Question

I read the following statement:

ArrayLists are unsynchronized and therefore faster than Vector, but less secure in a multithreaded environment.

I would like to know why unsynchronization can improve the speed, and why it will be less secure?

Answer 1

I will try to address both of your questions:

Improve speed

If the ArrayList were synchronized and multiple threads were trying to read data out of the list at the same time, the threads would have to wait to get an exclusive lock on the list. By leaving the list unsynchronized, the threads don’t have to wait and the program will run faster.

Unsafe

If multiple threads are reading and writing to a list at the same time, the threads can have unstable view of the list, and this can cause instability in multi-threaded programs.