Functional Java 8 - Introduction

Łukasz 
Biały
Functional 
Java 8 
 
Introduction
Scala / Java Developer
lbialy@virtuslab.com

What is functional
programming?

What is functional
programming?
• usage of pure functions

What is functional
programming?
• expressions instead of statements

What is functional
programming?
• widespread immutability

What is functional
programming?
• referential transparency

What is functional
programming?
• lazy evaluation

What is functional
programming?
• lazy evaluation
... and functional idioms!

What's the gain?
• More predictable execution (same arguments
always yield the same results)

What's the gain?
• Easier to reason about

What's the gain?
• Easier to enforce strict type correctness

What's the gain?
• more precise checks in compile time

What's the gain?
• more precise checks in compile time
• less bugs in runtime

How does this relate to Java at all?

How does this relate to Java at all?
java.lang.NullPointerException
at org.springsource.loaded.agent.SpringLoadedPreProcessor.tryToEnsureSystemClassesInitialized(SpringLoadedPreProcessor.java:362)
at org.springsource.loaded.agent.SpringLoadedPreProcessor.preProcess(SpringLoadedPreProcessor.java:128)
at org.springsource.loaded.agent.ClassPreProcessorAgentAdapter.transform(ClassPreProcessorAgentAdapter.java:102)
at sun.instrument.TransformerManager.transform(TransformerManager.java:188)
at sun.instrument.InstrumentationImpl.transform(InstrumentationImpl.java:424)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:2818)
at org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:1159)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1647)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:2818)
at org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:1159)
at org.springframework.util.ClassUtils.forName(ClassUtils.java:265)
at org.springframework.beans.factory.support.AbstractBeanDefinition.resolveBeanClass(AbstractBeanDefinition.java:419)
at org.springframework.beans.factory.support.AbstractBeanFactory.doResolveBeanClass(AbstractBeanFactory.java:1299)
at org.springframework.beans.factory.support.AbstractBeanFactory.access$000(AbstractBeanFactory.java:109)
at org.springframework.beans.factory.support.AbstractBeanFactory$4.run(AbstractBeanFactory.java:1265)
at org.springframework.beans.factory.support.AbstractBeanFactory$4.run(AbstractBeanFactory.java:1263)
at java.security.AccessController.doPrivileged(Native Method)
at org.springframework.beans.factory.support.AbstractBeanFactory.resolveBeanClass(AbstractBeanFactory.java:1263)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.predictBeanType(AbstractAutowireCapableBeanFactory.java:575)
at org.springframework.beans.factory.support.AbstractBeanFactory.isFactoryBean(AbstractBeanFactory.java:1347)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doGetBeanNamesForType(DefaultListableBeanFactory.java:358)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeanNamesForType(DefaultListableBeanFactory.java:327)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeansOfType(DefaultListableBeanFactory.java:437)
at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:626)
at com.xxx.core.web.WebApplicationContext.invokeBeanFactoryPostProcessors(WebApplicationContext.java)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:461)
at com.xxx.core.web.WebApplicationContext.refresh(WebApplicationContext.java)
...

Most common errors in Java
• Takipi monitoring service analysed error logs of over 1k
enterprise apps

enterprise apps
• 29,965,285 exceptions over 30 days

enterprise apps
• Just 10 exception types generated 97,3% of all errors

enterprise apps
• Two most common: NullPointerException &
NumberFormatException

enterprise apps
• Two most common: NullPointerException &
NumberFormatException
http://blog.takipi.com/we-crunched-1-billion-java-logged-errors-heres-what-causes-97-of-them/

http://blog.takipi.com/the-top-10-exceptions-types-in-production-java-applications-based-on-1b-events/

How can FP help?
• Functional style makes implicit rules governing code
explicit, often as a part of type signatures

How can FP help?
• Functional style avoids constructs that are not statically
analysable like throwing exceptions, therefore allowing
compiler to do it's job better

How can FP help?
• In concurrent and parallel programming lack of mutable
state removes whole classes of programming errors for free

How can FP help?
• In concurrent and parallel programming lack of mutable
state removes whole classes of programming errors for free
• Yields an elegant and concise code :)

