You're trying to create files /hello.txt/test_file. The leading slash means you want to put them in the root of your file system. From the exception I see that you are using a Linux or UNIX file system (perhaps Mac OS even). The thing is, on Linux and UNIX systems, not every user can write to any location. Usually all users have only access to their own home folder, and the /tmp folder, with the exception of the root user who can do anything he wants on the system.
So, if you want to write to the root of the file system (which you really really really shouldn't!) you'll need to run your code as a user who has more rights. On Ubuntu you need to put sudo before the command, e.g. sudo java IOTest.File_1, and that will still fail if you're not allowed to run sudo.