but when you add one element to an ArrayList..it will allocate dynamically size 1 right?
It depends on how you add an element. If you use the method add(Object) of the type List the list will be resized. If you use add(int, Object) you will get an IndexOutOfBoundsException.
please..tell me the default size of vector, hashtable and hashMap
You can easily adapt the above program to find this out.