How do I address unchecked cast warnings?

Question

Eclipse is giving me a warning of the following form:

Type safety: Unchecked cast from Object to HashMap

This is from a call to an API that I have no control over which returns Object:

HashMap<String, String> getItems(javax.servlet.http.HttpSession session) {
  HashMap<String, String> theHash = (HashMap<String, String>)session.getAttribute("attributeKey");
  return theHash;
}

I'd like to avoid Eclipse warnings, if possible, since theoretically they indicate at least a potential code problem. I haven't found a good way to eliminate this one yet, though. I can extract the single line involved out to a method by itself and add @SuppressWarnings("unchecked") to that method, thus limiting the impact of having a block of code where I ignore warnings. Any better options? I don't want to turn these warnings off in Eclipse.

Before I came to the code, it was simpler, but still provoked warnings:

HashMap getItems(javax.servlet.http.HttpSession session) {
  HashMap theHash = (HashMap)session.getAttribute("attributeKey");
  return theHash;
}

Problem was elsewhere when you tried to use the hash you'd get warnings:

HashMap items = getItems(session);
items.put("this", "that");

Type safety: The method put(Object, Object) belongs to the raw type HashMap.  References to generic type HashMap<K,V> should be parameterized.

Answer 1

A quick guess if you post your code can say for sure but you might have done something along the lines of

HashMap<String, Object> test = new HashMap();

which will produce the warning when you need to do

HashMap<String, Object> test = new HashMap<String, Object>();

it might be worth looking at

Generics in the Java Programming Language

if your unfamiliar with what needs to be done.

Answer 2

The problem lies in here:

... = (HashMap<String, String>)session.getAttribute("attributeKey");

The result of session.getAttribute(...) is an object which could be anything, but since you "know" it's a HashMap<String, String> you're just casting without checking it first. Thus, the warning. To be pedantic, which Java wants you to be in this case, you should retrieve the result and verify it's compatibility with instanceof.

Answer 3

If you are sure that the type returned by session.getAttribute() is HashMap then you can not typecast to that exact type, but rely on only checking the generic HashMap

HashMap<?,?> getItems(javax.servlet.http.HttpSession session) {  
    HashMap<?,?> theHash = (HashMap<?,?>)session.getAttribute("attributeKey");
    return theHash;
} 

Eclipse will then surprise warnings, but of course this can lead to runtime errors that can be hard to debug. I use this approach in not operation-critical contexts only.

Answer 4

If I have to use an API that doesn't support Generics.. I try and isolate those calls in wrapper routines with as few lines as possible. I then use the SuppressWarnings annotation and also add the type-safety casts at the same time.

This is just a personal preference to keep things as neat as possible.

Answer 5

This makes the warnings go away...

 static Map<String, String> getItems(HttpSession session) {
        HashMap<?, ?> theHash1 = (HashMap<String,String>)session.getAttribute("attributeKey");
        HashMap<String,String> theHash = (HashMap<String,String>)theHash1;
    return theHash;
}

Answer 6

Solution: Disable this warning in Eclipse. Don't @SuppressWarnings it, just disable it completely.

Several of the "solutions" presented above are way out of line, making code unreadable for the sake of suppressing a silly warning.