On most pre-iPhone cellphones you could install any .jar file, but there were 2 challenges:
Finding a .jar file that works on your phone was the biggest. Games often only supported a single screen resolution and so there were multiple .jar files for each game and you had to find the right one for your phone. Also sometimes even if you had the right screen resolution the .jar just crashed when you started it without any clue as to what's wrong (probably they needed more RAM or some platform specific code, but I was in high school back then so I didn't know much more about it)
In the early days, there was also the issue of how to get that .jar file to your phone. I distinctly remember old Nokia phones could download them through WAP (which was paid) or receive them through IrDA/Bluetooth, but themselves couldn't send applications through IrDA/Bluetooth (I think Sony Ericssons were the ones which could also send them....), This issue was later solved by microSD cards and USB cable transfers from PC.
I've never had much trouble finding compatible JAR/JAD files from legitimate sources. Usually, either your phone's model was auto detected to serve a compatible file, or you could select an appropriate version for apps that had resolution limitations.
Pirated JAR/JAD files were definitely hit and miss, but I don't think those are a great example.
Finding a .jar file that works on your phone was the biggest. Games often only supported a single screen resolution and so there were multiple .jar files for each game and you had to find the right one for your phone. Also sometimes even if you had the right screen resolution the .jar just crashed when you started it without any clue as to what's wrong (probably they needed more RAM or some platform specific code, but I was in high school back then so I didn't know much more about it)
In the early days, there was also the issue of how to get that .jar file to your phone. I distinctly remember old Nokia phones could download them through WAP (which was paid) or receive them through IrDA/Bluetooth, but themselves couldn't send applications through IrDA/Bluetooth (I think Sony Ericssons were the ones which could also send them....), This issue was later solved by microSD cards and USB cable transfers from PC.