Adding to List in Java: Tips and Tricks

If you're working with Java and trying to add elements to a List<? extends Number>, you may have encountered some issues. In order to add elements to this type of list, you need to use some tips and tricks.

First of all, it's important to understand that List<? extends Number> is a wildcard type. This means that you can't add any elements to it, because the compiler can't guarantee that the elements you're adding are of the correct type.

One solution is to use a List<Number> instead. This type allows you to add any element that extends Number. However, if you need to use a List<? extends Number>, you can use the following trick:

List<? extends Number> list = new ArrayList<>();
List<Integer> integerList = new ArrayList<>();
list = integerList;

In this example, we create a List<? extends Number> and an ArrayList<Integer>. We then assign the integerList to the list variable, which is of type List<? extends Number>. This works because List<Integer> is a subtype of List<? extends Number>.

Another trick you can use is to create a helper method that takes a List<? super T> parameter. This method can then add elements to the list, because the compiler knows that the list can hold elements of type T or any of its supertypes. Here's an example:

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

You can then call this method with a List<? extends Number> and an element of type Integer, for example:

List<? extends Number> list = new ArrayList<>();
addToList(list, 42);

In conclusion, adding elements to a List<? extends Number> in Java requires some tips and tricks, such as using a List<Number> instead or creating a helper method that takes a List<? super T> parameter. These solutions allow you to add elements to the list while still ensuring type safety.