Functional programming in Java 8

1. Functional programming in Java 8
07 juli 2016
Masudul Haque

2. Adapt : Learn functional programming

3. Java meets functional
programming (with
lambdas)

4. Java is not your grandma’s language anymore!

5. What is Lambda?

6. Before Lambda
List<Employee> employees = ...
double highestSalary = 0.0;
for (Employee e : employees) {
if (e.year == 2016) {
if (e.salary > highestSalary) {
highestSalary = e.salary;
}
}
}
• Client controls iteration
• Inherently serial: iterate from
beginning to end
• Not thread-safe

7. After Lambda
List<Employee> employees = ...
double highestSalary = employees.stream()
.filter(Employee e-> e.getYear() == 2016)
.map(e -> e.getSalary())
.max();
• More readable
• More abstract
• Less error-prone
• No reliance on mutable state
• Easier to make parallel

8. What is Lambda?
• Lambda expressions represent anonymous functions
– Like a method, has a typed argument list, a return type, a set of thrown exceptions, and
a body
– Not associated with a class
List<Employee> employees = ...
double highestSalary = employees.stream()
.filter(Employee e-> e.getYear() == 2016)
.map(e -> e.getSalary())
.max();
Predicate
Function

9. What is Lambda?
List<Employee> employees = ...
double highestSalary = employees.stream()
.filter(Employee e-> e.getYear() == 2016)
.map(e -> e.getSalary())
.max();
• Stream<T> filter(Predicate<? super T> predicate)
• Filter objects that match a Predicate object
• <R> Stream<R> map(Function<? super T, ? extends R> mapper)
• Map objects to another value as specified by a Function object
Predicate
Function

10. What is Functional programming?
• Functional languages view programs as an entity— called a function—
that accepts inputs and produces output
• Functions are connected together by their outputs to other functions’
inputs.
• Underlying approach: “Evaluate an expression. Then use the results for
something else.”

11. Example
Runnable r = new Runnable(){
public void run(){
System.out.println("Hello World");
}
};
Runnable r =()-> System.out.println("Hello World");

12. Stream

13. What is Stream?
• A Typed Interface.
public interface Stream<T> extends BaseStream<T,Stream<T>>
• Abstraction for specifying aggregate computations
– Not a data structure
– Can be infinite
• Simplifies the description of aggregate computations
– Exposes opportunitires for optimisation
– Fusing, laziness and parrallelism

14. What is Stream?
• What can I do with it?
Answer: efficiently process high volumes of data, but also small ones.
• What does efficiently mean?
Two things:
1. In parallel, to leverage multicore CPU
2. In pipeline, to avoid the computation of intermediate data structures

15. Stream Pipeline
A stream pipeline consists of three types of things
1. A source
2. Zero or more intermediate operations
3. A terminal operation
List<Employee> employees = ...
double highestSalary = employees.stream()
.filter(Employee e-> e.getYear() == 2016)
.map(e -> e.getSalary())
.max(); Intemediate
Operation
Source
Terminal
Operation
Intemediate
Operations

16. stream pipeline
Stream
source
Intermediate
operations
Terminal
operation
stream
stream
Examples:
IntStream.range(),
Arrays.stream()
Examples:
map(), filter(),
distinct(), sorted()
Examples:
sum(), collect(),
forEach(), reduce()

17. DoubleStream.
of(1.0, 4.0, 9.0)
map(Math::sqrt)
.peek(System.out::
println)
Stream
Source (with
elements 1.0,
4.0, and 9.0)
Intermediate
Operation 1
(maps to
element values
1.0, 2.0, and 3.0)
Intermediate
Operation 2
(prints 1.0, 2.0,
and 3.0)
.sum();
Terminal
Operation
(returns the
sum 6.0)
DoubleStream.of(1.0, 4.0, 9.0)
.map(Math::sqrt)
.peek(System.out::println)
.sum();

18. Stream.of (1, 2, 3, 4, 5)
.map(i -> i * i)
.peek(i -> System.out.printf("%d ",i))
.count();
prints: 1 4 9 16 25

19. Demo
long startTime = System.currentTimeMillis();
long numOfPrimes = LongStream.rangeClosed(2, 10_000_000)
.filter(PrimeNumbers::isPrime)
.count();
System.out.println(numOfPrimes);
long endTime = System.currentTimeMillis();
System.out.printf( " Duration : %f %n ",(endTime - startTime)/1000.0);

20. Parallel code
Serial code

21. long numOfPrimes =LongStream.rangeClosed(2, 10_000_000)
.filter(PrimeNumbers::isPrime)
.count();
System.out.println(numOfPrimes);
Prints 664579
8.501 seconds

22. Parallel code
Serial code
Let’s flip the switch by
calling parallel() function

23. long numOfPrimes =LongStream.rangeClosed(2, 10_000_000)
.parallel()
.filter(PrimeNumbers::isPrime)
.count();
System.out.println(numOfPrimes);
Prints 664579
2.041 seconds

24. Wow! That’s an awesome flip
switch!

25. Internally, parallel streams make
use of fork-join framework

26. Q & A

27. Contact
For any question
Email: M.Haque@cimsolutions.nl
Follow me in following site:
Stackoverflow
http://stackoverflow.com/users/1265660/masud
Github
https://github.com/masudcse05
Blog
https://masudulhaque.wordpress.com