Adding A New Line Every Three Digits

Question

Okay, I am trying to add a new line every three letters. The goal is to have 20 rows of randomized capital letters. With 3 capital letters per row.

I randomized the numbers and then converted them to a string. Then outputted the string. I have 60 random capital letters in one straight String, but I can't figure out how to add new lines, since each letter is randomized I can't tell java what to search for like I did before. I have tried using an if statement and add an "/n" at the end, but it just got messy and difficult. And the operator commands I couldn't figure out how to apply every three letters either.

Here is the code I have so far under the "wrightRandomCodesToFile"

package mp05;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.Random;
import java.util.Scanner;

public class Main {
    private static final int PRODUCT_COUNT = 20;
    private static final int CHARACTERS_PER_CODE = 3;
    private static int charA;
    private static int preChars;
    private static String preLetters;

    public static void main(String[] args) {
        /* (1) Generate three character alphabetic prefix codes. */
        writeRandomCodesToFile("prefix.txt", 'A', 'Z',
                              CHARACTERS_PER_CODE, PRODUCT_COUNT);
        // when you hit run, there is 3 numbers. six digits though.

        /* (2) Generate three character alphabetic suffix codes. */
        writeRandomCodesToFile("suffix.txt", 'A', 'Z',
                               CHARACTERS_PER_CODE, PRODUCT_COUNT);

        /* (3) Generate three character numeric inline codes. */
        writeRandomCodesToFile("inline.txt", '0', '9',
                               CHARACTERS_PER_CODE,  PRODUCT_COUNT);

        /* (4) Merge the two alphabetic and one numeric code to produce the
         * final product code. */
        mergeProductCodesToFile("prefix.txt", "inline.txt", "suffix.txt",
                                "productCode.txt");
    }// end main()


    /* (1),(2),(3) Generate the alphanumeric codes. These are three aphanumeric
     * character combinations generated randomly with each character in the
     * range A-Z | 0-9. Thus you will have codes from AAA-ZZZ | 000-999.
     *
     * Write numberOfCodesToGenerate codes to the specified file.
     */
    public static void writeRandomCodesToFile(String codeFile,
                                              char fromChar, char toChar,
                                              int numberOfCharactersPerCode,
                                              int numberOfCodesToGenerate) {
        Random rnd = new Random();
        charA = 65;
        for (int i = 1; i <= 20; i++) {
            try (
                PrintWriter fout = new PrintWriter(new File("prefix.txt")); //Creates File
            ) {
                preChars = charA + rnd.nextInt(26);
                preLetters = String.valueOf((char) preChars);
                System.out.print(preLetters);    

            } catch (FileNotFoundException e) {
                 System.err.println(e.getMessage());
            }
        }
    }// end writeRandomCodesToFile()

    /* (4) Merge the two alphabetic and one numeric code to produce the final
     * product code. The final product code will consist of <prefix><inline><suffix>.
     * Each code will be in the range AAA000AAA-ZZZ999ZZZ.
     *
     * Merge by using a prefix, an inline and a suffix code from their corresponding
     * files. Each individual code (pre, in, post) is used exactly once, and all values are used.
     *
     * Write these final product codes to the file specified in productFile.
     */
    public static void mergeProductCodesToFile(String prefixFile,
                                               String inlineFile,
                                               String suffixFile,
                                               String productFile) {


    }// end mergeProductCodesToFile()
}// end Main

Answer 1

import java.util.Random;
import java.io.PrintWriter;
class test{

    public static String randomString(String chars, int length) {
        Random rand = new Random();
        StringBuilder buf = new StringBuilder();
        for (int i=0; i<length; i++) {
            buf.append(chars.charAt(rand.nextInt(chars.length())));
        }
        return buf.toString().replaceAll("...","$0\n");
    }

    public static void main(String[] args ) {
        String myString = randomString("ABCDEFGHIJKLMONQRSTUVWXYZ",60);

        try {
            PrintWriter pw = new PrintWriter("fileame.txt");
            pw.println(myString);
            pw.close();
        } catch (Exception e) {}
    }
}

Answer 2

Ignoring your context and answering the question as per the title:

Given a long string, to insert a newline every 3 chars:

str = str.replaceAll("...", "$0\n");