Parking Lot

Ownership and transitions

Allocation policy and reservation are separate responsibilities. Rank compatibility by size, then choose the smallest compatible free kind and identifier. A ticket is a stable release handle: a repeated release must not free a later occupant. Validate the whole inventory before reserving anything. A scan is O(spots) per arrival; ticket lookup/release is O(1) with a hash map (ordered snapshots may require sorting).

Java boundaries

Use the supplied nested records as immutable value carriers. A record is only shallowly immutable: copy every retained mutable input and every outward list/map, including lists inside records. Absence is null; validation failures use IllegalArgumentException, missing-key purchase errors use NoSuchElementException, and invalid transaction boundaries use IllegalStateException. Java primitive types already exclude booleans from numeric inputs. Numeric values use long where signatures say long and all computations must fit signed 64-bit values. Calls in these service exercises are serialized.

Compatibility is an ordered relation; a smaller kind may consume a larger kind, but the reverse is invalid. Keep that policy explicit rather than depending on inventory insertion order:

import java.util.*;

class Example {
    public static void main(String[] args) {
        Map<String, Integer> rank = Map.of("motorcycle", 0, "car", 1, "van", 2);
        assert rank.get("car") <= rank.get("van");
        assert !(rank.get("van") <= rank.get("car"));
        long oldTicketId = 7, laterTicketId = 8;
        assert oldTicketId != laterTicketId;
    }
}

The compatibility check is separate from reservation. Releasing through a lifetime ticket ID prevents an old handle from targeting a later allocation of the same physical spot.

Design boundary

A full lot is a normal capacity failure, while an invalid kind or duplicate active vehicle is a request error. Neither may consume the next ticket number. This model assumes serialized calls; concurrent gates would need synchronization around selection plus commit, and physical occupancy would need its own source of truth.