1 package com.example;
2
3 class CharacterOps {
4
5 static Character firstCharacter(String string) {
6 return (string.length() > 0) ? string.charAt(0) : null;
7 }
8
9 static Integer getAlphabetPosition(Character character) {
10 int code = (int) character;
11
12 if (isLowerCase(code))
13 return code - 96;
14 else if (isUpperCase(code))
16 else
17 return null; // or throw exception?
18 }
19
20 private static boolean isLowerCase(int code) {
21 return code >= 97 && code <= 122;
22 }
23
24 private static boolean isUpperCase(int code) {
25 return code >= 65 && code <= 90;
26 }
27
28 }
Optionality 
of values

2
4
7 }
8
11
16 else
18 }
19
21 return code >= 97 && code <= 122;
22 }
23
26 }
27
28 }
Optionality 
of values
• optionality of value is not explicitly
known from method signature

2
4
7 }
8
11
16 else
18 }
19
21 return code >= 97 && code <= 122;
22 }
23
26 }
27
28 }
Optionality 
of values
• not feeling especially bright today?
have a NullPointerException in
runtime

2
4
7 }
8
11
16 else
18 }
19
21 return code >= 97 && code <= 122;
22 }
23
26 }
27
28 }
Optionality 
of values
runtime
• null is a subtype of every reference
type, so it will be propagated as  
a valid object value

2
4
7 }
8
11
16 else
18 }
19
21 return code >= 97 && code <= 122;
22 }
23
26 }
27
28 }
Optionality 
of values
runtime
• null is a subtype of every reference
type, so it will be propagated as  
a valid object value
• tracking null's origination point is so
much fun!

What we want: 
composability
2
3 import org.junit.Test;
4
5 import static com.example.CharacterOps.*;
6
7 public class CharOpsTest {
8
9 @Test
10 public void testComposability() {
11
12 getAlphabetPosition(firstCharacter("cat"));
13
14 getAlphabetPosition(firstCharacter(""));
15
16 }
17
18 }

What we want: 
composability
2
4
6
8
9 @Test
11
13
15
16 }
17
18 }
• feeding function a "cat" is
ﬁne, we get 3 as result

What we want: 
composability
2
4
6
8
9 @Test
11
13
15
16 }
17
18 }
at com.example.CharacterOps.getAlphabetPosition(CharacterOps.java:10)
at com.example.CharOpsTest.testComposability(CharOpsTest.java:14)

What we want: 
composability
2
4
6
8
9 @Test
11
13
15
16 }
17
18 }
• doesn't work so well for  
an empty string though
at com.example.CharacterOps.getAlphabetPosition(CharacterOps.java:10)
at com.example.CharOpsTest.testComposability(CharOpsTest.java:14)

2
4
6
7 public class ActualCharOpsTest {
8
9 @Test
10 public void testNullChecks() {
11
12 Character firstChar = firstCharacter("cat");
13
14 if (null == firstChar) {
15 // oops. what now? exception? propagate null?
16 } else {
17 // will throw NPE on digit :(
18 Integer position = getAlphabetPosition(firstChar);
19 // ...
20 }
21
22 }
23
24 }
What we get: 
explicit null checks

2
4
6
8
9 @Test
11
13
16 } else {
19 // ...
20 }
21
22 }
23
24 }
• you have to remember to
check for null explicitly or
face NPE hunting
What we get: 

2
4
6
8
9 @Test
11
13
16 } else {
19 // ...
20 }
21
22 }
23
24 }
face NPE hunting
• methods don't compose
anymore
What we get: 

2
4
6
8
9 @Test
11
13
16 } else {
19 // ...
20 }
21
22 }
23
24 }
face NPE hunting
• methods don't compose
anymore
• ugly :(
What we get: 

2
3 import java.util.Optional;
4
5 class SafeCharOps {
6
7 static Optional<Character> firstCharacter(
8 String string
9 ) {
10 return (string.length() > 0) ?
11 Optional.of(string.charAt(0)) :
12 Optional.empty();
13 }
14
15 static Optional<Integer> getAlphabetPosition(
16 Character character
17 ) {
19
20 return isLowerCase(code) ? Optional.of(code - 96) :
21 isUpperCase(code) ? Optional.of(code - 64) :
23 }
24
26 return code >= 97 && code <= 122;
27 }
28
31 }
32
33 }
Introducing  
Optional<T>

