java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class

Question

I'm trying to implement a trie in Java. When I test if my method "insert" works, I got the error below.

Exception in thread "main" java.lang.ClassCastException: 
class [Ljava.lang.Object; cannot be cast to class [LTrie$TreeNode; ([Ljava.lang.Object; 
is in module java.base of loader 'bootstrap'; [LTrie$TreeNode; is in unnamed module of loader 'app')
at Trie.insert(Trie.java:36)
at Trie.main(Trie.java:47)

I also went through some similar questions but still can't figure it out. I'm wondering why we can't cast the type from Object to TreeNode[] array.

My code is as follows. I know using dataIndexedCharMap by array wasted a lot of memory (HashTable or BST would be a better way), but I'm just trying to compare the memory and efficiency of these three methods but got some trouble.

class Trie {
    private static final int R = 128;
    public TreeNode root;

    private static class TreeNode {
        private boolean isKey;
        private dataIndexedCharMap next;
        private TreeNode(boolean b, int R) {
            isKey = b;
            next = new dataIndexedCharMap<TreeNode>(R);
        }
    }

    public static class dataIndexedCharMap<V> {
        private V[] items;
        public dataIndexedCharMap(int R) {
            items = (V[]) new Object[R];
        }
    }

    /** Initialize your data structure here. */
    public Trie() {
        root = new TreeNode(false, R);
    }

    /** Inserts a word into the trie. */
    public void insert(String word) {
        TreeNode curr = root;
        for (int i = 0; i < word.length(); i++) {
            TreeNode[] array = (TreeNode[]) curr.next.items;
            array[word.charAt(i)] = new TreeNode(false, R);
            curr = array[word.charAt(i)];
            if (i == word.length() - 1) {
                curr.isKey = true;
            }
        }
    }

    public static void main(String[] args) {
        Trie obj = new Trie();
        obj.insert("apple");
    } 
}

Answer 1

From your code it looks like dataIndexedCharMap is always going to have items of type TreeNode. In that case, there is no need to create an object array and cast it to TreeNode. You can create TreeNode[] array instead of Object[] array and remove all the generics that you have added.

public static class dataIndexedCharMap {
    private TreeNode[] items;
    public dataIndexedCharMap(int R) {
        items =  new TreeNode[R];
    }
}