I'm running Raspbian “wheezy”, a Debian based Linux distribution for the Raspberry Pi and I would like to know where I can change the desktop environment that boots with the OS. In particular, how can I replace LXDE with a custom QT or Java program?
You might be able to install Raspbian Lite to your Pi and install a different desktop, but this is not recommended because you may have WiFi connectivity problems. You may also be able to install a display manager with support for desktop switching, for example, GDM, then install a desktop. I don't think anyone has tried this before, and it may be extremely dangerous.