The implementation of Comparable.compareTo(Object) is faulty regarding the handling of version numbers where a build number is present but zero. Here's a unit test to demonstrate:
DefaultArtifactVersion v1 = new DefaultArtifactVersion("2.0");
DefaultArtifactVersion v2 = new DefaultArtifactVersion("2.0-0");
DefaultArtifactVersion v3 = new DefaultArtifactVersion("2.0-alpha1");
// v1 and v2 are equal
assertTrue( v1.compareTo(v2) == 0 );
assertTrue( v2.compareTo(v1) == 0 );
// v1 is newer than v3
assertTrue( v1.compareTo(v3) > 0 );
assertTrue( v3.compareTo(v1) < 0 );
// ergo, v2 should also be newer than v3
assertTrue( v2.compareTo(v3) > 0 ); // FAILS!! returns 0 (equal)
assertTrue( v3.compareTo(v1) < 0 ); // succeeds
This test demonstrates this behavior violates symmetry and transitivity.
Luckily, the fix is easy. A small change to the compareTo method to consider qualifiers before build numbers relieves this issue. I have a patch that I'll submit as soon as I have the issue number.