2
4
6
8 String string
9 ) {
13 }
14
17 ) {
19
23 }
24
26 return code >= 97 && code <= 122;
27 }
28
31 }
32
33 }
• optionality of return value
is explicit in type signature
Introducing  
Optional<T>

2
4
6
8 String string
9 ) {
13 }
14
17 ) {
19
23 }
24
26 return code >= 97 && code <= 122;
27 }
28
31 }
32
33 }
• optionality of return value
is explicit in type signature
• expressions used where
possible
Introducing  
Optional<T>

Composability
revisited
2
4
6
8
9 @Test
11
13
15
16 }
17
18 }

Composability
revisited
2
4
6
8
9 @Test
11
13
15
16 }
17
18 }
• doesn't compile :(

Composability
revisited
2
4
6
8
9 @Test
11
13
15
16 }
17
18 }
• getAlphabetPosition
accepts Character instances

Composability
revisited
2
4
6
8
9 @Test
11
13
15
16 }
17
18 }
• we have
Optional<Character> from 
ﬁrstCharacter call

Composability
revisited
2
4
6
8
9 @Test
11
13
15
16 }
17
18 }
• we have
Optional<Character> from 
ﬁrstCharacter call
• how to extract value in safe
manner?

2
4
6
7 import static com.example.SafeCharOps.*;
8
9 public class OptionalCharOpsTest {
10
11 @Test
12 public void testOptionalMethods() {
13
14 Optional<Character> firstChar =
15 firstCharacter("cat");
16
17 if (firstChar.isPresent()) {
18
19 Optional<Integer> alphabetPosition =
20 getAlphabetPosition(firstChar.get());
21
22 if (alphabetPosition.isPresent()) {
23 Integer position = alphabetPosition.get();
24
25 // ...
26 }
27
28 }
29
30 }
31
32 }
Imperative  
approach

2
4
6
8
10
11 @Test
13
16
18
21
24
25 // ...
26 }
27
28 }
29
30 }
31
32 }
• we've gained explicit
information about
optionality
Imperative  
approach

2
4
6
8
10
11 @Test
13
16
18
21
24
25 // ...
26 }
27
28 }
29
30 }
31
32 }
information about
optionality
• still looks like null checks :(
Imperative  
approach

2
4
6
8
10
11 @Test
13
16
18
21
24
25 // ...
26 }
27
28 }
29
30 }
31
32 }
information about
optionality
• still looks like null checks :(
• still breaks composability :(
Imperative  
approach

Functional  
approach
2
4
6
8
9 @Test
11
12 firstCharacter("cat")
13 .flatMap(SafeCharOps::getAlphabetPosition)
14 .ifPresent(value -> {
15 assert value == 3;
16 });
17
18 assert !firstCharacter("")
20 .isPresent();
21 }
22
23 }

• no null checks!
Functional  
approach
2
4
6
8
9 @Test
11
16 });
17
20 .isPresent();
21 }
22
23 }

• no null checks!
• clean functional
composition
Functional  
approach
2
4
6
8
9 @Test
11
16 });
17
20 .isPresent();
21 }
22
23 }

• no null checks!
composition
• type safety, IDE helps at
every step as type inference
is easy
Functional  
approach
2
4
6
8
9 @Test
11
16 });
17
20 .isPresent();
21 }
22
23 }

• no null checks!
composition
is easy
• easier to refactor
Functional  
approach
2
4
6
8
9 @Test
11
16 });
17
20 .isPresent();
21 }
22
23 }

• no null checks!
composition
is easy
• easier to refactor
• elegant & succinct
Functional  
approach
2
4
6
8
9 @Test
11
16 });
17
20 .isPresent();
21 }
22
23 }

2
3 import java.util.List;
4 import java.util.stream.Collectors;
5
6 class StreamsAndErrors {
7
8 static List<Integer> toNumbers(List<String> strings) {
9 return strings.stream()
10 .map(Integer::parseInt)
11 .collect(Collectors.toList());
12 }
13
14 }
Streams API 
revisited: errors
2
4
6
7 import static java.lang.System.out;
8 import static java.util.Arrays.asList;
9
10 public class StreamsTest {
11
12 @Test
13 public void testNumberConversion() {
14 List<String> strings = asList("1", "2", "3");
15 out.println(StreamsAndErrors.toNumbers(strings));
16 // prints '[1, 2, 3]'
17
18 List<String> itsaTrap = asList("1", "a", "3");
19 out.println(StreamsAndErrors.toNumbers(itsaTrap));
20 // NumberFormatException: For input string: "a"
21 }
22
23 }

