Home » JVM Languages » Scala » Memoization of Scala Streams

About Biju Kunjummen

Biju Kunjummen

Memoization of Scala Streams

I learnt the hard way that scala internally uses memoization with Streams.

This was my first attempt at a solution to Euler Problem 5

def from(n: Int): Stream[Int] = n #:: from(n + 1)

def isDivisibleByRange(n: Int, r: Range) = {
  r.forall(n % _ == 0)

val a = from(21)
val o = a.find(isDivisibleByRange(_, Range(2, 21)))
o match {
  case Some(i) => println(i)
  case None => println("Nothing found!")

I was a little mystified by why this code was throwing an OutOfMemoryError, realized thanks to Stackoverflow that since the answer to this problem is quite high 232792560, all the integers in this range will be memoized within the different nodes of the stream and hence the issue.

This is actually easy to see, let me first modify the stream generator function with a side effect:

def from(n: Int): Stream[Int] = {println(s"Gen $n"); n #:: from(n + 1)}
val s = from(1)

The second statement would not print anything.

Given this memoization behavior there are a few possible fixes, the simplest is to not keep a reference to the head of the stream anywhere and to use the find method of iterator instead:

from(1).iterator.find(isDivisibleByRange(_, Range(1, 21)))

On a related note, Java 8 streams are not memoized and a solution using Java 8 streams (admittedly can be improved massively) is the following:

public void testStreamOfInts() {
 Stream<Integer> intStream = Stream.generate(from(1));
 List<Integer> upto20 = IntStream.rangeClosed(1, 20).boxed().collect(Collectors.toList());
 Predicate<Integer> p = (i -> isDivisibleOverRange(i, upto20));
 Optional<Integer> o = intStream.filter(p).findFirst();
 o.ifPresent(i -> System.out.println("Found: " + i));

private Supplier<Integer> from(Integer i) {
 AtomicInteger counter = new AtomicInteger(0);
 return () ->  counter.incrementAndGet();

private boolean isDivisibleOverRange(Integer n, List<Integer> l) {
 return l.stream().allMatch(i -> n % i == 0);
Reference: Memoization of Scala Streams from our JCG partner Biju Kunjummen at the all and sundry blog.

Do you want to know how to develop your skillset to become a Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you our best selling eBooks for FREE!


1. JPA Mini Book

2. JVM Troubleshooting Guide

3. JUnit Tutorial for Unit Testing

4. Java Annotations Tutorial

5. Java Interview Questions

6. Spring Interview Questions

7. Android UI Design


and many more ....


Receive Java & Developer job alerts in your Area


Leave a Reply

1 Comment on "Memoization of Scala Streams"

Notify of
Sort by:   newest | oldest | most voted

you could also directly create an iterator:

def from(n: Int) = Iterator.iterate(n)(_ + 1)

The scaladoc is very explicit about Stream doing memoization by the way.