Question

我试图让用户输入超级类中汽车的座位数，但是提出了一种方法来打印子类“汽车”中的座位数。但是，当我声明存储用户输入的变量时，出现错误，指出该变量不可见，这是因为它在另一个类中。

import java.util.Scanner;

class Vehicle {

    Scanner s = new Scanner(System.in);
    private String color;
    private int noOfCylinders;
    private int noOfSeats;

    public Vehicle() {
        color = "Black";
        noOfCylinders = 0;
        noOfSeats = 1;
    }

    public Vehicle(String color, int noOfCylinders, int noOfSeats) {
        this.color = color;
        this.noOfCylinders = noOfCylinders;
        this.noOfSeats = noOfSeats;
    }

    public void getColor() {
        System.out.print("Enter color of vehicle: ");
        color = s.nextLine();
    }

    public String setColor() {
        return color;
    }

    public void getNoOfCylinders() {
        System.out.print("Enter number of cylinders: ");
        noOfCylinders = s.nextInt();
    }

    public int setNoOfCylinders() {
        return noOfCylinders;
    }

    public void getNoOfSeats() {
        System.out.print("Enter numer of seats: ");
        int noOfSeats = s.nextInt();
    }

    public String toString() {
        String information;
        information = "is " + color + " and it has " + noOfCylinders + " cylinders.";
        return information;
    }
}

public class CreateVehicle {
    public static void main(String[] args) {

        Car CarObject = new Car();
        Truck TruckObject = new Truck();

        CarObject.getColor();
        CarObject.setColor();
        CarObject.getNoOfCylinders();
        CarObject.setNoOfCylinders();
        CarObject.toString();
        CarObject.getNumOfSeats();

        TruckObject.getColor();
        TruckObject.setColor();
        TruckObject.getNoOfCylinders();
        TruckObject.setNoOfCylinders();
        TruckObject.toString();

        System.out.print(("\nThe car ")+CarObject.toString());
        System.out.print(("\nThe truck ")+TruckObject.toString());      
    }
}


class Car extends Vehicle{

    public void getNumOfSeats(){
        System.out.print("\nThe car has " + noOfSeats + " seats.");
    }
}


    class Truck extends Vehicle {
        public void printTowingCapacity() {
            System.out.print("\nThe car has " + towingCapacity + ".");
        }
    }

Answer 1

如果要将变量设为私有，则可以使用父类的公共函数getNoOfSeats（）。

class Car extends Vehicle{
   public void getNumOfSeats(){
      System.out.print("\nThe car has " + super.getNoOfSeats() + " seats.");
   }
}

或者只是将变量noOfSeats更改为受保护或受包保护，

Answer 2

noOfSeats是该类的私有成员，任何其他类都无法访问。如果要使它可以从继承的类访问，请使其对说明符的访问权限为protected。

Answer 3

基于OOP概念，即使在子类中也不能访问类外的私有变量。

您在这里声明

 private int noOfSeats;

然后您尝试在私有变量范围之外的child子类之外访问它。

要在子类中访问它，请设置noOfSeats变量

 public int noOfSeats;

或

 protected int noOfSeats;

在车辆类中。

如果要将noOfSeats变量设置为私有变量，请在车辆类（父类）上对public函数进行创建，然后从Car类（子类）中调用它。

您正在尝试访问

 System.out.print("\nThe car has " + towingCapacity + ".")

Truck类中的towingCapacity变量，您没有在Vehicle类或Truck类中声明它。

因此，首先声明它，然后使用它。

有关java访问修饰符的更多详细信息，请阅读本文

https://www.softwaretestingmaterial.com/access-modifiers-in-java/

如何覆盖在另一个已继承的类中被赋予值的变量

3 个答案: