Getting the ALT key working

If you are using KDE you may want to modify the ALT key behaviour:
Open Control Center --> Desktop --> Window Behaviour --> Actions

And change the modifier key to something else.

Otherwise you can use right alt key (Alt Gr) only.

Java Virtual Machine

When using Ubuntu with Gnome you could install JOSM from the repositories. If you haven't installed java please do so. Then you could start JOSM in the bash with the command josm. Sometimes there is an error like this:

No valid JVM found to run JOSM.
Please install one of those mentioned in README.Debian
or set the JAVACMD environment variable prior running josm.

You have to make a symbolic link in the folder /usr/bin/ like this:

  sudo ln -s /usr/bin/java /usr/bin/jamvm

Now you should be able to start JOSM.