Linkedlist.contains(x) can't generate test cases

[idiomaticrefactoring]

Hello,
when I use example/TestArray.java,I found when I replaced ArrayList in LinkedList.It can't generate testcases.
I am confused why it can't generate constraints just like ArrayList ?
it's my .jpf and source code:

    target=TestArray
    classpath=${jpf-symbc}/build/examples
     symbolic.method =testLinkedList(sym)
      symbolic.dp = z3
      symbolic.min_int=-100
      symbolic.max_int=100symbolic.debug=on
      search.multiple_errors=true
     listener = gov.nasa.jpf.symbc.sequences.SymbolicSequenceListener

public static void testLinkedList(int x) {		
	LinkedList<Integer> ll = new LinkedList<Integer>();
	 ll.add(0);
	 if(ll.contains(x)){
		 System.out.println("arrList.contains(x): " ); }
}