Generic collections. It's a bug?

Question

I'm new in Java 5 and I have seen that we can insert elements of any type in a typed collection!! , so, its not sure to use them? Have I to check all the elements in the collections received from parameters ?
public static void main(String argv[]){   
     Ejemplo e = new Ejemplo();      
     List lista1 = new ArrayList();
     lista1.add("this is not Ejemplo");
     List<Ejemplo> lista = lista1;
     lista.add(e);
     lista.add(e);
//ClassCastException
     for(Ejemplo i:lista){
          System.out.println(i);
}

Answer 1

That's why you'll get an "unchecked" warning from the compiler when you do the list assignment. It's warning you that there's no certainty that the list has the right type of contents.
The trouble with generics is that you can get a lot of these warnings, some that you really can't avoid. You need to look at them and be confident that the generic type is valid.
Of course, had you not been using generics, you have had to write the loop casting the element explicity to (Ejemplo), which would still have given you the ClassCastException.