Java assert - what I learnt today

I learnt something new today, the following I thought would be just fine:

File f = new File("foo.txt");

if( f.exists() )

assert f.delete();

That all looked good to me, right up to the point that I discovered that an assert expression isn't even evaluated if asserts are not enabled, had to change it to.

if( f.exists() ){

boolean deleted = f.delete();

assert deleted;

}