Ans. ArrayList is not thread-safe whereas Vector is thread-safe. - In the Vector class each method is surrounded by a synchronized block and thus making Vector class thread-safe. - Both the ArrayList and Vector hold onto their contents using an Array. - When an element is inserted into an ArrayList or a Vector, the object will need to expand its internal array if it runs out of room. - A Vector defaults to doubling the size of its array, while the ArrayList increases its array size by 50 percent.