For example when using Preconditions.checkArgument, is the error message supposed to reflect the passing case or the failing case of the check in question?
import static com.google.common.base.Preconditions.*;
void doStuff(int a, int b) {
checkArgument(a == b, "a == b");
// OR
checkArgument(a == b, "a != b");
}
For precondition checks, stating the requirement in the exception detail message is more informative than simply stating the facts. It also leads to a lot more naturally reading code:
The documentation provides the following example:
This allows constructs such as
if (count <= 0) { throw new IllegalArgumentException("must be positive: " + count); }to be replaced with the more compact
checkArgument(count > 0, "must be positive: %s", count);
Two things are of note:
Following that example, a better message would be:
checkArgument(a == b, "a must be equal to b"); // natural!
You can even go a step further and capture the values of a and b in the exception message (see Effective Java 2nd Edition, Item 63: Include failure capture information in detail messages):
checkArgument(a == b, "a must be equal to b; instead %s != %s", a, b);
The alternative of stating facts is less natural to read in code:
checkArgument(a == b, "a != b"); // awkward!
checkArgument(a == b, "a is not equal to b"); // awkward!
Note how awkward it is to read in Java code one boolean expression, followed by a string that contradicts that expression.
Another downside to stating facts is that for complicated preconditions, it's potentially less informative, because the use