Java - Get instances of abstract class by <class ? extends SuperClass> with parameters

Question

Excuse me if the Title is rather vague.

Allow me to elaborate my problem:

Imagine I have a class called "Car", which is an abstract class. Now imagine I have multiple instances of Car, for example Audi, Volvo, Ferrari etc.

I want to store these instances in an enum class, so I can easily retrieve them by the enum. The problem, however, is that every instance has an parameter in it's constructor, that I cannot put as a final attribute within the enum. I need to get the instances of the superclass (with 1 parameter) created from .class.

Pseudo code

/* This is my super class */
public abstract class Car{
   public Car(Object param){ }
}

/* This is my instance */
public class Volvo extends Car{ 
   public Volvo(Object param){
      super(param);
   }
}

/* This is my other instance */
public class Ferrari extends Car{ 
   public Ferrari(Object param){
      super(param);
   }
}

The code above is a proper display of the classes I made. Allright, now the enum class:

public enum CarType{

   VOLVO(Volvo.class), FERRARI(Ferrari.class);

   private Class<? extends Car> instance;
   private CarType(Class<? extends Car> instance){
       this.instance = instance;
   }

   /* This is what I tried, NOT working*/
   public Car getCarInstance(Object param){
       try{
          return Car.class.getConstructor(instance).newInstance(param);
       }catch(Exception e){
          /* I didn't do bugmasking, but all the exceptions would 
          make this  post look messy.*/
       }
   }
}

What I need as result: If I would've called 'CarType.VOLVO.getCarInstance("My parameter value"); ' it would be the same as 'new Volvo("my parameter value");'

Thanks in advance.

Answer 1

在getCarInstance更改行后，尝试执行以下操作：

return instance.getConstructor(Object.class).newInstance(param);

Answer 2

You don't need Car.class because you already specified the type in your enum constructor. By the way, don't call it instance , call it type .

Here we go:

public Car getCarInstance(Object param) {
    try {
        return type.getConstructor(Object.class).newInstance(param);
    } catch (ReflectiveOperationException e) {
        throw new IllegalStateException(e);
    }
}

The second thing (as you can see) is, how to retrieve the correct constructor. Read more about reflection if you want to know more.