Enum is one of the strongest features in Java. They are used as a list of constraints. An enum can contain a constructor, a method, a field etc. This link goes into some more details, but we're going to cover one important aspect of them here. 

They don't support generics.

Let's check the following code:

public enum Cars {
    AUDI(new Audi()),
    ACURA(new Acura());

    private Vehicle vehicle;

    Cars(Vehicle vehicle) {
        this.vehicle = vehicle;
    }
    public Vehicle getVehicle() {
        return vehicle;
    }
}

public abstract class Vehicle {
}

public class Acura extends Vehicle {
    public int getSerialNo() {
        return 134;
    }
}

public class Audi extends Vehicle{
    public String getName() {
        return "Audio";
    }
}

We have two different methods in the two classes. Now we can access the enum using the following:

public class Main {
    public static void main(String[] args) {
        Vehicle vehicle = Cars.ACURA.getVehicle();
    }
}

But there is no way we can call the getSerialNo() method from this.

To solve this problem, a new JEP (JDK Enhancement Proposal) has submitted.

With this JEP, we will able to do following:

public enum Cars<T extends Vehicle> {
    AUDI<Audi>(new Audi()),
    ACURA<Acura>(new Acura());

    private T vehicle;

    Cars(T vehicle) {
        this.vehicle = vehicle;
    }

    public T getVehicle() {
        return vehicle;
    }
}

And, definitely, we will able to call the getSerialNo() method like following:

Cars.ACURA.getVehicle().getSerialNo();