Introduction to Monads in Java. Introduction to Optional and Streams as monads. Trying out a small monad implementation.

1. Java 8 : Monads

2. Generics: Have Typed Containers
List list = new ArrayList();
list.add("one");
list.add(1);
list.add(1L);
list.add(new Object());
Before After
List<String> list = new ArrayList<>();
list.add("one");
list.add("1");
list.add(“1212”);
Typed ContainerContainer

3. A Thing we cannot do with containers
Transform container of String to a container of Integers

4. Monad: {Type Constructor, Bind, Return}
Optional Stream
Type Constructor of of
Bind map map
Return get collect
Example:
Stream.of(1,2,3,4,5).map(i -> i + 2).collect(Collectors.toList())
Optional.of("Test").map(s -> s.concat(" ").concat(" One")).get()
https://en.wikipedia.org/wiki/Monad_(functional_programming)

5. Use Case of Optional: Utility method to get
encrypted password in base 64
public String encodePassword1(String password) throws Exception {
return Optional.ofNullable(password)
.map(String::getBytes)
.map(MessageDigest.getInstance("SHA1")::digest)
.map(Base64.getEncoder()::encodeToString)
.orElse(null);
//.orElseThrow(IllegalArgumentException::new);
}
public String encodePassword2(String password) throws Exception{
if (password == null)
return password;
return Base64.encode(MessageDigest.getInstance("SHA1").digest(password.getBytes()));
}

6. Exercise
• Write a method that accept String (which can be null) and return
number of characters in the String. If null throw
IllegalArgumentException.

7. Use Cases of Stream: Intro
There many Stream type constructors
• Directly from Stream class
Stream.of(T…t)
• From Collections
List<String> list = new ArrayList<>();
list.stream();
• Primitive Streams
• IntStream
• IntStream interator
IntStream.iterate(1, i -> i++).limit(100) [ for(int i = 0; i < 100; i++) ]
• Random IntStream
ThreadLocalRandom.current().ints()
• LongStream (similar to intstream)
• Buffered Reader
• new BufferedReader(new FileReader("input.txt")).lines();

8. Use Case of Optional: Find π with random
numbers
long sampleSize = 1_000_000_000L;
long count = ThreadLocalRandom.current().doubles(0,1)
.limit(sampleSize)
.map(d -> Math.pow(d, 2D))
.map(d -> d + Math.pow(ThreadLocalRandom.current().nextDouble(0D,1D), 2D))
.map(Math::sqrt)
.filter(d -> d < 1D)
.count();
System.out.printf("Original: %s Computed: %s %n", Math.PI, 4D * count / sampleSize );
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https://codepen.io/andrewarchi/pen/mRMRed
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9. Side Note: Using Parallel Stream
StopWatch stopWatch = new StopWatch();
stopWatch.start();
long sampleSize = 1_000_000_000L;
long count = ThreadLocalRandom.current().doubles(0,1)
.limit(sampleSize)
.parallel()
.map(d -> Math.pow(d, 2D))
.map(d -> d + Math.pow(ThreadLocalRandom.current().nextDouble(0D,1D), 2D))
.map(Math::sqrt)
.filter(d -> d < 1D)
.count();
stopWatch.stop();
System.out.printf("Original: %s Computed: %s , Time: %s %n",
Math.PI,
4D * count / sampleSize,
stopWatch);

10. Side Note: Use of Core
Original: 3.141592653589793 Computed: 3.141606068 , Time: 0:00:04.342
With Parallel Streams
Original: 3.141592653589793 Computed: 3.141551928 , Time: 0:00:38.072
With Serial Streams
Base Line Processes Usage

11. Exercise
• Read File with one billion numbers and find the Average
https://tinyurl.com/y8k8dfkf

12. Monad As A Design Pattern
Structural Pattern
• Operation Chaining
• Apply Functions Regardless of the result of any of them
https://github.com/iluwatar/java-design-patterns/tree/master/monad

13. Try Monad
Optional for NullPointerException, Try For RuntimeException
import java.util.function.Function;
public class Try<I> {
private I i;
private Try(I i) {
this.i = i;
}
// Type Constructor
public static <O> Try<O> of(O instance) {
return new Try<O>(instance);
}
// Bind
public <O> Try<O> attempt(Function<I, O> function) {
try {
O o = function.apply(i);
return new Try<O>(o);
} catch (Throwable t) {
return new Try<O>(null);
}
}
// Return
public I get() {
return i;
}
// Return
public I getOrElse(I defaultValue) {
return i == null ? defaultValue : i;
}
}

14. Exercise
• Write a monad which uses provided default value if the value is null when
chaining.
String lname = DeafultM.of(user)
.map(User::getName, “Roy”)
.map(s -> lastNameMap.get(s), “James”)
.get();
• Write a monad which uses provided default value if the value is not
provided criteria use when chaining.
String lname = DeafultM.of(user)
.map(User::getName, s -> s.length() < 1, “Roy”)
.map(s -> lastNameMap.get(s), s.contains(“R”), “James”)
.get();

15. Things to study more
• Monad functions: zip, map, flatmap, sequence
• Monad Design Pattern: https://github.com/iluwatar/java-design-
patterns/tree/master/monad
• Monoids: https://en.wikipedia.org/wiki/Monoid
• Use of Optional: https://www.programcreek.com/java-api-
examples/?api=java.util.Optional
• Functor and Monad Examples in Plain Java.
https://dzone.com/articles/functor-and-monad-examples-in-plain-
java