Fail-Safe concept in collection

1. Difference between Hashtable and HashMap.

• Hashtable is synchronized, HashMap is not.
• Iterator in the HashMap is fail-safe (meaning, it throws ConcurrentModificationException when try to add a value to the collection while iterating), but the enumeration for Hashtable is not fail-safe.
• HashMap points null values, while Hashtable doesn't.

If you want to synchronize a HashMap write the following code.
Map map = Collections.synchronizedMap(new HashMap());

2. Vector vs ArrayList.

• Vector is synchronized, ArrayList is not
• Iterator that are returned by both classes are fail-safe. Enumeration returned by Vector are not.

If you want to synchronize an ArrayList write the following code.
List list = Collections.synchronizedList(new ArrayList());