2
5
7
12 }
13
14 }
• generic string to integer
conversion on collection level
Streams API 
revisited: errors
2
4
6
9
11
12 @Test
16 // prints '[1, 2, 3]'
17
21 }
22
23 }

2
5
7
12 }
13
14 }
• type signature tells nothing
about possible of failure
Streams API 
revisited: errors
2
4
6
9
11
12 @Test
16 // prints '[1, 2, 3]'
17
21 }
22
23 }

2
5
7
12 }
13
14 }
• type signature tells nothing
about possible of failure
• programmer knows (let's
hope so) about possibility of
failure and has to deal with it
explicitly
Streams API 
revisited: errors
2
4
6
9
11
12 @Test
16 // prints '[1, 2, 3]'
17
21 }
22
23 }

2
4 import java.util.Objects;
7
9
10 static List<Integer> filterOut(List<String> strings) {
12 .map(s -> {
13 try {
14 return Integer.parseInt(s);
15 } catch (NumberFormatException nfe) {
16 return null;
17 }
18 })
19 .filter(Objects::nonNull)
21 }
22
23 static Optional<List<Integer>> failOnFirst(
24 List<String> strings
25 ) {
26 try {
27 return Optional.of(
28 strings.stream()
30 .collect(Collectors.toList())
31 );
33 return Optional.empty();
34 }
35 }
36
37 }
Handling errors  
with try

2
7
9
12 .map(s -> {
13 try {
16 return null;
17 }
18 })
21 }
22
25 ) {
26 try {
28 strings.stream()
31 );
34 }
35 }
36
37 }
• ughh... null again :(
Handling errors  
with try

2
7
9
12 .map(s -> {
13 try {
16 return null;
17 }
18 })
21 }
22
25 ) {
26 try {
28 strings.stream()
31 );
34 }
35 }
36
37 }
• we have to ﬁlter nulls out lest
we create a disaster waiting to
happen
Handling errors  
with try

2
7
9
12 .map(s -> {
13 try {
16 return null;
17 }
18 })
21 }
22
25 ) {
26 try {
28 strings.stream()
31 );
34 }
35 }
36
37 }
• we have to ﬁlter nulls out lest
we create a disaster waiting to
happen
• using Optional tells caller that
this method might yield value,
but doesn't say anything about
errors that may happen inside
Handling errors  
with try

Handling errors  
with Try
2
3 import javaslang.control.Try;
4
7
9
10 static List<Try<Integer>> withErrs(List<String> strings) {
12 .map(s -> Try.of(() -> Integer.parseInt(s)))
14 }
15
16 static Try<List<Integer>> failOnFirst(
18 ) {
19 return Try.of(() -> strings.stream()
21 .collect(Collectors.toList()));
22 }
23
24 }

• type signature explicitly
informs us about the
possibility of error
Handling errors  
with Try
2
4
7
9
14 }
15
18 ) {
22 }
23
24 }

• type signature explicitly
informs us about the
possibility of error
• exceptions are caught
inside stream or in lambda
wrapping whole stream
processing
Handling errors  
with Try
2
4
7
9
14 }
15
18 ) {
22 }
23
24 }

2
3 import java.text.SimpleDateFormat;
4 import java.util.Date;
7
8 class StreamsAndCheckedExceptions {
9
10 private static SimpleDateFormat sdf =
11 new SimpleDateFormat("ddMMyyyy");
12
13 static List<Date> parseAll(List<String> strings) {
15 .map(sdf::parse) // won't compile!
17 }
18
19 }
Checked exceptions  
& Java 8 Streams: 
Rough edges

2
7
9
12
17 }
18
19 }
• ParseException is checked,
which breaks lambda
expression
Rough edges

2
7
9
12
17 }
18
19 }
• ParseException is checked,
which breaks lambda
expression
• solution using try-catch &
rethrow-as-unchecked block
is ugly and defeats the
purpose of lambdas
Rough edges

