如何實現抽象類的方法？ （Java）

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，我總是收到一個空指針異常，我不知道爲什麼。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

的問題是，你只創建了下面幾行車輛的陣列 -

Vehicle[] automobiles = new Vehicle[3]; 

您仍然需要通過新車初始化變量對象（或新汽車，因爲汽車是一個抽象類，不能被實例化），它們可以被訪問之前。

實施例 -

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

Answer 2

你的郵件代碼如下：

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

在這裏，您剛剛分配的數組，但元素中它仍然是空（和調用空對象的設置方法時，因此空指針異常），需要初始化以及類似：

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