Can java's assert statement allow you to specify a message?

Question

Seems likes it might be useful to have the assert display a message when an assertion fails.

Currently an AssertionError gets thrown, can you specify a custom message for it?

Can you show an example mechanism for doing this (other than creating your own exception type and throwing it)?

score 176 · Accepted Answer · edited Sep 24 '21 at 15:06

176

You certainly can:

assert x > 0 : "x must be greater than zero, but x = " + x;

edited Sep 24 '21 at 15:06

turbanoff

answered Nov 07 '08 at 20:12

Greg Hewgill

matt burns · Answer 2 · 2015-07-22T09:51:33.230

22

assert (condition) : "some message";

I'd recommend putting the conditional in brackets

assert (y > x): "y is too small. y = " + y;

Imagine if you came across code like this...

assert isTrue() ? true : false : "some message";

Don't forget this has nothing to do with asserts you'd write in JUnit.

edited Jul 22 '15 at 09:51

answered Jan 29 '09 at 17:02

matt burns

3

There is no point in wrapping every expression in brackets just because a ternary would otherwise be slightly confusing. If you happen to have a ternary expression as your assertion, you can wrap that. (Although I would argue that if you have a ternary expression as your assertion, you have probably messed up & at the very least you should extract the expression into a named method call for clarity.) – laszlok May 01 '18 at 09:25

score 12 · Answer 3 · answered Nov 07 '08 at 20:14

12

It absolutely does:

assert importantVar != null : "The important var was null!";

This will add "The important var was null" to the exception that is thrown.

answered Nov 07 '08 at 20:14

Jason Coco

score 8 · Answer 4 · answered Nov 07 '08 at 20:14

8

If you use

assert Expression1 : Expression2 ;

Expression2 is used as a detail message for the AssertionError.

answered Nov 07 '08 at 20:14

Bill the Lizard

Just adding that AssertionError is thrown if Expression1 is false. – Italo Borssatto Dec 12 '13 at 16:31

4 Answers4