Java generics class cast exception

Question

I am trying to create a class that processes comparables. I boiled down to the simplest code that gives an error when I try to instantiate the class. I get a couple of compile warnings (unchecked cast) but when I run this program it throws a classcast exception. I did look at some of the other questions on this topic but didnt come across something useful.

public class GD<Item extends Comparable<Item>> {
   private Item[] data;
   private final int MAX_SIZE = 200;
   public GD() {
      data = (Item[]) new Object[MAX_SIZE];
   }

   public static void main(String[] args) {
     GD<String> g = new GD<String>();
   }
}

Answer 1

The problem lies here:

data = (Item[]) new Object[MAX_SIZE];

You are instantiating an array of Object and then you try to cast it as an array of Item, which throws an exception because Object does not extend your Item class, because it does not implement Comparable. What you would like instead is:

data = new Item[MAX_SIZE];

But you can't do this because Item is a generic type. If you want to create objects (or arrays of objects) of this type dynamically, you need to pass the Class object to your GD class's constructor:

import java.lang.reflect.Array;

public class GD<Item extends Comparable<Item>> {
   private Item[] data;
   private final int MAX_SIZE = 200;
   public GD(Class<Item> clazz) {
      data = (Item[]) Array.newInstance(clazz, MAX_SIZE);
   }

   public static void main(String[] args) {
     GD<String> g = new GD<String>(String.class);
   }
}