It looks like you're trying to run Java 8 code on a Java 7 runtime. (Java is
backwards-compatible, but not forwards-compatible!
If you're triggering it from the command line you need to set the right
environment variables. On Trisquel there are helper applications that can
assist you. For example:
sudo update-alternatives --config java
This will show you all the versions of Java you have installed. Simply make
sure you select version 8 for your code to work.
If this doesn't fix your problem then I'm guessing you need to change the
configuration of your project in Geany. (I've not used that particular IDE
before, so I'm going by what the usual solution would be in Eclipse/IntelliJ
CE)
