SecretSanta.java

import java.io.File;
import java.io.FileNotFoundException;
import java.util.*;

public class SecretSanta {
    public static void main(String[] args) {
        File inputFile = new File("names.txt");
        List<String> names = readNamesFromFile(inputFile);
        Map<String, String> secretSanta = generateSecretSanta(names);
        printSecretSanta(secretSanta);
    }

    /**
     * Utility method to read from a file
     * @param inputFile
     * @return
     */
    private static List<String> readNamesFromFile(File inputFile) {
        List<String> names = new ArrayList<>();
        try {
            Scanner fileReader = new Scanner(inputFile);
            while (fileReader.hasNext()) {
                //Assuming one line per name
                names.add(fileReader.nextLine());
            }
        } catch (FileNotFoundException fileNotFoundException) {
            System.out.println("Input file not found");
        }
        return names;
    }

    /**
     * Generate a shuffled list from the original list and generate a random secret santa for each name
     * @param names
     * @return
     */
    private static Map<String, String> generateSecretSanta(List<String> names) {
        return zipListsToMap(names, getShuffledList(names));
    }

    /**
     * This method shuffles the given list of strings so that no string is in the original place
     * @param list input list of strings
     * @return shuffled list of strings
     */
    private static List<String> getShuffledList(List<String> list) {
        List<String> shuffled = new ArrayList<>(list);
        for (int i = 0; i < list.size() - 1; i++) {
            int randomIndex = getRandomNumber(i + 1, shuffled.size());
            swapNames(shuffled, i, randomIndex);
        }
        return shuffled;
    }

    /**
     * Combines two lists into a map, assuming that two lists are of same length
     * @param list1 list of keys
     * @param list2 list of corresponding values for keys in list1
     * @return Map
     */
    private static Map<String, String> zipListsToMap(List<String> list1, List<String> list2) {
        Map<String, String> map = new HashMap<>();
        for(int i = 0; i < list1.size(); i++) {
            map.put(list1.get(i), list2.get(i));
        }
        return map;
    }

    /**
     * Utility method to print a map
     * @param secretSanta a Map
     */
    private static void printSecretSanta(Map<String, String> secretSanta) {
        for (String name : secretSanta.keySet()) {
            System.out.println(name + " -> " + secretSanta.get(name));
        }
    }

    /**
     * Generate a random number between [min, max) - min inclusive, max exclusive
     * @param min : int
     * @param max : int
     * @return a random integer in the given range
     */
    public static int getRandomNumber(int min, int max) {
        return (int) ((Math.random() * (max - min)) + min);
    }

    /**
     * Swap two elements in a list at index1, and index2
     * @param names : List of strings
     * @param index1 : first  index
     * @param index2 : second index
     */
    private static void swapNames(List<String> names, int index1, int index2) {
        String temp = names.get(index1);
        names.set(index1, names.get(index2));
        names.set(index2, temp);
    }
}