Try to the rescue
2
4
9
11
14
15 static List<Try<Date>> parseAll(List<String> strings) {
17 .map(s -> Try.of(() -> sdf.parse(s)))
19 }
20
21 }
2
4
6
9
11
12 @Test
14 List<String> strings = asList("10042017", "23");
15 out.println(
16 StreamsAndCheckedExceptions.parseAll(strings)
17 );
18
19 // [
20 // Success(Mon Apr 10 00:00:00 CEST 2017),
21 // Failure(ParseException: Unparseable date: "23")
22 // ]
23 }
24
25 }

• problem of checked
exceptions solved 
Try to the rescue
2
4
9
11
14
19 }
20
21 }
2
4
6
9
11
12 @Test
15 out.println(
17 );
18
19 // [
22 // ]
23 }
24
25 }

• problem of checked
exceptions solved 
• we also gained explicit
information of possibility of
failure
Try to the rescue
2
4
9
11
14
19 }
20
21 }
2
4
6
9
11
12 @Test
15 out.println(
17 );
18
19 // [
22 // ]
23 }
24
25 }

Case for immutable data
• Mutable state makes it signiﬁcantly harder to reason about a system due
to multiple code branches that can lead to mutation of state

• Shared mutable state in concurrent environment requires
synchronisation which is difﬁcult and leads to subtle and  
hard to replicate bugs

• Immutable objects and persistent data structures have none of those
problems

problems
• it's trivial to protect invariants once instance is immutable

problems
• no need for synchronisation ever

problems
• no need for synchronisation ever
... so how to do this in Java?

Immutable objects 
in plain Java
2
3 import java.util.Set;
4
5 public class Office {
6
7 private final Set<Room> rooms;
8 private final Set<Employee> employees;
9
10 public Office(Set<Room> rooms, Set<Employee> employees) {
11 this.rooms = rooms;
12 this.employees = employees;
13 }
14
15 public Set<Room> getRooms() {
16 return rooms;
17 }
18
19 public Set<Employee> getEmployees() {
20 return employees;
21 }
22
23 }

in plain Java
• lots of getter noise1 package com.example;
2
4
6
9
13 }
14
16 return rooms;
17 }
18
21 }
22
23 }

in plain Java
• lots of getter noise
• if we want a builder, we have
to create one and make
constructor private
2
4
6
9
13 }
14
16 return rooms;
17 }
18
21 }
22
23 }

in plain Java
constructor private
• optional ﬁeld is painful as IDE
will cry that ﬁelds should
never have type Optional<T>
2
4
6
9
13 }
14
16 return rooms;
17 }
18
21 }
22
23 }

in plain Java
constructor private
• optional field is painful as IDE
will cry that fields should
never have type Optional<T>
• how can we get a copy of
instance with only one field
modified?
2
4
6
9
13 }
14
16 return rooms;
17 }
18
21 }
22
23 }

Enter Immutables
2
3 import javaslang.collection.Set;
4 import org.immutables.value.Value;
5
6 @Value.Immutable
7 @Value.Style(typeImmutable = "*")
8 abstract class AbstractOffice {
9 abstract Set<Room> rooms();
10
11 abstract Set<Employee> employees();
12
13 abstract boolean open();
14 }

• that's all!
Enter Immutables
2
5
6 @Value.Immutable
10
12
14 }

• that's all!
• highly customisable code
generation
Enter Immutables
2
5
6 @Value.Immutable
10
12
14 }

• that's all!
generation
• handles Optional<T> values
correctly
Enter Immutables
2
5
6 @Value.Immutable
10
12
14 }

• that's all!
generation
correctly
• handles default values
Enter Immutables
2
5
6 @Value.Immutable
10
12
14 }

• that's all!
generation
correctly
• Jackson / Gson integration out-
of-the-box
Enter Immutables
2
5
6 @Value.Immutable
10
12
14 }

• that's all!
generation
correctly
• Jackson / Gson integration out-
of-the-box
• provides with* methods
allowing painless modiﬁcation
of immutable instances
Enter Immutables
2
5
6 @Value.Immutable
10
12
14 }

