Question

I get a compile time error on countList method.

public static void countList( List<? extends Number> list, int count ){
        for( int i = 0; i < count; i++ ){
            list.set(i, i-1);
        }
}

public static void clearList( List<?extends Number> list){
    list.clear();
}

It says:
The method set(int, capture#2-of ? extends Number) in the type List is not applicable for the arguments (int, int)

What does this error message mean? Why can’t I set the elements in the list? Why is it OK to clear the list?

Answer 1

Because it’s a list of “Something that extends number, but I don’t know what.” You can’t go just putting integers in there, what if it’s actually a list of doubles?

List<Double> list = new ArrayList<Double>();
list.add(Double.valueOf(4.5);
countList(list, 1);
Double d = list.get(0); //what happens here?  You put an Integer in there!  class cast exception

You can clear the list because for that operation it doesn’t matter which subtype of Number is actually in there. clear is clear is clear.