Debian squeeze apparently does not include OpenJDK 7 yet.
Availability of OpenJDK 7 in Debian: http://packages.debian.org/search?keywords=openjdk-7-jdk
As you might know, early version of Java 7 had a number of issues. Ranging from excessive memory use with AWT to security issues (ok, IIRC these also affected Java 6).
So, at the time Debian squeeze was released, OpenJDK 7 was not ready to be used (not yet "stable").
So you have three options:
- Upgrade to the next version, wheezy
- Try to build the openjdk-7 package from wheezy on squeeze (should work, but it is not on backports yet!)
- Manually install openjdk-7
The first option has one big big big advantage: you get automatic security updates. And if you need more up to date software than is in current stable, you may have to use the next-stable version.
With the second option, you should at least have an easy way of making Java 7 your default java. With the unmanaged installation (option 3) this is much more tricky to get working reliably.
Anyway, I would go with the first option.