failNotSame message could emphasize reference equality

[sabi0]

When calling assertSame for two different instances of identical objects the failure message will be quite confusing: "expected same:<[1, 2]> was not:<[1, 2]>".
See

junit4/src/main/java/org/junit/Assert.java

Line 829 in 8c9e81d

fail(formatted + "expected same:<" + expected + "> was not:<" + actual

I think the clarity could be improved by adding identityHashCode to the message, like this:
"expected same:<[1, 2]@ba38e> was not:<[1, 2]@de440>"
Or maybe even with the class name:
"expected same:<java.util.List@ba38e [1, 2]> was not:<java.util.List@de440 [1, 2]>"

[panchenko]

There are cases when identityHashCode is not needed, like enum or when it's already present in toString(), like default implementation in Object. It should not be added always for sure, a good question if it's worth determining cases like Collection or leave the implementation as is.

[sabi0]

Well, technically enum values could also produce identical toString() output. And though one wouldn't normally do that such assumption can not be made JUnit code.

Instead of targeting specific cases why not just address the essence of the issue - identical toString() outputs?
How about this?

String expectedString = String.valueOf(expected);
String actualString = String.valueOf(actual);
if (expectedString.equals(actualString)) {
    if (expected != null) {
        expectedString = expected.getClass().getName() + '@' + System.identityHashCode(expected) + ' ' + expectedString;
    }
    if (actual != null) {
        actualString = actual.getClass().getName() + '@' + System.identityHashCode(actual) + ' ' + actualString;
    }
}
fail(formatted + "expected same:<" + expectedString + "> was not:<" + actualString + '>');

Then even cases like assertSame(null, "null") would produce a clear output:

expected same:< null> was not:<java.lang.String@de440 null>

[marcphilipp]

@sabi0 FWIW JUnit Jupiter includes the identity hash code if both strings are equal. https://github.com/junit-team/junit5/blob/b7897dbf9c70158f3fee76c56b9c1508b416968c/junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertionUtils.java#L115-L129