calculateMaxShipment.java

    public static int calculateMaxShipment(List<Shipment> shipmentList) {
        int highShipment = -1;
        int counter = 0;
        Map<Integer, Integer> hours = new TreeMap<>();
        for (Shipment shipment : shipmentList) {
            hours.put(shipment.in, hours.getOrDefault(shipment.in, 0) + 1);
            hours.put(shipment.out, hours.getOrDefault(shipment.out, 0) - 1);
        }
        
        for (Map.Entry<Integer, Integer> hourEntry : hours.entrySet()) {
            counter += hourEntry.getValue();
            if (counter > highShipment)
                highShipment = counter;
        }

        return highShipment;
    }

shipment.java

public class Shipment {
    final int in;
    final int out;
    public Shipment(int in, int out) {
        this.in = in;
        this.out = out;
    }
}