Array<addObject> objs = new Array<addObject>(20);

addObject obj = new addObject();
    objs.add(obj);

Array<addObjectPosition> positions = new Array<addObjectPosition>(20);

int size = objs.items[index].positions.size;

Caused by: java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Lcom.mgsoft.addObject;

...
// Convert to list
List<addObjectPosition> list = Arrays.asList(objs.items[index].positions);
// Get Size
int size = list.size();