Immutables
in action
2
3 import javaslang.collection.HashSet;
5
6 public class OfficeTest {
7
8 @Test
9 public void muchSafeSuchImmutableVeryWow() {
10
11 HashSet<Employee> employees = HashSet.of(
12 Employee.of("Boss")
13 );
14
15 HashSet<Room> rooms = HashSet.of(
16 Room.of("Boss' office")
17 );
18
19 Office office = Office.builder()
20 .employees(employees)
21 .rooms(rooms)
22 .open(true)
23 .build();
24
25 System.out.println(office);
26 // Office{
27 // rooms=HashSet(Room{name=Boss' office}),
28 // employees=HashSet(Employee{name=Boss}),
29 // open=true}
30
31 Office officeWithoutEmployees = office
32 .withEmployees(HashSet.empty())
33 .withOpen(false);
34
35 System.out.println(officeWithoutEmployees);
36 // Office{
38 // employees=HashSet(),
39 // open=false}
40 }
41 }

• named factory method can be
generated (of by default)
Immutables
in action
2
5
7
8 @Test
10
13 );
14
17 );
18
21 .rooms(rooms)
22 .open(true)
23 .build();
24
26 // Office{
29 // open=true}
30
34
36 // Office{
39 // open=false}
40 }
41 }

• builder class is generated by
default
Immutables
in action
2
5
7
8 @Test
10
13 );
14
17 );
18
21 .rooms(rooms)
22 .open(true)
23 .build();
24
26 // Office{
29 // open=true}
30
34
36 // Office{
39 // open=false}
40 }
41 }

default
• toString, hashCode and equals
are generated using best practice
methods
Immutables
in action
2
5
7
8 @Test
10
13 );
14
17 );
18
21 .rooms(rooms)
22 .open(true)
23 .build();
24
26 // Office{
29 // open=true}
30
34
36 // Office{
39 // open=false}
40 }
41 }

default
methods
• with(ﬁeld) methods make
obtaining modiﬁed instances a
breeze!
Immutables
in action
2
5
7
8 @Test
10
13 );
14
17 );
18
21 .rooms(rooms)
22 .open(true)
23 .build();
24
26 // Office{
29 // open=true}
30
34
36 // Office{
39 // open=false}
40 }
41 }

default
methods
• with(ﬁeld) methods make
obtaining modiﬁed instances a
breeze!
• nulls are not allowed by default,
so fail early rule is enforced
Immutables
in action
2
5
7
8 @Test
10
13 );
14
17 );
18
21 .rooms(rooms)
22 .open(true)
23 .build();
24
26 // Office{
29 // open=true}
30
34
36 // Office{
39 // open=false}
40 }
41 }

About that  
javaslang.collection.HashSet

About that  
• Javaslang provides immutable, persistent, purely functional
collections that match Java Collections

About that  
• Immutable, so every operation returns a new instance

About that  
• Persistent, so creation of new instance means that data is
shared between instances of collection and only the reference
to modiﬁed element is replaced

About that  
• Persistent, so creation of new instance means that data is
shared between instances of collection and only the reference
to modiﬁed element is replaced
• Functional, so all operations are referentially transparent 
(at least as long as values stored in collection are immutable!)

Javaslang  
Collections APIs

• classic, imperative
approach with
StringBuilder: for loop
and append calls
Javaslang  
Collections APIs

1 String join(String... words) {
2 StringBuilder builder = new StringBuilder();
3 for(String s : words) {
4 if (builder.length() > 0) {
5 builder.append(", ");
6 }
7 builder.append(s);
8 }
9 return builder.toString();
10 }
approach with
and append calls
Javaslang  
Collections APIs

6 }
8 }
10 }
approach with
and append calls
• declarative use of List
methods and fold
(reduce to accumulator
value)
Javaslang  
Collections APIs

6 }
8 }
10 }
2 return List.of(words)
3 .intersperse(", ")
4 .fold("", String::concat);
5 }
approach with
and append calls
methods and fold
value)
Javaslang  
Collections APIs

6 }
8 }
10 }
5 }
approach with
and append calls
methods and fold
value)
• a helper method!
Javaslang  
Collections APIs

6 }
8 }
10 }
5 }
1 List.of(words).mkString(", ");
approach with
and append calls
methods and fold
value)
• a helper method!
Javaslang  
Collections APIs

2
3 import javaslang.Tuple;
4 import javaslang.collection.HashMap;
6 import javaslang.collection.Map;
8
9 public class PhoneNumberData {
10
11 public static final Set<String> phoneNumbers =
12 HashSet.of(
13 "+48413422345",
14 "413572456",
15 "+48413456990",
16 "48225697246",
17 "+48224914634",
18 "48126434972",
19 "+48128275242"
20 );
21
22 public static final Map<String, String> areas =
23 HashMap.ofEntries(
24 Tuple.of("41", "Kielce"),
25 Tuple.of("22", "Warszawa"),
26 Tuple.of("12", "Kraków")
27 );
28
29 }
A more complex  
example...

