Installing default-jdk, uninstalling it, doing apt-get autoremove, and reinstalling it leaves you without /usr/bin/javac ?

Oh, and the host OS was Ubuntu 12.04.4 LTS, if that matters.

The postinst script for openjdk-6-jdk_6b27-1.12.6-1ubuntu0.10.04.4_amd64.deb says:

case "$1" in
configure)
    # obsolete tool
    if update-alternatives --list apt 2>/dev/null; then
        update-alternatives --remove-all apt || true
    fi

    if [ -z "$2" ]; then
        update_alternatives=y
    fi

...

https://www.debian.org/doc/debian-policy/ch-maintainerscripts.html says $2 is " most-recently-configured-version".

Thus it looks like a thinko ... the author of the postinst script thought $2 meant something else, or forgot that the postrm script removed the alternatives?

The same test appears to be present in ubuntu 12.04's openjdk-7-jdk_7u51-2.4.4-0ubuntu0.12.04.2_amd64.deb,
but I haven't verified yet whether the problem occurs there as well.

Running 'apt-get purge openjdk-6-jdk' instead of (or after) removing openjdk-6-jdk appears to work around
the problem; the next install does put /usr/bin/javac in place.

Correction: apt-get purge openjdk-6-jdk *after* uninstalling it does not work around it (it refuses to purge
because the package isn't installed).
The correct workaround if you know the package is uninstalled is sudo dpkg --purge openjdk-6-jdk.

The problem is present if the package is not installed but is configured, e.g. if you see:

$ dpkg-query -l openjdk-6-jdk
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Cfg-files/Unpacked/Failed-cfg/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Description
+++-=========================-=========================-==================================================================
rc openjdk-6-jdk 6b27-1.12.6-1ubuntu0.10.0 OpenJDK Development Kit (JDK)

instead of

$ dpkg-query -l openjdk-6-jdk
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Cfg-files/Unpacked/Failed-cfg/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Description
+++-=========================-=========================-==================================================================
un openjdk-6-jdk <none> (no description available)