如何实现抽象类的方法？ (爪哇)

Question

I am trying to use the methods from an abstract class that is implementing an interface. I keep getting a null pointer exception when I call upon a method and I am not sure why. Any ideas? Thanks.

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

The issue is that you have only created the array of vehicles in the following lines -

Vehicle[] automobiles = new Vehicle[3];

You still need to initialize the variables to objects by using new Vehicle (or new Automobile , since Vehicle is an abstract class and cannot be instantiated) , before they can be accessed.

Example -

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

Answer 2

Your code in mail is as below:

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

Here you just allocated array but element within it is still null (and hence null pointer exception when calling setter on null object) and needs to initialized as well like:

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