DropTable.java

package droptable;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Locale;

public class DropTable {
    private List<DropChance> table;
    private boolean sorted;

    public DropTable() {
        this.table = new ArrayList<>();
    }

    public void add(double chance, Item item) {
        sorted = false;
        table.add(new DropChance(chance, item));
    }

    public Item getDrop() {
        if(!sorted) {
            Collections.sort(table, (a, b) -> Double.compare(a.chance, b.chance));
            sorted = true;
        }

        double roll = Math.random();

        for(DropChance c : table) {
            if(roll <= c.chance) {
                return c.item;
            }
        }

        return null;
    }

    public class DropChance {
        private double chance;
        private Item item;

        public DropChance(double chance, Item item) {
            this.chance = chance;
            this.item = item;
        }
    }

    public static class Item {
        private String name;
        private Rarity rarity;

        public Item(String name, Rarity rarity) {
            this.name = name;
            this.rarity = rarity;
        }

        @Override
        public String toString() {
            return String.format(Locale.ROOT, "%s (%s)", name, rarity);
        }

        public enum Rarity {
            JUNK,
            COMMON,
            MAGIC,
            RARE,
            EPIC,
            LEGENDARY
        }
    }

    public static void main(String... argv) {
        Item mace = new Item("Source of Envy", Item.Rarity.LEGENDARY);
        Item sword = new Item("Awesome Sword of Awesomeness", Item.Rarity.EPIC);
        Item shield = new Item("Damn Fine Shield", Item.Rarity.RARE);
        Item pants = new Item("Leggings +1", Item.Rarity.MAGIC);
        Item common = new Item("Rusty Dagger", Item.Rarity.COMMON);
        Item junk = new Item("Rat's ears", Item.Rarity.JUNK);

        DropTable table = new DropTable();
        table.add(0.01, mace);
        table.add(0.05, sword);
        table.add(0.1, shield);
        table.add(0.2, pants);
        table.add(0.3, common);
        table.add(0.8, junk);

        for(int i = 0;i < 100;i++) {
            Item item = table.getDrop();

            if(item == null) {
                System.out.println("No drop :(");
            }
            else {
                System.out.println(item);
            }
        }
    }
}