Java trie implementation

Trie.java

public class Trie {
    private int childValueLength = 1;
    private Map<String, Node> children = new HashMap<>();

    public void add(String value) {
        add(this, value);
    }

    private void add(Trie node, String value) {
        if (node.childValueLength == value.length()) {
            if (!node.children.containsKey(value)) {
                node.children.put(value, new Node(value));
            } else {
                throw new IllegalStateException("Value already exists");
            }
        } else {
            String valuePart = value.substring(0, node.childValueLength);

            if (node.children.containsKey(valuePart)) {
                add(node.children.get(valuePart), value);
            } else {
                Node partNode = new Node(valuePart);

                node.children.put(valuePart, partNode);

                add(partNode, value);
            }
        }
    }

    public Node find(String value) {
        return find(this, value);
    }

    private Node find(Trie node, String value) {
        if (node.childValueLength > value.length()) {
            return null;
        }

        String valuePart = value.substring(0, node.childValueLength);

        if (node.children.containsKey(valuePart)) {
            Node child = node.children.get(valuePart);

            if (valuePart.length() == value.length()) {
                return child;
            } else if (valuePart.length() < value.length()) {
                return find(child, value);
            }
        }

        return null;
    }

    private static class Node extends Trie {
        private final String value;

        private Node(String value) {
            this.value = value;
            super.childValueLength = value.length() + 1;
        }

        @Override
        public String toString() {
            return value;
        }
    }
}