Instantiation with Generics

Question

Consider the following code:
public class InstantiateWithGenerics {
     public static <C> C possibleSelects(C from) {
          try {
               Class<C> clazz = (Class<C>) from.getClass();  // Displays typical type safety warning when casting
               return clazz.newInstance();
          } catch (Exception e) {    // newInstance() exceptions must be caught
               throw new RuntimeException(e);
}Why should the cast be unsafe?
Is there some other way to instantiate another object just like the one passed in?
This seems to not produce any errors or warnings:
     public static <C> C possibleSelects(Class<C> clazz, C from) { ... }
     MyClass baseObject = ...;
     MyClass myObject = possibleSelects(MyClass, baseObject);I'm basically passing it in as a class literal, but it seems ugly to have to add that to my method signature and the first way looks like it should be type-safe to me. What am I missing?

Answer 1

LMaccherone wrote:
PM>The erasure of C is java.lang.Object, so getClass() is going to return Class<? extends java.lang.Object> in this case.
I'm not sure why, but OK.Have a look at http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5004321
During the Java 5 beta, it didn't behave this way, and was reported as a bug. The current behavior was done intentionally.
Is there some other way to accomplish the goal or do you think the cast is acceptable in this situation? Since we know the "from" identifier is of type C, is should always be safe to cast it to type Class<C>, right?Sorta, it's actually a Class<? extends C> since the object being passed in could be a subtype of C.
Is there something situation where getClass() might return null?No.