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)?

Answer 1

You certainly can:

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

See Programming with Assertions for more information.

Answer 2

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.

Answer 3

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.

Answer 4

If you use

assert Expression1 : Expression2 ;

Expression2 is used as a detail message for the AssertionError.