Previously I would always have thought a Vector was good to use for non-descript objects when length was unknown. As far as I was aware I thought it was thread-safe too
What would change that it shouldn't be used anymore, and what is the alternative?
You should use ArrayList instead of Vector. Vector used internal synchronisation, but that is rarely good enough for actual consistency, and only slows down execution when it is not really needed.
Also see this stackoverflow question.
ArrayList is now the better class to use. Vector is now considered Legacy, and has the added performance overhead of being Thread-Safe.
You can use an ArrayList instead.
If you need a synchronized version, you can do something like:
ArrayList arrayList = new ArrayList();
List synchList = Collections.synchronizedList(arrayList);
Use ArrayList when you need a List implementation but don't need thread safety, and use CopyOnWriteArrayList when you need a List implementation that is thread safe.
Vector is a legacy collection class from Java 1.0. In Java 1.2 (long ago!), the Collections Framework was added which included new collection classes such as ArrayList and HashMap, which were intended to replace the legacy classes Vector and Hashtable.
As said before, the legacy collection classes had built-in synchronization, which is unnecessary for many applications. Synchronization has a performance overhead, so if it's not necessary, you shouldn't use it.
In some cases (when your program is multi-threaded, and multiple threads access the same data) you need to synchronize your collections. Some people would then use the old Vector or Hashtable classes, but a better way is to use a synchronization wrapper with for example an ArrayList:
// Your standard, unsynchronized list
List<String> data = new ArrayList<String>();
// Use this to put it into a synchronization wrapper
List<String> syncedData = Collections.synchronizedList(data);
See the API documentation of Collections.synchronizedList() (and other methods) for more information.