The document discusses Java 8 features such as monads, generics, and streams, showcasing examples of typed containers, optional handling, and stream operations. It provides practical use cases for encoding passwords and computing pi using random numbers and parallel streams. The content also includes exercises on writing methods employing monad patterns and encourages further study of related design patterns and concepts.

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