1.java

public static void main(String[] args) {
    AbstractPurchase purchases[] = {...};
    Arrays.sort(purchases);
    print(purchases);
    /* http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#binarySearch(byte[],%20byte)
     * binarySearch return index of the search key, if it is contained in the array; 
     * otherwise, (-(insertion point) - 1). Use it by create comparator without return 0
     * (equals) section. binarySearch with this comparator always return insertion point.
     */
    final long SEARCH_COST = 5300;
    AbstractPurchase comp = new  AbstractPurchase() {
        
        @Override
        public long getCost() {
            return SEARCH_COST + 1;
        }
    };
    
    int f = Arrays.binarySearch(purchases, comp);
    /* Insertion point reverse transformation */
    f = -f - 1;
    if (f < purchases.length && purchases[f].getCost() == SEARCH_COST) {
        System.out.println("RESULTS:");
        for (int i = f; purchases[i].getCost() == SEARCH_COST; i++) {
            System.out.println(purchases[i]);
        }
    } else {
        System.out.println("NOT FOUND");
    }
}