How to create a 64 bit Unique Integer in Java?

I need to create a 64 bit Unique Integer in Java so that collision chances are pretty less.

Is there any way, we can create a 64 bit integer in Java which is always Unique?

As of now I am using -

long number = System.nanoTime();

Is this the right way to generate 64 bit Unique Integer in Java or is there anything else I can try?

UPDATE:-

How about doing this way? Will this be unique?

UUID number = UUID.randomUUID();
long uniqueNumber = number.timestamp();