我不确定为什么getCost（）;正在返回一个错误

Question

我目前正在为学校做一个小项目，我不知道为什么这会导致问题。

package tripcalculator;

import java.util.Scanner;

public class Trip {
    Scanner kbd = new Scanner(System.in);
    private int distance;
    public final double MILEAGE = 0.14;
    public final double COST_PER_LITRE = 1.29;
    public void getLitresUsed() {

    }

//constructor with distance parameter passed
    public Trip(int distance) {
    }

//default construtor
    public Trip() {
         System.out.println("Enter distance travelled: ");
         distance = kbd.nextInt(); 
    }

//getter and setter
    public int getDistance() {
        return distance;
    }

    public void setDistance(int distance) {
        this.distance = distance;
    }

    public double getLitresUsed(int distance){
        double litresUsed = MILEAGE * distance;
        return litresUsed;
    }

    public double getCost(double litresUsed){
        double cost = litresUsed * COST_PER_LITRE;
        return cost;
    }


    public String toString(String litresUsed) {
        getLitresUsed();
        getCost();
        String output = "Trip Details\n" + "\n" + "Distance: " + distance + " km\n" + "\n" + "Litres Used: " + litresUsed + "\n" + "\n" + "Cost: $" + cost;
        return output;
    }

}

如你所见，getCost存在问题，我不确定为什么会导致问题。在我的主要内容中，我有以下代码：

package tripcalculator;

public class TripCalculator {

    public static void main(String[] args) {
        Trip trip1 = new Trip();
        trip1.getLitresUsed();
        showTrip(trip1);
    }

    public static void showTrip(Trip trip1) {
        System.out.println(trip1.toString());
    }
}

这是一个程序，用于计算旅行的总成本，我不知道为什么它没有从getCost返回值，如果有人可以解释这个我会非常感谢这样的！

Answer 1

插口！

我重构了你的程序。仔细阅读并分析我的评论。

trip.Java:

package tripcalculator;

import java.util.Scanner;

public class Trip {
    Scanner kbd = new Scanner(System.in);
    private double distance; // This should be a double value
    final double MILEAGE = 0.14; // Let;s make constants private also. They are not needed for the world.
    final double COST_PER_LITRE = 1.29;

    // public void getLitresUsed() { // What was the purpose of this? Removed.
    //
    // }

    // constructor with distance parameter passed
    // public Trip(int distance) { // Actually, this constructor does not do
    // anything. Removed.
    // }

    // default constructor
    public Trip() {
        System.out.println("Enter distance travelled: ");
        distance = kbd.nextInt();
    }

    // getter and setter
    public double getDistance() {
        return distance;
    }

    // public void setDistance(int distance) { // We do not need setter method as
    // distance is being set when constructing Trip object.
    // this.distance = distance;
    // }

    public double getLitresUsed() { // distance is class variable. No need in method parameter distance. Removed.
        double litresUsed = MILEAGE * distance;
        return litresUsed;
    }

    public double getCost() { // litresUsed is being calculated by method. So, parameters has been removed and
                                // changed to method output.
        double cost = getLitresUsed() * COST_PER_LITRE;
        return cost;
    }

    @Override // Actually, we're overriding method here. Be careful.
    public String toString() {
        // getLitresUsed(); // These methods' outputs have been moved to String
        // generation.
        // getCost();
        String output = "Trip Details\n" + "\n" + "Distance: " + distance + " km\n" + "\n" + "Litres Used: "
                + getLitresUsed() + "\n" + "\n" + "Cost: $" + getCost();
        return output;
    }

}

trip calculator.Java:

package tripcalculator;

public class TripCalculator {

    public static void main(String[] args) {
        Trip trip1 = new Trip();
        // trip1.getLitresUsed(); // No need in calling this method from outside.
        showTrip(trip1);
    }

    public static void showTrip(Trip trip1) {
        System.out.println(trip1.toString());
    }
}

祝好运！