs/01-overview-of-Java-8-foundations.pdfOverview of Programming Paradigms in Java 8 • Object-oriented programming is an “imperative”

Overview of Java Object-Oriented Programming Language …schmidt/cs891f/2017-PDFs/L1-pt1-overview-of-OO-Java.pdf• Object-oriented Java programs also perform actions & contain logic

Java 8 Functional Interfacesschmidt/cs891f/2019-PDFs/java... · 2019. 9. 2. · •Recognize foundational functional programming features in Java 8, e.g., •Lambda expressions •Method

Overview of Advanced Java 8 CompletableFuture Features ...schmidt/cs891f/2018-PDFs/... · 2 Learning Objectives in this Part of the Lesson •Understand advanced features of completable

Java security: Java security, Part 1: Crypto basics · PDF fileJava security: Java security, Part 1: Crypto basics ... Java security, Part 1: Crypto basics ... Providers add additional

Reactive Programming & Java 8 Completable Futuresschmidt/cs891f/2018-PDFs/... · •Resilient •Exception methods help systems be resilient to crippling failures Reactive Programming

Java 8 Functional Interfaces - Vanderbilt University › ~schmidt › cs891f › 2019-PDFs › java...Learning Objectives in This Lesson •Recognize foundational functional programming

Motivating the Need for Java 8 Completable Futures …schmidt/cs891f/2018-PDFs/04...6 •Pros of async calls with Java futures •May leverage parallelism more effectively with fewer

Background on Concurrency & Parallelism in Java (Part 1)schmidt/cs891f/2018-PDFs/01-background … · Learning Objectives in this Part of the Lesson •Understand the meaning of the

Overview of Java 8 Streams (Part 5) - Vanderbilt …schmidt/cs891f/2018-PDFs/09-Java...2 Learning Objectives in this Part of the Lesson •Understand the structure & functionality

Overview of Java’s Support for Inheritance & Polymorphism ...schmidt/cs891f/2017-PDFs/L1-pt4-polymorphism.pdf• Polymorphism in Java occurs when a reference to a super class is