Hello Tayitu Betule,
I am studying for the SCJP exam, and I am not an expert.
To reply to your post I think the code prints out "done" because the assertions are ignored,
there is nothing that tell us that the assertions are enabled, thus at run time these lines of code are not executed.
To enable assertions it needs to be specified with a flag in the command line:
java -enableassertions AssertTest
or with the short flag, which is equivalent
java -ea AssertTest
Regards,
Giorgio