shell_map is a normal function. The trick is when you call shell_map new my_map shell_map will copy itself to a new function named my_map, so, instead of having an object instance, what you have is a function instance.
When you add data to your map using my_map put "somekey" "somevalue", a variable will be added to the global scope under the name of my_map_DATA_somekey, for this reason, key strings can only be made of letters, digits and underscores.
When retrieving data with my_map get somekey the value from the global variable my_map_DATA_somekey will be returned, this way the resulting functionallity works pretty much like a HashMap.
Each method name is actually the "$1" argument given to shell_map function, method arguments are the $2, $3...