Java 如何实现抽象类的方法？（爪哇）

我试图使用一个抽象类中的方法来实现一个接口。当我调用一个方法时，我总是得到一个空指针异常，我不知道为什么。有什么想法吗？谢谢

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
}

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

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
}

---

您在邮件中的代码如下：

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

在这里，您只分配了数组，但数组中的元素仍然为null（因此在对null对象调用setter时出现null指针异常），并且需要初始化，如下所示：

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

---

问题是您仅在以下行中创建了车辆阵列-

Vehicle[] automobiles = new Vehicle[3];

您仍然需要使用new Vehicle（或者new Automobile，因为Vehicle是一个抽象类，无法实例化）将变量初始化为对象，然后才能访问它们

范例-

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

---

请在使用[0]之前将新对象分配给它。就像这样做——汽车；自动设置车辆名称（“a”）；欢迎，没关系，每个人都会犯错，从中学习很重要。