Java 8 DOs and DON'Ts - javaBin Oslo May 2015

Java 8 and .
1
javaBin Oslo – 12.05.2015
Fredrik Vraalsen
fredriv vraalsen@iterate.no

Java 8 – what’s new?
Lambdas
Method handles
Extension methods
Streams
Optional
3

Java 8 – what’s new?
Lambdas (anonymous functions)
Method handles
Extension methods
Streams
Optional
4

Java 7 vs 8
Collections.sort(people, new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
});
6

Java 7 vs 8
}
});
vs
sort(people, (x, y) -> x.getName().compareTo(y.getName()));
7

Java 7 vs 8
}
});
vs
sort(people, comparing(person -> person.getName()));
8

Java 7 vs 8
}
});
vs
sort(people, comparing(Person::getName));
9

Java 7 vs 8
}
});
vs
people.sort(comparing(Person::getName));
10

Streams
© Fredrik Vraalsen 2008

Old fashioned imperative
12
List<RoadData> filtered = new ArrayList<>();
int count = 0;
for (Iterator<RoadData> i = roadData.iterator();
i.hasNext() && count < 10; ) {
RoadData data = i.next();
if (nameQuery.matches(data.getName())) {
filtered.add(data);
count++;
}
}

Streams
13
roadData.stream()
.filter(data -> nameQuery.matches(data.getName()))
.limit(10)
.collect(toList());

Streams – pipelines
14
roadData.stream()
.limit(10)
.collect(toList());
cat roadData.txt | grep … | head > output.txt

Map<String, List<Article>> articlesByCategory =
Learn you a Stream API
16

articleTransports.stream()
17

.map(transport -> convertToArticle(transport))
18

.collect(groupingBy(article -> article.getCategory()));
19

Method handles
20

.map(this::convertToArticle)
Method handles
21

Method handles
22
.collect(groupingBy(Article::getCategory));

Naming, caching, counting
23
.collect(groupingBy(Article::getCategory));

Naming, caching, counting
24
.map(this::transportToArticle)
.collect(groupingBy(Article::category));

Extract functions
25
.map(transportToArticle)
.collect(groupingBy(Article::category));
Function<ArticleTransport, Article> transportToArticle =
transport -> convertToArticle(transport);

Lambdas everywhere!!!1!
links.stream()
.map(link -> link.build())
.forEach(abderaElement::addLink);
26

Old school ifs and fors
links.stream()
.map(link -> link.build())
.forEach(abderaElement::addLink);
for (LinkBuilder lb : links) { 
abderaElement.addLink(lb.build()); 
}
27

Parsing nested JSON
"suggest": {
"tag_suggest": [
{
"length": 5,
"offset": 0,
"options": [
{
"freq": 25,
"score": 0.8,
"text": "fakta"
}
],
"text": "fanta"
}
]
} 28

List<String> terms = new ArrayList<>(); 
suggestions.getAsJsonObject().getAsJsonArray(“tag_suggest”) 
.forEach(tag -> tag.getAsJsonObject().getAsJsonArray("options") 
.forEach(option -> terms 
.add(option.getAsJsonObject().getAsJsonPrimitive("text") 
.getAsString()))); 
29

for (JsonElement tag : suggestions.getAsJsonObject().getAsJsonArray("tag_suggest")) { 
for (JsonElement option : tag.getAsJsonObject().getAsJsonArray("options")) { 
terms.add(option.getAsJsonObject().getAsJsonPrimitive("text").getAsString()); 
} 
} 
30

for (JsonElement tag : getAll("tag_suggest", suggestions)) { 
for (JsonElement option : getAll("options", tag)) { 
terms.add(getString("text", option)); 
} 
} 
31

List<String> terms = getAll("tag_suggest", suggestions).stream() 
.flatMap(tag -> getAll("options", tag).stream()) 
.map(option -> getString("text", option)) 
.collect(Collectors.toList());
32

Readability ≫ Style / LOC
Always code as if the person who ends up maintaining your code
is a violent psychopath who knows where you live.
http://c2.com/cgi/wiki?CodeForTheMaintainer

OH NOES
NullPointerException!!?!

