This document discusses gradually adopting functional programming techniques in Java. It provides examples of using streams, lambdas, and functional data structures to improve key performance indicators like code quality, developer productivity, and performance. Frameworks like Vavr aim to bring functional programming concepts like immutability and pure functions to Java development for benefits like reduced verbosity and improved testability. Bytecode examples demonstrate how functional Java code can improve performance at the bytecode level compared to imperative styles. Overall the document argues for embracing functional programming in Java to gain advantages in areas like garbage collection and ease of testing.

1. KPI driven JAVA(functional) development
Gradual journey from imperative to functional development in JAVA
Anirban Bhattacharjee – Architect @ Walmart Labs

2. About my
functional
exposure
Writing java for
living … since
JDK 4 (mostly
OO way in the
last decade)
Came across
clojure/scala
around
2010…and used
in production
Learning and
using F# since
last year
@work
Occasional
tinkering with
OCaml and
Haskell

3. Problem Statement
• We have tons of code written in JAVA
• Where will I find developer?
• One company one tech stack…
• Performance, Monitoring, Support?
• Interop, library support.
CAN I STILL WRITE FUNCTIONAL CODE, IMPROVE KPI, MAKE MY LEADERSHIP HAPPY

4. Today’s JAVA
OOO -> FP
Functional programming is mainstream
since JDK 8 (GA – 2014)
We could use the power of FP ever since
then
Powerful Lambdas/Streams
Collection Enhancements
Function, Supplier, Consumer

5. Growing eco-system
• Many promising and production ready libraries
• Clean APIs
• Reduction of verbosity, testable and concise code
• Great dev tools
• Rapid learning from other functional programming languages

6. KPI improvements (target areas)
using FP in java
SONAR
DEVELOPER
PRODUCTIVITY
Time to market
GC, PERF
IMPROVEMENT

7. Journey to FP
• Birds eye view of functional java
• Additional powerful frameworks
 vavr
 functional java
 streamex
 Atlassian fugue
 misc…

8. Functional Java : Stream basics
Loop
List<String> countries = new
ArrayList<>();
for (String s : allCountries) {
if (s.startsWith(“I")) {
countries.add(s);
}
}
Stream
List<String> countries
= allCountries.stream()
.filter(a -> a.startsWith(“I"))
.collect(toList())

9. Functional Java : Stream illustrated
Origin EndOp 1 Op 2 Op n
Data Pipeline Start Data Pipeline End
Intermediate
Operations

10. Functional Java : Map, reduce, collect
List<String> list = Stream.of("India", "Iceland",“Peru“,"Italy")
.filter(s -> s.startsWith("I"))
.map(String::toUpperCase)
.sorted()
.collect(toList())
India Iceland ItalyPeru
India Iceland Italy
INDIA ICELAND ITALY
ICELAND INDIA ITALY
ICELAND INDIA ITALY
1
2
3
4
5
1
2
3
4
5

11. Intermediate Operation : Filter
Ind It UKJP
Ind It
1
2
Stream<String> startsWithI = Stream.of("Ind", "It", "JP", "UK")
.filter(s -> s.startsWith("I"));
1
2

12. Functional Java : Intermediate Operation
Ind It UKJP
Ind It
1
2
Stream<String> startsWithI = Stream.of("Ind", "It", "JP", "UK")
.limit(3);
1
2
JP

13. Functional Java : Intermediate Operation
Ind It UKJP
Ind It
1
2
Stream<String> startsWithI = Stream.of("Ind", "It", "JP", "UK", "UK")
.distinct();
1
2
JP
UK
UK

14. Functional Java : Intermediate Operation
Ind It UKUS
Ind It
1
2
Stream<String> startsWithI = Stream.of("Ind", "It", “US", "UK")
.sorted();
1
2
UK US

15. Functional Java : Intermediate Operation
Ind It UKUS
Ind It
1
2
Stream<String> startsWithI = Stream.of("Ind", "It", “US", "UK")
.sorted(Comparator.comparing(String::length));
1
2
UK US

16. Functional Java : Terminal Operation
Ind It UKJP
Ind
It
1
2
Stream<String> startsWithI = Stream.of("Ind", "It", "JP", "UK")
.forEach(System.out.println());
1
2
JP
UK

17. Functional Java : Downstream Collectors
Ind It UKJP
Ind It
1
2
Stream<String> startsWithI = Stream.of("Ind", "It", "JP", "UK”, "UK” )
.collect(Collectors.toSet());
1
2
JP UK
UK

18. Functional Java : Custom Downstream collector
return Collector.of(
() -> new ArrayList<T>(), // supplier
(a, e) -> { a.add(e); }, // accumulator
(a, b) -> { a.addAll(b); return a; }, // combiner
a -> { // finisher
a.sort(comp);
DA da = downstream.supplier().get();
for (T e : a)
downstream.accumulator().accept(da, e);
return downstream.finisher().apply(da);
});
Stream<Locale> locales =
Stream.of(Locale.getAvailableLocales());
var result = locales.collect(
groupingBy(Locale::getCountry,
mapping(Locale::getDisplayName,
sorting(String::compareTo,
joining(",")))));

19. Functional Java : Collection enhancemetns
map.forEach((k, v) -> System.out.println(k + v));for (Map.Entry<String,String> entry : map.entrySet())
System.out.println(entry.getKey() + entry.getValue());
for (Map.Entry<String,String> entry : map.entrySet())
entry.setValue(entry.getValue().toLowerCase());
map.replaceAll((k, v) -> v.toLowerCase());
map.computeIfAbsent(str, x -> HashsSet::new).add(i);Set<Integer> set = multimap.get(str);
if (set == null) {
set = new HashSet<>();
multimap.put(str, set);
}
set.add(i);

20. Functional Java : More Collection Examples
Contents

21. Functional Java : Consumer, Supplier, Functions
Contents

22. Functional Java : Byte Code
• Is Functional java only syntactic sugar?
• Byte code improvements
• Example of Byte code of BEFORE and AFTER

23. Functional Java : Byte Code
• Is Functional java only syntactic sugar?
• Byte code improvements
• Example of Byte code

24. Vavr

25. Vavr : basics
• New functional data-structures (persistent) with superior
performance
• Pure Functions, monads, superior exception management
• Less verbosity
• Great Functional semantics

26. Vavr : Examples
• LIST, TREE, SET and the cool APIs
•

27. Vavr : Examples
• CHECKED FUNCTION, CONSUMER, PREDICATE
•

28. Vavr : Examples
• CHECKED FUNCTION, CONSUMER, PREDICATE
•

29. Other frameworks: Streamex, Fugue etc..
• CHECKED FUNCTION, CONSUMER, PREDICATE
•

30. Million Dollar Question: KPI improvement
• SONAR (detailed examples)

31. Million Dollar Question: KPI improvement
• Developer Productivity -REPL
• Developer Productivity - IDE support

32. Million Dollar Question: KPI improvement
• All about GC

33. Million Dollar Question: KPI improvement
• Ease of testing and performance