2
8
10
12 HashSet.of(
13 "+48413422345",
14 "413572456",
15 "+48413456990",
16 "48225697246",
17 "+48224914634",
18 "48126434972",
19 "+48128275242"
20 );
21
27 );
28
29 }
• phoneNumbers: phone
numbers in different
formats
A more complex  
example...

2
8
10
12 HashSet.of(
13 "+48413422345",
14 "413572456",
15 "+48413456990",
16 "48225697246",
17 "+48224914634",
18 "48126434972",
19 "+48128275242"
20 );
21
27 );
28
29 }
formats
• areas: maps preﬁxes to area
names
A more complex  
example...

2
8
10
12 HashSet.of(
13 "+48413422345",
14 "413572456",
15 "+48413456990",
16 "48225697246",
17 "+48224914634",
18 "48126434972",
19 "+48128275242"
20 );
21
27 );
28
29 }
formats
• areas: maps preﬁxes to area
names
• task: group numbers by
area name
A more complex  
example...

Processing!1 package com.example;
2
4 import javaslang.Tuple2;
10
11 import static java.util.function.Function.identity;
12 import static com.example.PhoneNumberData.*;
13
14 public class GroupNumbersTest {
15
16 @Test
17 public void processNumbers() {
18 Map<String, Set<String>> map = phoneNumbers
19 .filter(num -> num.length() >= 9)
20 .map(number -> Tuple.of(number, number))
21 .map(tuple ->
22 tuple.map(
23 num -> num.replaceFirst("^+?48", ""),
24 identity()
25 )
26 )
27 .groupBy(tuple -> tuple._1().substring(0, 2))
28 .bimap(
29 key -> areas.get(key).getOrElse("Nieznany"),
30 value -> value.map(Tuple2::_2)
31 );
32
33 System.out.println(map);
34 }
35
36 }

Processing!
• ﬁlter out numbers that are too
short
2
10
13
15
16 @Test
21 .map(tuple ->
22 tuple.map(
24 identity()
25 )
26 )
28 .bimap(
31 );
32
34 }
35
36 }

Processing!
short
• we need to have original number
and still process it - Tuples!
2
10
13
15
16 @Test
21 .map(tuple ->
22 tuple.map(
24 identity()
25 )
26 )
28 .bimap(
31 );
32
34 }
35
36 }

Processing!
short
• strip international code from
processed tuple entry, don't do
anything to original
2
10
13
15
16 @Test
21 .map(tuple ->
22 tuple.map(
24 identity()
25 )
26 )
28 .bimap(
31 );
32
34 }
35
36 }

Processing!
short
• group tuples by area code
obtained from processed tuple
entry
2
10
13
15
16 @Test
21 .map(tuple ->
22 tuple.map(
24 identity()
25 )
26 )
28 .bimap(
31 );
32
34 }
35
36 }

Processing!
short
• group tuples by area code
obtained from processed tuple
entry
• map keys and values to either
area name and set of original
phone numbers
2
10
13
15
16 @Test
21 .map(tuple ->
22 tuple.map(
24 identity()
25 )
26 )
28 .bimap(
31 );
32
34 }
35
36 }

Results:
HashMap( 
(Kielce, HashSet(+48413456990, +48413422345, 413572456)),  
(Kraków, HashSet(+48128275242, 48126434972)),  
(Warszawa, HashSet(+48224914634, 48225697246)) 
)

Results:
HashMap( 
(Kielce, HashSet(+48413456990, +48413422345, 413572456)),  
(Kraków, HashSet(+48128275242, 48126434972)),  
)
... neat!

Results:
HashMap( 
(Kielce, HashSet(+48413456990, +48413422345, 413572456)),  
(Kraków, HashSet(+48128275242, 48126434972)),  
)
... neat! 
But what if that was an inﬁnite stream of phone numbers?

