How to use Java 8 Functional Programming to Generate an Alphabetic Sequence
I’ve stumbled upon an interesting Stack Overflow question by user “mip”. The question was:
I’m looking for a way of generating an alphabetic sequence:
A, B, C, ..., Z, AA, AB, AC, ..., ZZ.
This can be quickly recognised as the headings of an Excel spreadsheet, which does precisely that:
So far, none of the answers employed any Java 8 functional programming, which I accepted as a challenge. We’re going to use jOOλ, because the Java 8 Stream API does not offer enough functionality for this task.
But first, let’s decompose the algorithm in a functional way. What we need are these components:
- A (reproducible) representation of the alphabet
- An upper bound, i.e. how many letters we want to produce. The requested sequence goes to
ZZ
, which means the upper bound would be 2 - A way to combine each letter of the alphabet with the previously generated combined letters in a cartesian product
Let’s look into some code:
1. Generating the alphabet
We could be writing the alphabet like this:
List<String> alphabet = Arrays.asList("A", "B", ..., "Z");
but that would be lame. Let’s generate it instead, using jOOλ:
List<String> alphabet = Seq .rangeClosed('A', 'Z') .map(Object::toString) .toList();
The above generates a “closed” range (Java-8-Stream-speak for a range with inclusive upper bound) of characters between A
and Z
, maps characters to strings and collects them into a list.
So far so good. Now:
2. Using an upper bound
The requested sequence of characters includes:
A .. Z, AA, AB, .. ZZ
But we could easily imagine to extend this requirement generally to produce the following, or even more.
A .. Z, AA, AB, .. ZZ, AAA, AAB, .. ZZZ
For this, we’ll use again rangeClosed()
:
// 1 = A .. Z, 2 = AA .. ZZ, 3 = AAA .. ZZZ Seq.rangeClosed(1, 2) .flatMap(length -> ...) .forEach(System.out::println);
The idea here is to produce a new stream for each individual length in the range [1 .. 2]
, and to flatten those streams into one single stream. flatMap()
is essentially the same as a nested loop in imperative programming.
3. Combine letters in a cartesian product
This is the trickiest part: We need to combine each letter with each letter length
times. For this, we’ll use the following stream:
Seq.rangeClosed(1, length - 1) .foldLeft(Seq.seq(alphabet), (s, i) -> s.crossJoin(Seq.seq(alphabet)) .map(t -> t.v1 + t.v2)) );
We’re using again rangeClosed()
to produce values in the range [1 .. length-1]
. foldLeft()
is the same as reduce()
, except that foldLeft()
is guaranteed to go from “left to right” in a stream, without requiring the folding function to be associative. Whew.
In other, more understandable words: foldLeft()
is nothing else but an imperative loop. The “seed” of the loop, i.e. the loop’s initial value, is a complete alphabet (Seq.seq(alphabet)
). Now, for every value in the range [1 .. length-1]
, we produce a cartesian product (crossJoin()
) between the letters “folded” so far and a new alphabet, and we concatenate each combination into a single new string (t.v1
and t.v2
).
That’s it!
Combining everything
The following simple program prints all the values from A .. Z, AA .. ZZ, AAA .. ZZZ
to the console:
import java.util.List; import org.jooq.lambda.Seq; public class Test { public static void main(String[] args) { int max = 3; List<String> alphabet = Seq .rangeClosed('A', 'Z') .map(Object::toString) .toList(); Seq.rangeClosed(1, max) .flatMap(length -> Seq.rangeClosed(1, length - 1) .foldLeft(Seq.seq(alphabet), (s, i) -> s.crossJoin(Seq.seq(alphabet)) .map(t -> t.v1 + t.v2))) .forEach(System.out::println); } }
Disclaimer
This is certainly not the most optimal algorithm for this particular case. One of the best implementations has been given by an unnamed user on Stack Overflow:
import static java.lang.Math.*; private static String getString(int n) { char[] buf = new char[(int) floor(log(25 * (n + 1)) / log(26))]; for (int i = buf.length - 1; i >= 0; i--) { n--; buf[i] = (char) ('A' + n % 26); n /= 26; } return new String(buf); }
Unnecessary to say that the latter runs much much faster than the previous functional algorithm.
Reference: | How to use Java 8 Functional Programming to Generate an Alphabetic Sequence from our JCG partner Lukas Eder at the JAVA, SQL, AND JOOQ blog. |
I found this interesting, and since I have not hard of Jool, I decided to try this code in a new Maven project. I have a compile error. I looked through the code and did not find this IntStream method wrapped by Seq.
Error:(29, 36) java: cannot find symbol
symbol: method rangeClosed(char,char)
location: interface org.jooq.lambda.Seq
I am using:
org.jooq
jool
0.9.7
Yes, the timing of the article is a bit unfortunate as we haven’t released this code yet. You’ll find it on GitHub: https://github.com/jOOQ/jOOL