public class Roller {

    public static Random random = new Random();

    public static int roll(ItemRoll[] arr) {
        double sum = 0;
        for (int i = 0; i < arr.length; i++)
            sum += arr[i].getRate();

        double t;
        double[] percent = new double[arr.length];

        for (int i = 0; i < arr.length; i++) {
            t += arr[i].getRate() / sum;
            percent[i] = t;
        }

        double r = random.nextDouble();
        for (int i = 0; i < arr.length; i++) {
            if (r <= arr[i].getRate()) {
                return arr[i];
            }
        }

        return null; //Never happend
    }

}

@AllArgsConstructor
public class ItemRoll {


    @Getter private final ItemStack item;
    @Getter private final double rate;

}