Enabling assertion in eclipse

I think all of us(who have tried to compile and run the code in chapter 13, page 307). Here output won't be same as the book states.
The problem is..assertion is not by default enabled in eclipse. So your program would run to negative values without throwing assertion exception.
Don't worry..there is always a way.
Go to eclipse Window>preference>java>installed JRE's>
select the one and click edit. In the window that appears, in the text box named 'Default VM arguments' write "-ea", without the quote. save it and close it.
Now run that program again, isn't it showing the result same as the book?

in chapter 13, page 307).

which book?

Java, The Complete Reference 7th edition by Herbert Schildt. thnx.