/**
  * Test removeAttribute(Serializable key), when key is null, IllegalArgumentException is expected.
  */
 public void testRemoveAttribute_KeyIsNull() {
   try {
     tester.removeAttribute(null);
     fail("when key is null, IllegalArgumentException is expected.");
   } catch (IllegalArgumentException e) {
     // good
   }
 }
 /**
  * Test setAttribute(Serializable key, Serializable value), when value is null,
  * IllegalArgumentException is expected.
  */
 public void testSetAttribute_ValueIsNull() {
   try {
     tester.setAttribute("key", null);
     fail("when value is null, IllegalArgumentException is expected.");
   } catch (IllegalArgumentException e) {
     // good
   }
 }