Optional<T>
Wrapper object
Make explicit if a value is present or absent (empty)
35

Optional example
public Optional<Image> getImage(Article article)
---
36

Optional example
---
Optional<Image> image = getImage(article);
37

Check presence
---
if (image.isPresent()) {
doSomething(image.get());
}
38

Check presence
---
image.ifPresent(img -> doSomething(img));
39

Check presence
---
getImage(article).ifPresent(image -> doSomething(image));
40

Default / fallback value
---
Image image = getImage(article).orElse(defaultImage);
41

Optional in / Optional out
public Optional<String> getCaption(Optional<Image> image)
---
Optional<String> caption = getCaption(image);
42

Optional in / Optional out
public String getCaption(Image image)
---
Optional<String> caption = ???
43

Reaching into the void …
---
Optional<String> caption = image.map(img -> getCaption(img));
44

Reaching into the void …
---
Optional<String> caption = getImage(article)
.map(img -> getCaption(image));
45

Bye bye NullPointerException?
Marker – this may not return/contain value
Public APIs
Not a general solution to NPEs
46

Optional + Streams = true?
48
myStream.map(v -> functionReturningOptional(v))

49
Stream<Optional<T>>

50
Stream<Optional<T>> Stream<T> ???

Guava Optional
51
Iterable<T> presentInstances(Iterable<Optional<T>> optionals)

52
---
List<Image> images = articles.stream() // Stream<Article>

53
---
List<Image> images = articles.stream() // Stream<Article> 
.map(article -> getImage(article)) // Stream<Optional<Image>>

54
---
.filter(Optional::ifPresent)
.map(Optional::get) // Stream<Image>

55
---
.map(Optional::get) // Stream<Image> 
.collect(toList());

ﬂatMap to the rescue!
flatMap = map + flatten
Stream<Stream<T>> Stream<T>
56

flatMap = map + flatten
Stream<Optional<T>> Stream<T> ???
57

Easy to create helper functions
Stream<T> toStream(Optional<T> optValue)
58
http://stackoverﬂow.com/questions/22725537/using-java-8s-optional-with-streamﬂatmap

59
---
.map(Optional::get) // Stream<Image> 
.collect(toList());

Optional + Streams = true
60
---
.flatMap(article -> toStream(getImage(article))) // Stream<Image> 
.collect(toList());

Optional + Streams = true
61
---
.flatMap(article -> getImage(article).stream()) // Stream<Image> 
.collect(toList());
Java 9
https://bugs.openjdk.java.net/browse/JDK-8050820

Take one stream and pass it around…
62
public Stream<Image> getImage(Article article)
---
.flatMap(article -> getImage(article)) // Stream<Image> 
.collect(toList());

No method handle for you!
63
private Article elementToArticle(Map contentById, Element el)
---
List<Article> articles = elements.stream() 
.map(el -> elementToArticle(contentById, el)) 
.collect(toList())

64
http://en.wikipedia.org/wiki/Malabar_matthi_curry#/media/File:Meen_curry_2_(cropped).JPG

Yummy currying!!
65
Mapper mapper = new Mapper(contentById);
---
.map(el -> mapper.elementToArticle(el)) 
.collect(toList())

Yummy currying!!
66
Mapper mapper = new Mapper(contentById);
---
.map(mapper::elementToArticle) 
.collect(toList())

Performance

Streams – performance
List<Article> frontpageArticles =
frontpage.getArticleIds().stream()
.map(id -> fetchArticle(id))
.collect(toList());
68

This one goes to 11!
List<Article> frontpageArticles =
frontpage.getArticleIds().parallelStream()
.map(id -> fetchArticle(id))
.collect(toList());
69

