javac rebuilds files based on relative timestamps on the java and class files. This fails in two interesting and important cases:

First, clock skew in a network could make it so that a newer .java file had an older timestamp than the .class files. As customers work more over networks this will be a problem.

Second, some build configuration systems backdate files when you cancel edit on a file. This would lead to an older .java file than .class file, but the .class file should be rebuilt anyway. Atria does this, and some systems layered on RCS or SCCS do it, too.

We should store the actual timestamp of the .java file in the .class file and recompile if the current .java timestamp is different. In effect the timestamp can act analagously to a quick checksum on the file.