Overview of Java 8 Streams (Part 2) - Vanderbilt …schmidt/cs891f/2018-PDFs/06-Java...2 Learning Objectives in this Part of the Lesson • Understand the structure & functionality

Overview of Java 8 Streams (Part 2)

Douglas C. [email protected]

www.dre.vanderbilt.edu/~schmidt

Professor of Computer ScienceInstitute for Software Integrated Systems

Vanderbilt University Nashville, Tennessee, USA

mailto:[email protected]

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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

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operations• These operations apply to both

sequential & parallel streams

Input x

Output f(x)

Output g(f(x))

Intermediate operation (Behavior f)



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

Input x

Output f(x)

Output g(f(x))




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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

https://github.com/douglascraigschmidt/LiveLessons/tree/master/SimpleSearchStream

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Overview of SimpleSearchStream Example

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• 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()


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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…


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List<String> …

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

Search Words

stream()

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List<String> …


Search Words

stream()

Convert collection to a (sequential) stream

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Output a stream of words to find


…Stream<String>

List<String> …


Search Words

stream()

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Input a stream of words to find



Stream<String>

List<String> …

…


Search Words

stream()

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Stream<String>

List<String> …

…


Search Words

stream()

Search for the word in the input string

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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

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Input a stream of search results




Stream<SearchResults>

Stream<String>

List<String> …


Search Words

…

…

stream()

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Stream<String>

List<String> …


Search Words

…

…

stream()

Remove empty search results from the stream

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Output a stream of non-empty search results





Stream<String>


List<String> …


Search Words

…

…

stream()

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Input a stream of non-empty search results




collect(toList())


Stream<String>


List<String> …


Search Words

…

…

stream()

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collect(toList())


Stream<String>


List<String> …


Search Words

…

…

stream()

Trigger intermediate operation processing

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collect(toList())


Stream<String>


List<SearchResults>

List<String> …


Search Words

…

…

stream()

Return a list of search results

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• 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

https://www.ibm.com/developerworks/library/j-java-streams-3-brian-goetz/index.html

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Overview of Common Stream Aggregate

Operations

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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)))

https://en.wikipedia.org/wiki/Higher-order_function

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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)))

https://en.wikipedia.org/wiki/Higher-order_function

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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

What

How

Output h(g(f(x)))

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

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Overview of Common Stream Aggregate Operations

Input x

Output f(x)

Output g(f(x))




• There are two types of aggregate operations

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• 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))




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

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

etc.


Input x

Output f(x)

Output g(f(x))




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• 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

https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#map-java.util.function.Function-

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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

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


Stream<String>

List<String> …


Search Words

…

…

stream()

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Stream<String>

List<String> …


Search Words

…

…

stream()

map() may transform the type of elements it processes

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Search Words

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

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

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• 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))

https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#filter-java.util.function.Predicate-

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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))

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Filter out empty SearchResults




Stream<String>


List<String> …


Search Words

…

…

stream()

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

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Filter out empty SearchResults




Search Words

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


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• 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)))

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

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• 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)))

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• 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)))

https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#collect-java.util.stream.Collector-

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collect(toList())


Triggers intermediate operation processing




Stream<String>


List<SearchResults>

List<String> …


Search Words

…

…

stream()

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


collect(toList())

Create a list of SearchResults





Search Words

stream()

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• 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))

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• 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

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

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End of Overview of Java 8 Streams (Part 2)

Documents

Overview of Java 8 Streams (Part 2) - Vanderbilt …schmidt/cs891f/2018-PDFs/06-Java...2 Learning Objectives in this Part of the Lesson • Understand the structure & functionality