Solution to the Happy Numbers problem in Java.

HappyNumbers.java

import java.util.HashSet;
import java.util.Scanner;
import java.io.*;

public class HappyNumber {
    //Note that we did not account for the fact that all permutations of a happy number's digits
    //must also form happy numbers since addition commutes. This is mentioned in the README.
    private final static long ten = 10;
    static long squareSum(long number) {
        long total = 0;
        long digit;
        while (number > 0) {
            digit = (number % ten);
            total += (digit * digit);
            number /= ten;
        }
        return total;
    }

    public static boolean checkFile(String fileName) {
        File file = new File(fileName);
        if (file.exists()) {
            return false;
        }
        //So the file does not already exist. Now we guarantee that the parent directory exists.
        file.getParentFile().mkdirs();
        return true;
    }

    public static void main(String[] args) {
        HashSet<Long> happyNumbers = new HashSet<>();

        Scanner in = new Scanner(System.in);
        System.out.println("Enter how many numbers you'd like to compute: ");
        int limit = in.nextInt();
        in.nextLine();
        System.out.println("Enter the absolute path of the output file you'd like to make: ");
        String fileName = in.nextLine();
        
        if (!checkFile(fileName)) {
            System.out.println("File already exists. Now exiting program.");
            System.exit(0);
        }

        TextOutput textOutput = new TextOutput(fileName);
        happyNumbers.add((long) 1);
        textOutput.writeLine(String.valueOf(1));

        int count = 1;
        long current = 2;
        HashSet<Long> deadEnds = new HashSet<>();

        while (count < limit) {
            HashSet<Long> currentSet = new HashSet<>();
            long number = current;
            
            while (number != 1) {
                currentSet.add(number);
                number = squareSum(number);
                if (deadEnds.contains(number) || currentSet.contains(number)) {
                    //Then the number cycles and will never ever find happiness.
                    deadEnds.addAll(currentSet);
                    break;
                }
                if (happyNumbers.contains(number)) {
                    happyNumbers.addAll(currentSet);
                    textOutput.writeLine(String.valueOf(current));
                    count++;
                    break;
                }
            }
            current++;
        }
        textOutput.close();
        in.close();
    }
}


class TextOutput {
    private BufferedWriter bw;
    private FileWriter fw;

    public TextOutput(String fileName) {
    	try {
    		this.fw = new FileWriter(fileName);
        this.bw = new BufferedWriter(this.fw);
        } catch (IOException e) {
        	e.printStackTrace();
        }
    }

    public void writeLine(String line) {
        try {
            this.bw.write(line + "\n");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void close() {
    	try {
    		this.bw.close();
    	    this.fw.close();
    	} catch (IOException e) {
    		e.printStackTrace();
    		}
    }
}