"Volatile” keyword

Consider this example:

int i = 5;
System.out.println(i);

The compiler may optimize this to just print 5, like this:

System.out.println(5);

However, if there is another thread which can change i, this is the wrong behaviour. If another thread changes i to be 6, the optimized version will still print 5. The volatile keyword prevents such optimization and caching, and thus is useful when a variable can be changed by another thread.