Hello.
Since Tekkit v3 was introduced a requirement on Java 7 was made. As a developer I know how important backwards compatibility is, and requiring a version of Java some can't get for one mod is pretty crazy.
I've been building Tekkit versions compatible with Java 1.6 since 3.0.4 with very minimal changes. I recommend that the developers take note and too do the same, then everyone is happy.
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The _diff has the changed files over the official version of Tekkit. Also of note, only NetherOres was compiled with Java 7 (with no target to 1.6 -- it doesn't use 1.7 features). CodeChickenCore only contains a fix for where it is hardcoded to only accept CRLF line endings not LF.
So, given how small of a change it is to make NetherOres work on 1.6 (-target it or edit it manually) I believe the tekkit devs should seriously consider fixing this. It will make everyone happier that they aren't forced to install a specific version of Java
:)