### Question:
UndefinedNode extends AbstractExprNode { @Override public Object accept( FilterVisitor visitor ) { return null; } private  UndefinedNode(); @Override boolean isLeaf(); @Override Object accept( FilterVisitor visitor ); @Override boolean isSchemaAware(); @Override String toString(); static final UndefinedNode UNDEFINED_NODE; }### Answer:
@Test public void testAccept() throws Exception { assertNull( undefinedNode.accept( new FilterVisitor() { public Object visit( ExprNode node ) { return null; } public boolean isPrefix() { return false; } public List<ExprNode> getOrder( BranchNode node, List<ExprNode> children ) { return null; } public boolean canVisit( ExprNode node ) { return false; } } ) ); }