[Junit-devel] assertEquals: String compare with context diff

[<gu...@st...>]

The method assertEquals(String, String) is very useful, but breaks
down with large strings.  The problem is that, if the strings differ
in only a few characters, or start to differ more than a few lines in,
the error message is almost completely useless.  

expected:<HUGE STRING> but was:<NEARLY IDENTICAL HUGE STRING>

When you add in line breaks it gets impossible to "eyeball" the error
message and know what's going wrong.  It also breaks up the format of
the output report with miles of strings, possibly including line
breaks.

I've written some code that fixes this.  The output for a failed test
with message "HTML Report" is:

HTML Report: strings differ at character 22
Expected: ...>Poll:</b> dummy<br>\n<b>Question:</b> Vote for your favor...
  Actual: ...>Poll:</b> dummy<br>\r\n<b>Question:</b> Vote for your favo...

Note that you can tell right off the bat that the problem is a
difference in line endings (since my code escapes those).  This
particular bug was actually impossible for me to track down using the
normal failure message, since line endings are invisible.

I'd like to contribute the code to JUnit.  I'm just not sure where you
want to put it.

I see three options:

1. Change the behavior of the existing Assert.assertEquals(String, String)

	(I don't think this would break anybody's tests, and I doubt
	anyone has written code that depends on the actual format of
	the string error messages.  I could be wrong, of course.)

2. Add a new assertEqualsDiff to class Assert

	(This would mean that the benefit of the new messages would
	not be given to existing tests without massive test code
	rewrites.)

3. Make a new DiffTestCase that overrides assertEquals

	(Same problem as above -- existing test code would need to be
	rewritten.  It's also a sloppy substitute for mixins...)

If you guys want the code, let me know, and I'll download the JUnit
source and give you a patch.

 - A

P.S.  I've attached the current version below.  I don't think this is
the final version, though -- I want it to be smarter about diffs close
to the start and end of the strings.

-- 
Alex Chaffee                       mailto:al...@jg...
jGuru - Java News and FAQs         http://www.jguru.com/alex/
Creator of Gamelan                 http://www.gamelan.com/
Founder of Purple Technology       http://www.purpletech.com/
Curator of Stinky Art Collective   http://www.stinky.com/


    void assertEqualsDiff(String message, String expected, String actual)
    {
	if (!expected.equals(actual)) {
	    if (expected.length() < 20 || actual.length() < 20) {
		assertEquals(message, expected, actual);
		return;	// we'll get an exception, but just in case
	    }
	    int at = strdiffat(actual, expected);
	    String expectedContext =
		Utils.abbreviate(expected.substring(at - 20), 60);
	    String actualContext =
		Utils.abbreviate(actual.substring(at - 20), 60);
	    assertTrue(message + ": strings differ at character " + at + "\n" +
		       "Expected: ..." + Utils.jsEscape(expectedContext) + "\n" +
		       "  Actual: ..." + Utils.jsEscape(actualContext) + "\n",
		       false);
	}
    }

    /**
     * returns the index where the strings diverge<p>
     * strdiff("i am a machine", "i am a robot") -> 7<p>
     * @return -1 if they are the same
     *
     **/
    public static int strdiffat(String s1, String s2) {
	int i;
	for (i=0; i<s1.length() && i<s2.length(); ++i) {
	    if (s1.charAt(i) != s2.charAt(i)) {
		break;
	    }
	}
	if (i<s2.length() || i<s1.length())
	    return i;
	return -1;
    }

for abbreviate() and jsEscape() see http://www.purpletech.com/code/src/com/purpletech/util/Utils.java