#vulnerable code 
                                public HashSet<Constraint> getConstraints() {

        HashSet<Constraint> constraints = new HashSet<Constraint>();

        Set<String> ids = variablesHashMap.keySet();
        for (String s : ids) {
            Domain d = variablesHashMap.get(s).dom();
            ArrayList<Constraint> c = d.constraints();
            constraints.addAll(c);
        }

        return constraints;
    } 
                            #location 7 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public HashSet<Constraint> getConstraints() {

        HashSet<Constraint> constraints = new HashSet<Constraint>();

        for (Var v : variablesHashMap.values()) {
            Domain d = v.dom();
            ArrayList<Constraint> c = d.constraints();
            constraints.addAll(c);
        }

        return constraints;
    }