Probability of x possible outcomes [duplicate]

Answer 1

Use a NavigableMap, which will allow you to retrieve the correct outcome with one clean and simple lookup. (And internally, this uses an efficient O(log n) lookup—not that your maps will be large enough to matter.)

import java.util.NavigableMap;
import java.util.TreeMap;
import static java.util.concurrent.ThreadLocalRandom.current;

final class LoadedDie {

  public static void main(String... argv) {

    /* One-time setup */
    NavigableMap<Integer, String> loot = new TreeMap<>();
    int cumulative = 0;
    loot.put(cumulative += 20, "Gold");
    loot.put(cumulative += 30, "Iron");
    loot.put(cumulative += 50, "Coal");

    /* Repeated use */
    System.out.println(loot.higherEntry(current().nextInt(cumulative)).getValue());
    System.out.println(loot.higherEntry(current().nextInt(cumulative)).getValue());
  }

}

Answer 2

Here's one way to do it.

public static String getOutcome(Map<String, Integer> possibilities) {
    int x = (int) (Math.random() * 100);

    for (Map.Entry<String, Integer> possibility : possibilities.entrySet()) {
        if (x <= possibility.getValue()) {
            return possibility.getKey();
        }
        x -= possibility.getValue();
    }

    // unreachable if probabilities are correctly mapped
    return null;
}