我有一個 Event 類，它是由 Concert、Play 和 Meeting 實現(xiàn)的。我處于單獨的門票類別中，出售給相應(yīng)的“活動”。在 Ticket 中，我需要 getPrice()，它從 10 開始，并且在預(yù)訂活動（具有不同價格因素）時需要乘以價格因素。我嘗試簡單地調(diào)用一個靜態(tài) PRICE（即門票的基本價格）并調(diào)用 event.getPriceFactor() ，它返回 null，我顯然可以在 Ticket 中實例化此事件。public abstract class Event {    private String description;    protected int ticketsSold;    private int eventId;    private double priceFactor;    private static int counter = 1;    private static final int CAPACITY = 5;    private ObservableList<Ticket> tickets =     FXCollections.observableArrayList();    /**     * Stores the description and price factor and assigns a unique id to the event.     * The constructor also allocates the array tickets.     *      * @param description a description of this Play     * @param priceFactor the price factor for this Play     *      */    public Event(String description, double priceFactor) {        this.description = description;        this.priceFactor = priceFactor;        this.eventId = computeSerialNumber();    }    /**     * Receives the description and stores that and a price factor of 1.0. Besides,     * it assigns a unique id to the event. The constructor also allocates the array     * tickets.     *      * @param description a description of this Play     *      */    public Event(String description) {        this(description, 1.0);    }    public double getPriceFactor() {        return priceFactor;}------------------------------------------------------------------------------public class Ticket {    private static int counter = 1;    private int serialNumber;    private double price;    private static double PRICE = 10.0;    private Event event;我期待 Ticket.PRICE (10.) * event.getPriceFactor() (1.0) 返回 10，但它返回 null。