Java nested generic type mismatch

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If you want to be able to call fourth with a List<List<String>> argument, then you’ll need to change your signature to this:

private static void fourth(List<? extends List<?>> a){
    System.out.println("List of a List of anything ");
}

The above will work because unlike List<List<?>>, List<? extends List<?>> is compatible with List<List<String>>. Think of it this way:

List<List<String>> original = null;
List<? extends List<?>> ok  = original; // This works
List<?> ok2                 = original; // So does this
List<List<?>> notOk         = original; // This doesn't

List<Integer> original      = null;
List<? extends Number> ok   = original; // This works
List<?> ok2                 = original; // So does this
List<Number> notOk          = original; // This doesn't

The reasoning is simple. If you had

private static void fourth(List<List<?>> a) {
    List<?> ohOh = Arrays.asList(new Object());
    a.add(ohOh);
}

And then if you could call that method as such:

List<List<String>> a = new ArrayList<List<String>>();
fourth(a);
String fail = a.get(0).get(0); // ClassCastException here!

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