### Question:
SchemaFactoryImpl implements SchemaFactory, SchemaResolver { @Override public void addPostProcessor( final SchemaResolverPostProcessor postProcessor) { if (postProcessor == null) { throw new IllegalArgumentException( "PostProcessor must not be null."); } postProcessors.add(postProcessor); } 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 testAddNullPostProcessor() { schemaFactory.addPostProcessor(null); }