How to make a method thread safe without using synchronized keyword?

..oops i thought the post is for C#

You can use Lock Keyword, Monitor Class and InterLocked

Although not as simple as using the synchronized keyword, you can overcome some of its limitations using an 'explicit lock' in Java 5 or later.

Check this link for an explanation of what these are:

http://www.javamex.com/tutorials/synchronization_concurrency_9_locks.shtml