Cannot cast HashMap type to object which is HashMap itself and has HashMap object inside

Question

I had to modify current code to fit into Spring MVC. I had

HashMap hashmap = new HashMap();
request.setAttribute("dslrErrors", hashmap);

Now I modified the method to pass HashMap back to (method in) controller:

showHTMLResponse method:

@RequestMapping(value = "/s")
public String showHTMLResponse(@ModelAttribute("dslrs") DSLR dslrs[],
                               @ModelAttribute("dslr") DSLR dslr,
                               @ModelAttribute("dslrErrors") HashMap<?> dslrErrors,
                               @ModelAttribute ("dslrform") DSLRForm dslrForm,
                               @RequestParam("id") String paramId,
                               @RequestParam("action") String paramAction,
                               Model model){
// stuff
...
HashMap<String,Object> dslrHashMap = getDSLRById(paramId);
        dslr = (DSLR) dslrHashMap.get("dslr");
        dslrForm = (DSLRForm)dslrHashMap.get("dslrForm");
        dslrErrors = (HashMap<>)dslrHashMap.get("dslrErrors");
...
}

getDSLRById method:

    ...
     HashMap<String,Object> map = new HashMap<String, Object>();
     map.put("dslr", dslr);
     map.put("dslrform", dslrForm);
     map.put("dslrErrors", new HashMap());    

     return map;
    ...

Problem:

This line: dslrErrors = (HashMap<>)dslrHashMap.get("dslrErrors");

and this:

@ModelAttribute("dslrErrors") HashMap<?> dslrErrors

How to properly cast HashMap element/object and assign to Model so the View would be able to access it?

Answer 1

The following line does not compile, due to HashMap taking two generic parameters (as said by Tom G in a comment)

@ModelAttribute("dslrErrors") HashMap<?> dslrErrors;

Simply said, I would expect something along the lines of:

@ModelAttribute("dslrErrors") HashMap<?,?> dslrErrors;

The first parameter is the key, the second parameter is the value.

An example of how the above would like in an instantiation situation:

HashMap<?,?> myMap = new HashMap<String, String>();

And one last note to add to that; unless you need hashmap specific methods, prefer to use the interface as a best practise, as it makes it easier to switch implementation without changing alot of code

Map<?,?> myMap = new HashMap<String, String>();
@ModelAttribute("dslrErrors") Map<?,?> dslrErrors;

As for the other line of code;

dslrErrors = (HashMap<>)dslrHashMap.get("dslrErrors");

I can think of 2 ways ontop of my head (don't have an editor in front of me at the moment to verify:

    dslrErrors = (HashMap)dslrHashMap.get("dslrErrors"); //no diamond <> operator in the cast
    dslrErrors = (HashMap<Object, Object>)dslrHashMap.get("dslrErrors"); //note how we specify the type in the diamond operator here

I hope this helps you. Feel free to ask if any question remains.

Some source about programming to an interface: Program to an interface