MapWritable and SortedMapWritable - Writable problems

When using the Writable interface for MapWritable and SortedMapWritable there are two errors:

• in readFields, if there are a number of entries of a class that is not one of the "predefined" classes, the following exception is thrown:
  java.lang.IllegalArgumentException: Class <not predefined class name> already registered
• readFields did not set the number of non-predefined classes. Consequently, making a copy of a copy that had entries of non-predefined classes would fail because the second copy would not receive the mapping from id to class and a NullPointerException would be thrown