2
6 import rx.Observable;
7
8 public class StreamingPhoneNumberData {
9
10 public static final Observable<String> phoneNumbers =
11 Observable.from(new String[]{
12 "+48413422345",
13 "413572456",
14 "+48413456990",
15 "48225697246",
16 "+48224914634",
17 "48126434972",
18 "+48128275242"
19 });
20
26 );
27
28 }
Event streams  
with RxJava

2
6 import rx.Observable;
7
8 public class StreamingPhoneNumberData {
9
10 public static final Observable<String> phoneNumbers =
11 Observable.from(new String[]{
12 "+48413422345",
13 "413572456",
14 "+48413456990",
15 "48225697246",
16 "+48224914634",
17 "48126434972",
18 "+48128275242"
19 });
20
26 );
27
28 }
Event streams  
with RxJava
• Observable<T> is  
a 0 .. n collection of
events (potentially
inﬁnite!)

2
6 import rx.observables.GroupedObservable;
7
8 import static com.example.PhoneNumberData.areas;
9 import static com.example.StreamingData.phoneNumbers;
12
13 public class RxGroupPhoneNumbersTest {
14
15 @Test
17 phoneNumbers
20 .map(tuple ->
21 tuple.map(
23 identity()
24 )
25 )
27 .map(grouped ->
28 GroupedObservable.from(
29 areas.get(grouped.getKey())
30 .getOrElse("Unknown"),
31 grouped.map(Tuple2::_2)
32 )
33 )
34 .forEach(grouped -> grouped.forEach(number ->
35 out.println(
36 grouped.getKey() + ": " + number
37 )
38 )
39 );
40 }
41
42 }
Event streams  
with RxJava:  
reuse your lambdas!

2
7
12
14
15 @Test
17 phoneNumbers
20 .map(tuple ->
21 tuple.map(
23 identity()
24 )
25 )
27 .map(grouped ->
32 )
33 )
35 out.println(
37 )
38 )
39 );
40 }
41
42 }
Event streams  
with RxJava:  
reuse your lambdas!
• highlighted code is the same
as code used on Javaslang
collection!

2
7
12
14
15 @Test
17 phoneNumbers
20 .map(tuple ->
21 tuple.map(
23 identity()
24 )
25 )
27 .map(grouped ->
32 )
33 )
35 out.println(
37 )
38 )
39 );
40 }
41
42 }
Event streams  
with RxJava:  
reuse your lambdas!
• highlighted code is the same
as code used on Javaslang
collection!
• we are operating on
potentially inﬁnite stream, so
groupBy returns Observable
of GroupedObservables - a
stream of streams of values
grouped by key!

Use cases of 
Reactive Extensions

Use cases of 
Reactive Extensions
• Streams of events

Use cases of 
Reactive Extensions
• Parallel processing

Use cases of 
Reactive Extensions
• Asynchronous programming

Use cases of 
Reactive Extensions
• Bonus: functional error handling

Use cases of 
Reactive Extensions
• Bonus: functional error handling
• Bonus: reactive streams speciﬁcation

Functional Programming in
Java: not needed, right?

• Microservices need linear scalability on instance level,
which is hard to achieve using thread-per-request
model, asynchronous frameworks offer this  
out-of-the-box

• Microservices need linear scalability on instance level,
which is hard to achieve using thread-per-request
model, asynchronous frameworks offer this  
out-of-the-box
• Reactive, event-driven programming model has already
been included into latest Java version - reactive streams
interfaces made it to JDK9!

Also - Spring 5:
1 @GetMapping("/accounts/{id}/alerts")
2 public Flux<Alert> getAccountAlerts(@PathVariable Long id) {
3
4 return this.repository.getAccount(id)
5 .flatMap(account ->
6 this.webClient
7 .perform(get("/alerts/{key}", account.getKey()))
8 .extract(bodyStream(Alert.class))
9 );
10 }

And Vert.x:
1 server.requestStream()
2 .toObservable()
3 .subscribe(request ->
4 request.toObservable()
5 .lift(RxHelper.unmarshaller(MyPojo.class))
6 .map(Marshaller::toMongoDocument)
7 .flatMap(jsonObject -> mongo.insertObservable(MY_POJOS, jsonObject))
8 .subscribe(
9 () -> request.response()
10 .setStatusCode(200)
11 .end(),
12 err -> request.response()
13 .setStatusCode(500)
14 .end()
15 )
16 );

Functional Java 8 - Introduction

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