我正在尝试使用实现接口的抽象类中的方法。当我调用一个方法时,我一直得到一个空指针异常,我不知道为什么。有任何想法吗?感谢。
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;
}
}
////////////////////////////////////////////// < / p>
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
}
答案 0 :(得分:2)
问题在于您只在以下行中创建了车辆数组 -
Vehicle[] automobiles = new Vehicle[3];
在使用新的Vehicle(或新的Automobile,因为Vehicle是一个抽象类,无法实例化)之前,您仍然需要将变量初始化为对象,然后才能访问它们。
示例 -
Vehicle[] automobiles = new Vehicle[3];
automobiles[0] = new Automobile();
automobiles[0].setVehicleName("Corvette");
答案 1 :(得分:1)
您的邮件代码如下:
Vehicle[] automobiles = new Vehicle[3];
automobiles[0].setVehicleName("Corvette");
这里你只是分配了数组但是其中的元素仍然是null(因此在调用null对象上的setter时因为空指针异常)并且需要初始化,如:
automobiles[0] = new ....;
//then access method from within automobiles[0]