### 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); }