public void setAttribute(String key, String value)
        {
            if (attributes == null)
                attributes = new HashMap<String, String>();

            attributes.put(key, value);
        }
Test ser/deser of polymorphic graph objects (references and cyclic dependencies).

@author David Yu
@created Jan 19, 2011
protostuff-runtime/src/test/java/io/protostuff/runtime/PolymorphicRuntimeGraphTest.java