Single-stepping does not reach every Java statement.
     
     We have a Java class similar to the following pseudo-code:
     
        1: void foo ()
        2: {
        3: bar ();
        4: x = 1;
        5: y = 2;
        6: }
     
     I set a breakpoint at line 3, and execute to that line. If from there
     I repeatedly single-step, I would expect to reach line 4 after the
     subroutine `bar' returns. This does not happen, however. After
     stepping an indeterminate number of times (it seems to vary),
     execution will proceed as though it were continuing from a breakpoint,
     not stepping. If I have a breakpoint set at line 5, it will hit that
     breakpoint without ever reporting a single-step to line 4.

This sounds familiar to 4046670, which has been fixed, but they are using sun.tools.debug.RemoteThread's "step" method instead of its "next" method.