Totals, Counts, and Extremes in an ArrayList

Unboxing in a total

A grocery receipt is stored as ArrayList<Double> prices, holding 2.49, 5.00, 1.25, and 3.75.

double total = 0.0;
for (double p : prices)
{
    total += p;
}

Each element is a Double, unboxed into the double variable p as the loop hands it over. The loop adds the four prices, so total ends at 12.49. Over a double[] this loop would be unchanged. An indexed version uses size() and get(i) where an array uses length and brackets.

A count over 3.00

int over = 0;
for (double p : prices)
{
    if (p > 3.00)
    {
        over++;
    }
}

Only the 5.00 and the 3.75 pass the test, so over is 2.

For the average price, total / prices.size() divides a double by an int, so it keeps the fraction and gives 3.1225.

The cheapest item, by index

double cheapest = prices.get(0);
for (int i = 1; i < prices.size(); i++)
{
    if (prices.get(i) < cheapest)
    {
        cheapest = prices.get(i);
    }
}

Each prices.get(i) returns a Double, which is unboxed for the comparison with cheapest. The 1.25 replaces the 2.49, and nothing later is smaller, so cheapest ends at 1.25. The seed prices.get(0) needs a list that isn't empty. On an empty list, get(0) throws an IndexOutOfBoundsException.