Which is more efficient, basic mutex lock or atomic integer?

Question

For something simple like a counter if multiple threads will be increasing the number. I read that mutex locks can decrease efficiency since the threads have to wait. So, to

Answer 1

The atomic variable classes in Java are able to take advantage of Compare and swap instructions provided by the processor.

Here's a detailed description of the differences: http://www.ibm.com/developerworks/library/j-jtp11234/