### Question:
RadixTreeNode { void addChildNode(RadixTreeNode node) { if (node == null || node._key == null || node._key.isEmpty()) { throw new IllegalArgumentException(_strings.get("errorInEmptyNullKeyInstertion")); } char c = node._key.charAt(0); if (_childNodes == null) { _childNodes = new HashMap<Character, RadixTreeNode>(); } RadixTreeNode oldNode = _childNodes.put(c, node); if (oldNode != null) { _logger.log(Level.WARNING, SLogger.CHILD_NODE_EXISTS, new Object[] {this.toString(), oldNode.toString(), node.toString()}); oldNode._parentNode = null; } node._parentNode = this; } RadixTreeNode(String key, String value); @Override String toString();  }### Answer:
@Test(expectedExceptions = IllegalArgumentException.class) public void testAdditionForNullNode() { RadixTreeNode rootNode = new RadixTreeNode(UNIT_TEST, UNIT_TEST); rootNode.addChildNode(null); }
@Test(expectedExceptions = IllegalArgumentException.class) public void testAdditionForEmptyChildKey() { RadixTreeNode rootNode = new RadixTreeNode(UNIT_TEST, UNIT_TEST); RadixTreeNode firstChildNode = new RadixTreeNode("", ""); rootNode.addChildNode(firstChildNode); }
@Test(expectedExceptions = IllegalArgumentException.class) public void testAdditionForNullChildKey() { RadixTreeNode rootNode = new RadixTreeNode(UNIT_TEST, UNIT_TEST); RadixTreeNode firstChildNode = new RadixTreeNode(null, ""); rootNode.addChildNode(firstChildNode); }