dtonhofer

dtonhofer

Functional Programming in Java, Second Edition: Code for "Chapter 7, Ininite Collections" in two files

SimpleTimer.java

package chapter7;

import java.util.function.Supplier;

public abstract class SimpleTimer {

    public record Result<T>(long duration_ms, T result) {}

    public static <T> chapter7.SimpleTimer.Result<T> timeIt(Supplier<T> supplier) {
        long start = System.currentTimeMillis();
        T result = supplier.get();
        long stop = System.currentTimeMillis();
        return new Result<T>(stop - start, result);
    }
}

BeingLazy_InfiniteCollections.java

package chapter7;

import org.junit.jupiter.api.Test;

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import static java.util.stream.Collectors.toList;
import static org.junit.jupiter.api.Assertions.assertEquals;

public class BeingLazy_InfiniteCollections {

    // If CPU cycles are of no importance...
    // "a desparate attempt", p.133

    public static boolean isPrimeVeryGorilla(final int number) {
        return number > 1 &&
                IntStream.rangeClosed(2, (int) Math.sqrt(number))
                        .noneMatch(divisor -> number % divisor == 0);
    }

    // This doesn't work as it causes an infinite recursion
    // "a desparate attempt", p.134, but with different method & parameter names

    public static List<Integer> primesUpwardsFrom_notworking(final int number) {
        if (isPrimeVeryGorilla(number)) {
            List<Integer> morePrimes = primesUpwardsFrom_notworking(number + 1);
            morePrimes.add(0, number);
            return morePrimes;
        } else {
            return primesUpwardsFrom_notworking(number + 1);
        }
    }

    // "lazy/fpij/Primes.java", p.135, but with different method & parameter names

    private static int firstPrimeAfter(final int number) {
        if (isPrimeVeryGorilla(number + 1)) {
            return number + 1;
        } else {
            return firstPrimeAfter(number + 1);
        }
    }

    // "lazy/fpij/Primes.java", p.135, but with different method & parameter names

    private static List<Integer> primesUpwardsFrom(final int fromNumber, final int count) {
        return Stream.iterate(firstPrimeAfter(fromNumber - 1), BeingLazy_InfiniteCollections::firstPrimeAfter)
                .limit(count)
                .collect(toList());
    }

    // Testing primesUpwardsFrom()
    // https://en.wikipedia.org/wiki/List_of_prime_numbers
    // https://oeis.org/A000040

    @Test
    public void generatePrimes() {
        final var shall = List.of(101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173,
                179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281);
        final List<Integer> primesAfter100 = primesUpwardsFrom(100, shall.size());
        final String txt = primesAfter100.stream().map(it -> Integer.toString(it)).collect(Collectors.joining(",\n"));
        System.out.println(txt);
        assertEquals(shall, primesAfter100);
    }

    // Testing primesUpwardsFrom()
    // Similar to "lazy/fpij/Primes.java" on p.136

    @Test
    public void runBookPrimes() {
        List<Integer> l1 = primesUpwardsFrom(1, 10);
        List<Integer> l2 = primesUpwardsFrom(100, 5);
        assertEquals(List.of(2, 3, 5, 7, 11, 13, 17, 19, 23, 29), l1);
        assertEquals(List.of(101, 103, 107, 109, 113), l2);
        System.out.println("10 primes from 1: " + l1);
        System.out.println("5 primes from 100: " + l2);
    }


    // It's quite slow!

    @Test
    void runBookPrimesWithTimer() {
        final int limit = 1_000_000;
        // not an actual test, just printout
        final SimpleTimer.Result<List<Integer>> stRes =
                SimpleTimer.timeIt(
                        () -> primesUpwardsFrom(2, limit)
                );
        final List<Integer> result = stRes.result();
        final String range = (result.isEmpty()) ? "" : "in range [" + result.get(0) + "," + result.get(result.size() - 1) + "] ";
        System.out.println("Using book code: finding " + result.size() + " primes " + range + "took " + stRes.duration_ms() + " ms");
    }

}

Where Next?

Popular Pragmatic Bookshelf topics Top

jimschubert
In Chapter 3, the source for index introduces Config on page 31, followed by more code including tests; Config isn’t introduced until pag...
New
mikecargal
Title: Hands-On Rust (Chap 8 (Adding a Heads Up Display) It looks like ​.with_simple_console_no_bg​(SCREEN_WIDTH*2, SCREEN_HEIGHT*2...
New
lirux
Hi Jamis, I think there’s an issue with a test on chapter 6. I own the ebook, version P1.0 Feb. 2019. This test doesn’t pass for me: ...
New
herminiotorres
Hi! I know not the intentions behind this narrative when called, on page XI: mount() |&gt; handle_event() |&gt; render() but the correc...
New
gilesdotcodes
In case this helps anyone, I’ve had issues setting up the rails source code. Here were the solutions: In Gemfile, change gem 'rails' t...
New
New
digitalbias
Title: Build a Weather Station with Elixir and Nerves: Problem connecting to Postgres with Grafana on (page 64) If you follow the defau...
New
Charles
In general, the book isn’t yet updated for Phoenix version 1.6. On page 18 of the book, the authors indicate that an auto generated of ro...
New
andreheijstek
After running /bin/setup, the first error was: The foreman' command exists in these Ruby versions: That was easy to fix: gem install fore...
New
SlowburnAZ
Getting an error when installing the dependencies at the start of this chapter: could not compile dependency :exla, "mix compile" failed...
New

Other popular topics Top

Exadra37
Please tell us what is your preferred monitor setup for programming(not gaming) and why you have chosen it. Does your monitor have eye p...
New
DevotionGeo
I know that -t flag is used along with -i flag for getting an interactive shell. But I cannot digest what the man page for docker run com...
New
Rainer
Not sure if following fits exactly this thread, or if we should have a hobby thread… For many years I’m designing and building model air...
New
AstonJ
In case anyone else is wondering why Ruby 3 doesn’t show when you do asdf list-all ruby :man_facepalming: do this first: asdf plugin-upd...
New
Exadra37
Oh just spent so much time on this to discover now that RancherOS is in end of life but Rancher is refusing to mark the Github repo as su...
New
PragmaticBookshelf
Build highly interactive applications without ever leaving Elixir, the way the experts do. Let LiveView take care of performance, scalabi...
New
foxtrottwist
A few weeks ago I started using Warp a terminal written in rust. Though in it’s current state of development there are a few caveats (tab...
New
mafinar
This is going to be a long an frequently posted thread. While talking to a friend of mine who has taken data structure and algorithm cou...
New
PragmaticBookshelf
Build efficient applications that exploit the unique benefits of a pure functional language, learning from an engineer who uses Haskell t...
New
hilfordjames
There appears to have been an update that has changed the terminology for what has previously been known as the Taskbar Overflow - this h...
New

Latest in Functional Programming in Java, Second Edition

Functional Programming in Java, Second Edition Portal

Sub Categories: