Collection safer than standard list with generic type?

Question

I use generics in Java but it isn't so good as I thought

public static void add(List l, Object o) {
    l.add(o);
}

public static void main(String[] args) throws Exception {
    List<Integer> list = new ArrayList<Integer>();
    add(list, "1.23");
    add(list, 1.23);
    System.out.println(list);
}

All this compiles and works. When I get a value from list an exception is thrown.

Can it be safer in Java 6?

Answer 1

I suggest using the standard Collections:

List<Integer> checked = Collections.checkedList(list, Integer.class);

then just work on checked. A ClassCastException will be thrown during the insertion of any non-compliant instance - sooner (thus better) than before (i.e. during retrieval).

N.B. check your compiler messages, I'd bet you have some warnings printed in there mentioning unsafe/unchecked code.. Your problem is exactly what the compiler is trying to tell you. If you control the signature of add, make it generic - it will give you compile-time safety.

public static <T> void add(List<T> list, T t) {
    list.add(t);
}

Answer 2

I don't think so. If that static add() method is a fact of life (something you don't control), there's not much you can do, because Java generics are implemented so that they are compatible with Java < 5.0.

However, if you are able to change the signature of the add() method, you can simply enforce the type check so you get compiler errors for the code in your main() method:

public static <T> void add(List<? super T> list, T object) {
    list.add(object)
}