How do I address unchecked cast warnings?

Solution 1:

The obvious answer, of course, is not to do the unchecked cast.

If it's absolutely necessary, then at least try to limit the scope of the @SuppressWarnings annotation. According to its Javadocs, it can go on local variables; this way, it doesn't even affect the entire method.

Example:

@SuppressWarnings("unchecked")
Map<String, String> myMap = (Map<String, String>) deserializeMap();

There is no way to determine whether the Map really should have the generic parameters <String, String>. You must know beforehand what the parameters should be (or you'll find out when you get a ClassCastException). This is why the code generates a warning, because the compiler can't possibly know whether is safe.

Solution 2:

Unfortunately, there are no great options here. Remember, the goal of all of this is to preserve type safety. "Java Generics" offers a solution for dealing with non-genericized legacy libraries, and there is one in particular called the "empty loop technique" in section 8.2. Basically, make the unsafe cast, and suppress the warning. Then loop through the map like this:

@SuppressWarnings("unchecked")
Map<String, Number> map = getMap();
for (String s : map.keySet());
for (Number n : map.values());

If an unexpected type is encountered, you will get a runtime ClassCastException, but at least it will happen close to the source of the problem.