如何實現抽象類的方法？ (爪哇)

Question

我正在嘗試使用實現接口的抽象類中的方法。 當我調用一個方法時，我不斷收到空指針異常，但我不知道為什么。 有任何想法嗎？ 謝謝。

package start;
public class Automobile extends Vehicle {     // code with main method
public static void main(String[] args) {
         Vehicle[] automobiles = new Vehicle[3];
         automobiles[0].setVehicleName("Corvette");
    }
}

///////////////////////////////////////////////// ////////////////////////

package start;

public abstract class Vehicle implements Movable {

    String name = "Unidentified"; // variables for vehicles
    String manufacturer = "Factory";
    String car = "Unknown";
    int yearOfManufacture = 0000;
    int horsepower = 0;
    static int instances = 0;

    int passengers = 0; // variables for methods below
    int speed = 0;

    public int getNoPassengers() { // returns how many passengers there are
        instances = instances + 1;
        return passengers;
    }

    public void setNoPassengers(int noPassengers) { // sets the number of passengers
        instances = instances + 1;
        passengers = noPassengers;
    }

    public int getTopSpeed() { // returns how fast a movable vehicle is
        instances = instances + 1;
        return speed;
    }

    public void setTopSpeed(int topSpeed) { // changes the speed of a movable vehicle
        instances = instances + 1;
        speed = topSpeed;
    }

    public void setVehicleName(String title) { // changes the name of a vehicle
        instances = instances + 1;
        name = title;
    }

    public String getVehicleName(String car){
        return car;
    }

    public void setManufacturer(String creator) { // changes the manufacturer
        instances = instances + 1;
        manufacturer = creator;
    }

    public String getManufacturer(String type){
        return type;
    }
}

//////////////////////////////////////////////

package start;

interface Movable {      // interface

    int getNoPassengers(); // returns how many passengers there are

    void setNoPassengers(int noPassangers); // sets the number of passengers

    int getTopSpeed(); // returns how fast a movable vehicle is

    void setTopSpeed(int topSpeed); // changes the speed of a movable vehicle
}

Answer 1

問題是您只在以下幾行中創建了車輛數組 -

Vehicle[] automobiles = new Vehicle[3];

在訪問變量之前，您仍然需要使用 new Vehicle （或 new Automobile ，因為 Vehicle 是一個抽象類並且無法實例化）將變量初始化為對象。

例子 -

Vehicle[] automobiles = new Vehicle[3];
automobiles[0] = new Automobile();
automobiles[0].setVehicleName("Corvette");

Answer 2

您在郵件中的代碼如下：

Vehicle[] automobiles = new Vehicle[3];
automobiles[0].setVehicleName("Corvette");

在這里，您剛剛分配了數組，但其中的元素仍然為空（因此在空對象上調用 setter 時出現空指針異常）並且需要初始化，例如：

automobiles[0] = new ....;
//then access method from within automobiles[0]