类不能强制转换为 java.lang.Comparable

Question

First of all, I know some people has already asked similar questions, but I'm not sure if this is the same as what they asked.

I have a class named Vehicle which has an attribute named location . On the other side, I have a class named Road which has an ArrayList attribute of Vehicles. I want to order the list, named _vehicles , using location but in descending order, so I created this class into the Road class:

class CompareVehicles implements Comparator<Vehicle> {

    @Override
    public int compare(Vehicle o1, Vehicle o2) {
        if (o1.getLocation() < o2.getLocation()) return 1;
        if (o1.getLocation() > o2.getLocation()) return -1;
        return 0;
    }
}

And an attribute named _vehicleComparator which its type is CompareVehicles . But, when I execute the method _vehicles.sort(_vehicleComparator) , I get this error:

java.lang.ClassCastException: class simulator.model.Vehicle cannot be cast to class java.lang.Comparable (simulator.model.Vehicle is in unnamed module of loader 'app'; java.lang.Comparable is in module java.base of loader 'bootstrap')
    at java.base/java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:320)
    at java.base/java.util.ComparableTimSort.sort(ComparableTimSort.java:188)
    at java.base/java.util.Arrays.sort(Arrays.java:1316)
    at java.base/java.util.Arrays.sort(Arrays.java:1510)
    at java.base/java.util.ArrayList.sort(ArrayList.java:1717)
    at simulator.model.Road.advance(Road.java:115)
    at simulator.model.TrafficSimulator.advance(TrafficSimulator.java:44)
    at simulator.control.Controller.run(Controller.java:54)
    at simulator.launcher.Main.startBatchMode(Main.java:136)
    at simulator.launcher.Main.start(Main.java:144)
    at simulator.launcher.Main.main(Main.java:156)

Should Vehicle implement the Comparable interface, even if I'm using a Comparator? Or am I missing something?

Thank you in advance

Answer 1

You appear to be passing null to List.sort instead of your Comparator . In this case, and some others, a null Comparator is interpreted as meaning use the elements' Comparable interface.