Some assignments offer a solution 'demo' that you can run to see the program's expected behavior. On many machines, all you have to do is download the .jar file, then double-click it to run it. But on some Macs, it won't work; your operating system will say that it doesn't know how to launch the file. If you have that issue, download the file, go to the Downloads folder in your Finder, right-click on the file, and click Open, and then press Confirm.
Some Mac users see a security error such as, "cs106b-hw1-life-demo.jar can't be opened because it is from an unidentified developer." To fix this, go to System Preferences → Security & Privacy. You will see a section about downloaded apps. You should see a prompt asking if you would like to allow access to our solution JAR. Follow the steps and then you should be able to open the demo.