Introducing the addAll Null Checkers in Java

In Java, the addAll method is used to add a collection of objects to another collection. However, if the collection being added is null, a NullPointerException will be thrown at runtime. To avoid this, there are several null checkers available in Java that can be used to check whether the collection being added is null before adding it to the target collection.

Using addAll with an if-statement

One way to check for null before calling addAll is to use an if-statement. Here is an example:

List<String> list1 = new ArrayList<>();
List<String> list2 = null;

if(list2 != null) {
    list1.addAll(list2);
}

In this example, we first create an empty ArrayList called list1. We then create another ArrayList called list2, but set it to null. Finally, we use an if-statement to check if list2 is null before calling addAll.

While this method works, it can be tedious to write an if-statement every time addAll is called with a collection that might be null.

Using Objects.requireNonNull()

Java provides a utility method called Objects.requireNonNull(), which can be used to check if an object is null. Here is an example:

List<String> list1 = new ArrayList<>();
List<String> list2 = null;

Objects.requireNonNull(list2, "Collection to be added cannot be null");

list1.addAll(list2);

In this example, we first create an empty ArrayList called list1. We then create another ArrayList called list2, but set it to null. Finally, we use Objects.requireNonNull() to check if list2 is null before calling addAll.

If list2 is null, Objects.requireNonNull() will throw a NullPointerException and the message "Collection to be added cannot be null" will be displayed.

Using Collections.EMPTY_LIST

Java provides a constant called Collections.EMPTY_LIST, which is an empty immutable List. This can be used to avoid a NullPointerException when adding an empty List to another List. Here is an example:

List<String> list1 = new ArrayList<>();
List<String> list2 = Collections.EMPTY_LIST;

list1.addAll(list2);

In this example, we first create an empty ArrayList called list1. We then create a constant List called list2 using Collections.EMPTY_LIST. Finally, we call addAll to add list2 to list1.

Using Collections.EMPTY_LIST in this way avoids the need for a null check.

Conclusion

When using the addAll method in Java, it's important to check whether the collection being added is null. This can be done using an if-statement, Objects.requireNonNull(), or Collections.EMPTY_LIST. By using one of these methods, you can avoid a NullPointerException and ensure that your code is more robust.