Get a random object from a given map containing objects and their probabilities

example-output.txt

1,000,000 iterations took 271ms to calculate

ten chances generated 181764 times = 18%
nine chances generated 163178 times = 16%
eight chances generated 146151 times = 14%
seven chances generated 127031 times = 12%
six chances generated 109405 times = 10%
five chances generated 91076 times = 9%
four chances generated 72484 times = 7%
three chances generated 54386 times = 5%
two chances generated 36190 times = 3%
one chances generated 18335 times = 1%

RandomObjectFromProbabilities.java

package com.scarsz.playground;

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Random;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * Made by Scarsz
 *
 * @in /dev/hell
 * @on 3/5/2017
 * @at 11:21 AM
 */
public class RandomObjectFromProbabilities {

    /**
     * {@link Random} object to generate random numbers from.
     */
    private static Random random = new Random();

    /**
     * The map of all our possible random objects to be generated along with their chance of being selected.<br>
     * Lower numbers have a lower chance of being selected & vice versa.
     */
    private static Map<Object, Integer> items = new LinkedHashMap<Object, Integer>() {{
        put("one chances", 1);
        put("two chances", 2);
        put("three chances", 3);
        put("four chances", 4);
        put("five chances", 5);
        put("six chances", 6);
        put("seven chances", 7);
        put("eight chances", 8);
        put("nine chances", 9);
        put("ten chances", 10);
    }};

    /**
     * Get a random object from the given list with given probabilities of being selected
     * @param items Items to choose from
     * @return Randomly selected key of an entry from the given map based on the entry's probability (entry value)
     */
    private static Object getRandomObjectFromProbabilities(Map<Object, Integer> items) {
        if (items == null) throw new NullPointerException("Null items map supplied!");
        if (items.size() == 0) throw new RuntimeException("Not enough items to generate from! (0)");
        
        int totalSum = items.values().stream().mapToInt(Integer::intValue).sum();
        int count = random.nextInt(totalSum) + 1;

        Object currentObject = null;
        while (count > 0) {
            for (Map.Entry<Object, Integer> entry : items.entrySet()) {
                currentObject = entry.getKey();
                count -= entry.getValue();
                if (count <= 0) return currentObject;
            }
        }

        // it should never reach this point, but, if it does, default to the first given item
        return currentObject != null ? currentObject : items.keySet().iterator().next();
    }

    public static void main(String[] args) {
        long startMs = System.currentTimeMillis();
        long iterations = 1000000;

        Map<String, AtomicInteger> results = new HashMap<>();
        for (int i = 0; i < iterations; i++) {
            String result = (String) getRandomObjectFromProbabilities(items);
            if (results.containsKey(result)) results.get(result).incrementAndGet();
            else results.put(result, new AtomicInteger(1));
        }

        System.out.println(iterations + " iterations took " + (System.currentTimeMillis() - startMs) + "ms to calculate\n");
        for (Map.Entry<String, AtomicInteger> result : results.entrySet())
            System.out.println(result.getKey() + " generated " + result.getValue() + " times = " + ((int) (((double) result.getValue().intValue() / iterations) * 100)) + "%");
    }

}