Question

So I have a map:

Map<String, Class> format = new HashMap<String, Class>();

And I would add elements to it like this:

format.put("Vendor Number", Integer.class);
format.put("Vendor Dispatch", Date.class); 
....

I have a generic method as follows:

public static <T> T verifyType(String name, Class<T> type) {
    if (type == Integer.class) {
        return type.cast(new Integer(Integer.parseInt(name)));
    }
             ......
    return null;
}

Now this piece of code works great with no compiler issues:

Integer i = verifyType("100",Integer.class);

But, when I try this:

    Integer i = verifyType("100",format.get("Vendor Number"));

OR 

    Class type = Integer.class
    Integer i = verifyType("100",type);

Compiler shows me this warning:
Type safety: Unchecked invocation verifyType(String,Class) of the generic method verifyType(String, Class)

That leaves me puzzled… please help…

Answer 1

Change:

Class type = Integer.class
Integer i = verifyType("100",type);

to

Class<Integer> type = Integer.class
Integer i = verifyType("100",type);

By only declaring the type as ‘Class’, you’re losing the generic parameter and the verifyType() method can’t infer the class, thus the unchecked warning.

This problem:

Map<String, Class> format = new HashMap<String, Class>();
format.put("Vendor Number", Integer.class);
format.put("Vendor Dispatch", Date.class);
Integer i = verifyType("100",format.get("Vendor Number"));

can’t really be solved due to type erasure. The compiler can’t infer the type based on a generic parameter that is gone by runtime. This is because Java generics are little more than smoke and mirrors for casting.