Splitting a string at every n-th character

You could do it like this:

String s = "1234567890";
System.out.println(java.util.Arrays.toString(s.split("(?<=\\G...)")));

which produces:

[123, 456, 789, 0]

The regex (?<=\G...) matches an empty string that has the last match (\G) followed by three characters (...) before it ((?<= ))

Java does not provide very full-featured splitting utilities, so the Guava libraries do:

Iterable<String> pieces = Splitter.fixedLength(3).split(string);

Check out the Javadoc for Splitter; it's very powerful.

import java.util.ArrayList;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        for (String part : getParts("foobarspam", 3)) {
            System.out.println(part);
        }
    }
    private static List<String> getParts(String string, int partitionSize) {
        List<String> parts = new ArrayList<String>();
        int len = string.length();
        for (int i=0; i<len; i+=partitionSize)
        {
            parts.add(string.substring(i, Math.min(len, i + partitionSize)));
        }
        return parts;
    }
}

As an addition to Bart Kiers answer I want to add that it is possible instead of using the three dots ... in the regex expression which are representing three characters you can write .{3} which has the same meaning.

Then the code would look like the following:

String bitstream = "00101010001001010100101010100101010101001010100001010101010010101";
System.out.println(java.util.Arrays.toString(bitstream.split("(?<=\\G.{3})")));

With this it would be easier to modify the string length and the creation of a function is now reasonable with a variable input string length. This could be done look like the following:

public static String[] splitAfterNChars(String input, int splitLen){
    return input.split(String.format("(?<=\\G.{%1$d})", splitLen));
}

An example in IdeOne: http://ideone.com/rNlTj5

Late Entry.

Following is a succinct implementation using Java8 streams, a one liner:

String foobarspam = "foobarspam";
AtomicInteger splitCounter = new AtomicInteger(0);
Collection<String> splittedStrings = foobarspam
                                    .chars()
                                    .mapToObj(_char -> String.valueOf((char)_char))
                                    .collect(Collectors.groupingBy(stringChar -> splitCounter.getAndIncrement() / 3
                                                                ,Collectors.joining()))
                                    .values();

Output:

[foo, bar, spa, m]

Splitting a string at every n-th character

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