Randomly getting value with percentage of chance in Java

Question

I would like to be able to return (and in our case, for the example, display) an item of an enumeration, according to a certain predefined percentage of chance for each item. An illustration in code of this enumeration just below :

package me.lucas.test;

public enum Item {

    ITEM_1("Item 1", 30),
    ITEM_2("Item 2", 30),
    ITEM_3("Item 3", 10),
    ITEM_4("Item 4", 65);

    private final String name;
    private final int percentage;

    Item(String name, int percentage) {
        this.name = name;
        this.percentage = percentage;
    }

    public String getName() {
        return name;
    }

    public int getPercentage() {
        return percentage;
    }

}

Also, these items will be stored in a List.

package me.lucas.test;

import java.util.ArrayList;
import java.util.List;

public class Program {

    private final List<Item> items = new ArrayList<>();

    /**
     * Displays an item among those in the list, according to their percentage chance of appearing.
     * @param items The items
     */
    public void displayItem(List<Item> items) {
        Item item;

        item = ?

        System.out.println("Selected item : " + item.getName() + " with percentage : " + item.getPercentage());
    }

}

Namely that, as in this example, two items can have the same percentage of chance to appear.

I also looked at the following topic (Java Random Percentage Chance) but in my case, the percentages are not predefined in the expressions directly but in the enumeration.

So I would appreciate your help. Thank you in advance for taking the time to help me.

Answer 1

Visually speaking you implement a Wheel of Chance

The wheel will land on any position 1 to 100. Now you just need to read out, which possibilities are at this position. If the sum is < 100 there may be nothing in this position. If the sum is > 100 there may be more than one item in this position.

All Percentages are wrapping from 1 to 100 - achieved by modulo. The random Number you generate is between 1 and 100, or in this case between 0 and 99 (see Javadoc of Random).

In Code this could look as follows:

public class RandomExample<T extends Object> {
    List<Item> items = new LinkedList<>();
    Random rand = new Random();
    
    public void addItem(int percentage, T object) {
        items.add(new Item(percentage, object));
    }
    
    public List<Item> getRandom() {
        List<Item> result = new ArrayList<>();
        int hit = rand.nextInt(100);
        items.forEach(i -> {
            if(i.isHit(hit)) {
                result.add(i);
            }
        });
        return result;
    }
    
    public class Item {
        private int percentage;
        private T object;
        private int hitLow;
        private int hitHigh;
        
        public Item(int percentage, T object) {
            this.percentage = percentage;
            this.object = object;
            int sum = 0;
            for(Item i : items) {
                sum+=i.getPercentage();
            }
            hitLow = sum % 100;
            hitHigh = (sum+percentage) % 100;
        }
        
        public int getPercentage() {
            return percentage;
        }
        
        public T getObject() {
            return object;
        }
        
        public boolean isHit(int hitVal) {
            boolean isHit = hitLow == hitHigh; //exactly 100%
            isHit |= hitLow <= hitVal && hitHigh >= hitVal;
            return isHit;
        }
    }

}

The method RandomExample.Item.isHit(int) still needs an extra condition to handle chances correctly, which wrap around 100 (eg. hitLow = 90 and hitHigh = 10). But you get the concept and should be able to adapt it to your needs.

Another question you need to ask yourself is: what happens if somebody passes a percentage greater than 100. My intuition says you should throw an exception. But that is for you to decide and implement.

Answer 2

BE SIMPLE!!!

1. Create an array.
2. Put into this array all items, percentage count each.
3. Shuffle the array.
4. Retrieve random element form this array.

---

public class Foo {

    public static void main(String... args) {
        Item[] items = generateItemArray();
        Random random = new Random();

        while (true) {
            System.out.println(items[random.nextInt(items.length)]);
        }
    }

    private static Item[] generateItemArray() {
        List<Item> items = new ArrayList<>();

        for (Item item : Item.values())
            for (int i = 0; i < item.percentage; i++)
                items.add(item);

        Collections.shuffle(items);
        return items.toArray(Item[]::new);
    }

    public enum Item {
        ITEM_1("Item 1", 30),
        ITEM_2("Item 2", 30),
        ITEM_3("Item 3", 10),
        ITEM_4("Item 4", 65);

        private final String name;
        private final int percentage;

        Item(String name, int percentage) {
            this.name = name;
            this.percentage = percentage;
        }

    }
}