类不是Java的抽象类？涉及的界面使用

Question

我正在为家庭作业创建一个程序，该程序使用界面计算火车旅行的成本：

Train Class:

public class Train implements MassTransit {



public void getCapacity() {

    int capacity = 100;



}//end get Capacity


public int getRoundTripCost(int leave, int return_time){
    int cost = 0;
    cost = (return_time - leave) * 100;
    return cost;


} //end getRoundtrip

public static void main(String[] args) {

    Train train = new Train();
    train.getRoundTripCost(20,25);


}//end of main method



}//end train class

以及质量传递法

public interface MassTransit {

public void getCapacity();
public void getRoundTripCost(int leave, int return_time);


}//end of MassTransit interface

当我试图编译Train类时，我得到的错误是“Train.java:6:error:Train不是抽象的，并且不会覆盖MassTransit中的抽象方法getRoundTripCost（int，int）”

及

Train.java:19:错误：Train中的getRoundTripCost（int，int）无法在MassTransit中实现getRoundTripCost（int，int）

我是一个完全的Java新手，我仍然不熟悉Java接口。我做错了什么

Answer 1

public int getRoundTripCost(int leave, int return_time){
    int cost = 0;
    cost = (return_time - leave) * 100;
    return cost;


} //end getRoundtrip

上面重写的方法返回类型应该是void而不是int。您的接口定义了返回类型为void的方法

public void getRoundTripCost(int leave, int return_time);

根据java tutorial

An instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass overrides the superclass's method.