Error creating and casting an array of objects

Question

I am trying to create an iterator in which I am creating an array of objects. I have to type cast them as Generics array creation is disallowed.

I am getting a run time error

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [LRandomizedQueueList$Node;

The full code of the iterator is shown below. I don't know what I am doing wrong.

   private class RandomizedQueueIterator implements Iterator<Item> {
       private Node current = first;
       private Node[] ca = (Node[])new Object[size()];
       //private Node[] ca = new Node[size()];

       private RandomizedQueueIterator() {
           if (first == null) throw new NoSuchElementException();
           for (int j = 0; j < size(); j++) {
               ca[j] = current;
               current = current.next;
           }                        
           StdRandom.shuffle(ca);
           current = ca[0];
       }

       public boolean hasNext()  { return current != null; }

       public Item next() {
           if (current == null) throw new NoSuchElementException();
           Item item = current.item;
           current = current.next; 
           return item;
       }
       public void remove() {
           throw new UnsupportedOperationException("This method is not supported");
       }
   }

I appreciate any help in understanding this error.

Why are you creating it as a new Object[size()]? You can just do new Note[size()]... — Pokechu22
– Pokechu22, Commented Oct 19, 2014 at 6:41
Because I cannot create an array of generics. Only an array of Object can be created. If I try to do new Node[size()], it gives me an error. — Ramesh Manian
– Ramesh Manian, Commented Oct 19, 2014 at 6:43
Node is not a generic type here. As the exception message indicates, it's a nested class of RandomizedQueueList — JB Nizet
– JB Nizet, Commented Oct 19, 2014 at 6:44
@RameshManian If Node is a generic type parameter, you should clearly state it in your question. The code you posted doesn't show it. — Eran
– Eran, Commented Oct 19, 2014 at 6:45

BobTheBuilder · Accepted Answer · 2014-10-19 06:42:05Z

1

Use:

private Node[] ca = new Node[size()];

There is no need to cast the array when you create it. You can just create an array of Node.

answered Oct 19, 2014 at 6:42

BobTheBuilder

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JB Nizet · Accepted Answer · 2014-10-19 06:42:42Z

1

The exception is clear: you can't cast an Object[] to Node[]. Object[] is not a subclass of Node[].

Replace the code by

private Node[] ca = new Node[size()];

answered Oct 19, 2014 at 6:42

JB Nizet

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3 Comments

Ramesh Manian Over a year ago

This is not allowed in Java. It gives the error; Error: /Users/admin/algs4/assignment2/RandomizedQueueList.java:133: generic array creation

JB Nizet Over a year ago

Edit your question and post a complete example reproducing the problem. The exception message indicates that Node is a nested class of RandomizedQueueList, and not a generic type.

Ramesh Manian Over a year ago

Ah, I see. I did not realize that I could not create an array of Node, which is indeed a class inside of RandomizedQueueList.

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