ADT02 - Java 8 Lambdas and the Streaming API

Java 8 Lambdas and the Streaming API

Michael RemijanSystem Architect,

Federal Reserve Bank St. Louis

Level: Intermediate

75

Where I work?• Federal Reserve Bank St. Louis• Since 2014

http://github.com/mjremijan http://slideshare.net/mjremijan

What I do?• Java Enterprise (Java EE)

– System Architect– Developer

• Since 1999• Industries

– B2C and B2B commerce, manufacturing, astronomy, agriculture, telecommunications, national defense, healthcare, and financial areas


What I do? (cont.)• Author

– EJB 3 In Action Second Edition• 2014

• Blogger– http://mjremijan.blogspot.com


What I do? (cont.)• Adjunct Instructor

– Since 2009• Java I & II


Where to find [email protected]@mjremijanhttp://mjremijan.blogspot.comhttps://github.com/mjremijanhttp://www.slideshare.net/mjremijan


mailto:[email protected]

http://mjremijan.blogspot.com/

https://github.com/mjremijan

http://www.slideshare.net/mjremijan

Where’s this material?https://github.com/mjremijan/live360-streamshttp://www.slideshare.net/mjremijan




What we’re going to talk about• Functional programming in Java 8• Lambdas

– What is a Lambda?– How do they relate to objects?

• Stream API– What is a Stream?– How are they used to process collections/lists


What’s the environment?NetBeans 8.1

Java 8– jdk1.8.0_65_x64


http://blog.takipi.com/5-features-in-java-8-that-will-change-how-

you-code/

LambdasBrief Introduction


http://viralpatel.net/blogs/lambda-expressions-java-tutorial/

Before Java 8: Objects• Objects are needed for

everything• Cannot use functions w/o

an Object


After Java 8: Lambdas• Enter Lambdas• Allow functions/methods

to be used outside an Object

• Pass lambdas (methods) around

– Not objects!


StreamsBrief Introduction


http://www.deadcoderising.com/java-8-no-more-loops/

Before Java 8: Iterator• Collections heavily follow

the Gang of Four iterator pattern

• java.util.Iterator• java.lang.Iterable<T>• External access to a

collection’s contents


After Java 8: Streams• Enter Streams• Access to collection

contents now internalized• Pass Lambda to apply for

each element.


General stream usageCollection .stream() .intermediateOperation() .intermediateOperation() .intermediateOperation() ... .terminalOperation()

.stream() .intermediateOperation() .intermediateOperation() .intermediateOperation() ... .terminalOperation()

;

String str = ints .stream() .map(i -> String.valueOf(i)) .reduce((s1, s2) -> s1 + ", " + s2) .get();


Intermediate operations• Operation performed on each stream

element• Results in a new stream• Lazy

– A terminal operation is needed

• Examples– Filter– Map– FlatMap– Sorted– Distinct– Limit

ints.stream() .map(i -> String.valueOf(i)) .reduce((s1, s2) -> s1 + ", " + s2) .get();


Terminal operations• Eager

– This is what starts the process

• Transform stream into something else– Collection/List– Map– Single value

• Examples– ForEach– Average– Count– Max– Min– Reduce

String str = ints .stream() .map(i -> String.valueOf(i)) .reduce((s1, s2) -> s1 + ", " + s2) .get();

– Collect– toArray– findFirst– findAny– anyMatch– allMatch


Specialized streams• Stream<T>

– Basis for starting all streaming operations

• Specialized Streams– IntStream– LongStream– DoubleStream

• Provide specialized terminal operations– average()– sum()– min()– max()– …


Example: forEach• Terminal Operator• forEach

– Execute the lambda on each element of the stream


Example: min & max• Specialized

Terminal Operator• min, max

– Returns the min or max element of the stream…maybe!


Example: min & max (cont.)• Just Stream<T>


Example: sum & average• Specialized

Terminal Operator• sum

– the total amount resulting from the addition of two or more numbers, amounts, or items.

• average– a number expressing the central or

typical value in a set of data, in particular the mode, median, or (most commonly) the mean, which is calculated by dividing the sum of the values in the set by their number.


Example: sum & average (cont.)• Just Stream<T>• Sum

– Reduce to a single value

• Average– Can’t do it…directly.


Example: reduce• Terminal Operator• reduce

– Turn all the stream elements into a single value of the same type

• Product of numbers– Multiply all the numbers

together– 3628800


Example: sum of squares• Terminal Operator• Sum of squares

– Square each number then get the sum.

• Unfortunately wrong– 379

• 3 + (10*10) = 103• 103 + (6*6) = 139• 139 + (1*1) = 140• …


Example: sum of squares (cont.)• Terminal Operator • Sum of squares• Correct!

– 385

• 0 + (3*3) = 9• 9 + (10*10) = 109• 109 + (6*6) = 145• …


Example: CSV string• Terminal Operator • Comma Separated Values

– tabular data, each record consists of one or more values, separated by commas.

• Stream<String>– Needed to reduce to a single

String value


Example: filter• Intermediate Operator• filter

– Filter stream elements by some test

– Resulting stream typically smaller than original

• Evens


Example: odds• Intermediate Operator• Odds


Example: primes• Intermediate Operator• Primes

– Kind of


Example: sorted• Intermediate Operator• sorted

– Jumble stream items around into a different order

– Resulting stream has same number of elements as orginal

• Low-to-high


Example: high-to-low• Intermediate Operator• High-to-low


Example: in 4-8 and sorted• Intermediate Operator• Between 4-8 inclusive,

sorted in reverse order


Example: map• Intermediate Operator• map

– Convert / map elements of the stream into something else.

– Resulting steam has same number of elements as original

• To squares


Example: to halves• Intermediate Operator• To halves


Example: to String• Intermediate Operator• To strings


Example: to Widget• Intermediate Operator• Map to my own Widget

object

• Create Widget– Basic bean


• Intermediate Operator• To widgets

Example: to Widget (cont.)


Example: Putting things together• How to translate your

processing into intermediate and terminal operations?

• Get all odds, square them, then sort low-to-high.


Example: int to Invoice• Invoice ID values• Find invoices for the last 6

months of the year, sorted by highest billing amount to lowest.

• Create Invoice – Basic bean


Example: int to Invoice (cont.)• Create InvoiceFinder

– Business logic to get invoices– SQL– Web Service– REST Service– etc…


Example: int to Invoice (cont.)• Map• Filter• Sort• Collect


Example: Beyond int• All examples so far

– Same array of int values

• int values are…– Boring?

• A Person is…– Interesting?


Example: Beyond int (cont.) • Create PersonFinder

– Small database of person data

– Find All


Example: max age• Find max age• Person::getAge

– Method Reference– Used to refer directly to an

existing method instead of creating a method with a lambda expression


Example: in their 30’s• Find people in

their 30’s


Example: 1st in Chicago• Find the first

person, listed alphabetically by last name, who lives in Chicago


Example: sort & sub-sort• List all people

sorted first by city then sub-sorted by last name reversed.


Summary• Lambda

– Brings functional programming to Java– Introduced in Java 8– Allow functions/methods to be used outside an Object

• Pass lambdas (methods) around

– Not objects!

• Stream<T>– New method of collection processing for Java– Introduced in Java 8– Heavily reliant on lambdas– Intermediate and terminal operations


Email regular expression

- http://emailregex.com/

Summary (cont.)• We just scratched the surface

– Can get much more complicated

• How do I learn more?– StackOverflow!


(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])

Software

ADT02 - Java 8 Lambdas and the Streaming API