### Question:
SchemaFactoryImpl implements SchemaFactory, SchemaResolver { @Override public void registerSchemaSource(final SchemaSource schemaSource) { if (schemaSource == null) { throw new IllegalArgumentException( "Schema source must not be null."); } if (LOG.isDebugEnabled()) { LOG.debug("Register schema source for system id {}", schemaSource .getSource().getSystemId()); } schemaSources.add(schemaSource); } private  SchemaFactoryImpl(); @Override void registerSchemaSource(final SchemaSource schemaSource); @Override void addPostProcessor(
            final SchemaResolverPostProcessor postProcessor); @Override Schema getSchema(); @Override void refresh(); static final SchemaFactoryImpl INSTANCE; }### Answer:
@Test(expectedExceptions=IllegalArgumentException.class) public void testRegisterNullSchemaSource() { schemaFactory.registerSchemaSource(null); }