Question

I’m creating a program for a homework assignment that figures out the cost for a train trip that uses an interface:

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

And the MassTransit Method

public interface MassTransit {

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


}//end of MassTransit interface

When I try to compile the Train class, the errors I get is “Train.java:6: error: Train is not abstract and does not override abstract method getRoundTripCost(int,int) in MassTransit”

And

“Train.java:19: error: getRoundTripCost(int,int) in Train cannot implement getRoundTripCost(int,int) in MassTransit”

I’m a complete Java newbie, and I’m still unfamiliar with Java interfaces. What did I do wrong?

Answer 1

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


} //end getRoundtrip

Above overriden method return type should be void not int. Your interface defined method with return type void.

public void getRoundTripCost(int leave, int return_time);

As per 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.