Generic class constructor failing with a class cast exception error at runtime

Question

I'm trying to make my own generic linkedlist data type but am having trouble with the constructor and am getting a class cast exception. Anyone know how to fix this and WHY this is happening?

Here is my relevant code:

public class SinglyLinkedList<E> {

   private LinkedListNode<E>[] linkedListNodeList;

   public SinglyLinkedList()
   {
       linkedListNodeList = (SinglyLinkedListNode<E>[]) new Object[10];
   }

}

The offending line is the implementation line in the constructor.

Here is my SinglyLinkedListNode class:

public class SinglyLinkedListNode<E> extends LinkedListNode<E>{

   private E data;

   public SinglyLinkedListNode(E data)
   {
       this.data = data;
   }
}

And my LinkedListNode class is simply an empty (for now) abstract class that SinglyLinkedListNode extends.

Here is the compiler error I'm receiving:

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [LabstractDataTypes.SinglyLinkedListNode; at abstractDataTypes.SinglyLinkedList.(SinglyLinkedList.java:23)

The type of new Object[10] is Object[]. It's not SinglyLinkedListNode<E>[]. Casting an object to a type doesn't change the type of the object. You can't cast a Car to a Bird and hope that the car will transform itself to a bird. You can cast an Animal to a Bird if the animal actually is a bird. That said, why would you need an array of nodes in a linked list? Have you understood what a linked list is? — JB Nizet
– JB Nizet, Commented Nov 23, 2013 at 13:39

isnot2bad · Accepted Answer · 2013-11-23 13:41:18Z

2

An Object[] is not compatible with any other array (e.g. String[]). But this will work:

public class SinglyLinkedList<E> {

   private LinkedListNode<E>[] linkedListNodeList;

   public SinglyLinkedList()
   {
       linkedListNodeList = (SinglyLinkedListNode<E>[]) new SinglyLinkedListNode<?>[10];
   }
}

Note that it is also not possible to use new SinglyLinkedListNode<E>[10], as generic arrays can't be created in general:

E[] myArray = new E[10]; // doesn't work, if E is a generic type parameter

answered Nov 23, 2013 at 13:41

isnot2bad

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