Notification of changes should be stored in block with a copyright? How it should look like?
The apache license is both clear and a tad vague and not very prescriptive for this:
4. Redistribution. You may reproduce and distribute copies of the
Work or Derivative Works thereof in any medium, with or without
modifications, and in Source or Object form, provided that You
meet the following conditions:
(b) You must cause any modified files to carry prominent notices
stating that You changed the files; and
Here if I modify the sources.... a notification comment would not end up in compiled Java class files or course: I would need to handle both source and binaries somehow.
For this, I would do the following to meet both the spirit and the letter of the license (and this may be a tad more than strictly needed):
In the source, I would add a note in the header comment stating that I modified the code, with a rough date and possibly a high level description of the modification. Doing that in a header comment at the top would be rather prominent to me. (Stating a date and the nature of the modification is not strictly needed, but I feel this is the right thing to do). It can look like anyway you want: the license does not say anything on the format (which is a good thing IMHO).
If the source did not had an Apache notice, I would add one as a comment header too. (even though this is may not be clear in the license, if the copyright/notice was missing I would add this for clarity and avoid any ambiguity)
If there was a NOTICE, I would mention there that the code was modified by me too, so this would be even more prominent. (even though this is may not be strictly needed by the license). If there were none, I would create one, so I could bundle it with the source and binaries. (even though this is may not be strictly needed by the license if there was no notice in the first place)
Finally I would ensure the license and notice file both end up in my sources and binaries redistributions (if I were to redistribute them).