com.sun.crypto.provider.DESKeyFactory contains the following code:

   protected SecretKey engineGenerateSecret(KeySpec keySpec)
throws InvalidKeySpecException {
DESKey desKey = null;

try {
if (keySpec == null ||
!(keySpec instanceof DESKeySpec)) {
throw new InvalidKeySpecException
("Inappropriate key specification");
}

If an instance of javax.crypto.spec.SecretKeySpec is passed in to DESKeyFactory.engineGenerateSecret(), it throws an InvalidKeySpecException which may not be necessary.