If you run this code you will see a page with 3 checkboxes one for each hobby and a submit button. The checkboxes are populated by the hobbyList modelAttribute. Notice I pulled this out into its own method. I could have populated this in the get method as well, however I would have had to change its type from List to ArrayList in the parameter list as Spring would otherwise try to instantiate an interface. I like coding to interfaces so I showed you the other approach.
The User object's hobbies attribute is bound to this checkbox form element so when you click submit if you set a break point in the userSubmit method you will see that the hobbies String on the User object will contain whatever values were checked on the JSP. After that the program will bomb as I am returning "someOtherView" which is of course not defined but you get the idea.