Starvation
Common F/J thread pool
Workarounds?
Execute within explicit F/J pool
Use CompletableFuture
70

CompletableFuture
Chaining of async futures and actions
Waiting for all or any future(s)
Explicitly complete (like Promise)
Control of executor service
71
http://blog.krecan.net/2014/03/18/how-to-specify-thread-pool-for-java-8-parallel-streams/
http://www.nurkiewicz.com/2013/05/java-8-completablefuture-in-action.html

To parallelStream or not to …
Batch?
parallelStream FTW!
Interactive? Concurrency?
CompletableFuture FTW!
72

Taking the happy path!
Try<RestResponse<ResolvedUri>> resolveURI(URI uri)
---
public ResolvedSectionUri resolveSectionUri(String uri) {
return client.resolveURI(uri))
??? Try<RestResponse<ResolvedUri>> 
}
73

---
return client.resolveURI(uri)) 
.map(response -> response.getEntity()) 
??? Try<Optional<ResolvedUri>> 
}
74

---
.map(response -> response.getEntity().get()) 
??? Try<ResolvedUri> 
}
75

---
.map(response -> (ResolvedSectionUri) response.getEntity().get()) 
??? Try<ResolvedSectionUri> 
}
76

---
.orElse(null);
}
77

---
.orElseGet(() -> { 
log.warn("Cannot resolve section from URI: {}", uri); 
return null; 
});
}
78
https://github.com/lambdista/try

More cool stuff in Java 8
String join
Collection removeIf (≈ filter)
Map getOrDefault, putIfAbsent, replace, forEach, etc.
java.util.Optional
java.util.stream.Collectors
count, sum, average, min, max, groupingBy, etc.
java.nio.file.Files lines
79

Functional programming
Simpler code
More robust
Better performance

Higher level
Declarative
Less verbose
Less bugs?
81
List<RoadData> filtered = new ArrayList<>();
int count = 0;
for (Iterator<RoadData> i = roadData.iterator();
i.hasNext() && count < 10; ) {
RoadData data = i.next();
if (nameQuery.matches(data.getName())) {
filtered.add(data);
count++;
}
}
roadData.stream()
.limit(10)
.collect(toList());
vs.

Pure functions
82
Data In Data Out
Transformation
No external interactions

Functional core
83
————- —— —
—— —- — —- —-
—————
———
—— —- — —- —
————
—— — — — ——-
——- —- — ————
—- ——— ———
———- —— ——
—— - —-
———- — —- — -
Pure functionsI/O I/O

Building blocks

What’s missing?

What’s missing?
Immutability
Value types
Data structures (lists, maps, etc.)
Concurrency
86

What’s the big deal?
Functional programming is all about values!
And transformations (functions) computing new values
Parallellism vs. Concurrency
Robustness
Testability
87

Some help to be found
Immutable collections
Google Guava, FunctionalJava, clj-ds
Concurrency mechanisms
Akka (Actors, STM)
88

github.com/krukow/clj-ds
PersistentVector<Person> people = Persistents.vector(
new Person("Fredrik", 39),
new Person("Hedda", 3));
89

PersistentVector<Person> morePeople =
people.plus(new Person("Johannes", 5));
90

PersistentVector<Person> morePeople =
people.plus(new Person("Johannes", 5));
morePeople.stream()
.forEach(p -> System.out.println(p.getName()));
91

Why use X instead?
Java 8 ready for enterprise dev?
JBoss AS, WebSphere – Nope
WildFly, GlassFish, WebLogic, Jetty, Tomcat – OK?
Important things are still missing from Java 8
Clojure and Scala available on JDK 6+!
92

Questions?
fredriv
vraalsen@iterate.no

Java 8 DOs and DON'Ts - javaBin Oslo May 2015

Java 8 DOs and DON'Ts - javaBin Oslo May 2015

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