1.java

public static void main(String[] args) {
    Commodity commodity = new Commodity("Milk", 10000);
    AbstractPurchase purchases[] = {...};
    Arrays.sort(purchases);
    print(purchases);
    /* Search */
    int finder = purchases.length;
    int result = -1;
    /* AbstractPurchase instance with cost returns search value.
     * No needed to write own comparator because AbstractPurchase implements 'comparable'.
     */
    final int SEARCH_PRICE = 1800000;
    AbstractPurchase comp = new AbstractPurchase() {
        @Override
        public long getCost() {
            return SEARCH_PRICE;
        }
    };
    /* Find first occurrence.
     * Each iteration find in [0, last_match) part array. */
    while ((finder =  Arrays.binarySearch(purchases, 0, finder, comp)) >= 0) {
        result = finder;
    }
    if (result >= 0) {
        int f = result;
        int l = result + 1;
        /* In result stored first match. Find last match. */
        while (l < purchases.length && purchases[l].compareTo(purchases[f]) == 0) {
            l++;
        }
        /* Print interval */
        System.out.printf("[%d, %d)", f, l);
    } else {
        System.out.println("Not found");
    }
}