What does the list interface in Java allow?

The list interface in Java is designed to represent an ordered collection of elements, which allows for duplicates. This means that you can store multiple instances of the same element within a single list, and the elements maintain their order based on the sequence in which they were added. This ordering is important because it allows for indexed access—elements can be retrieved using their position, identified by an integer index.

For example, if you add the numbers 1, 2, and then 1 again to a list, the list will preserve the order as [1, 2, 1]. You can access each occurrence of 1 independently by their index. This characteristic makes the list interface particularly useful in scenarios where the sequence of insertion is significant, and maintaining duplicates is necessary for the application.

The other options do not accurately describe the behavior of the list interface. It doesn't enforce uniqueness of elements, nor does it store key-value pairs or restrict the contained elements to only null values. Instead, its primary purpose is to provide a structured way to store, retrieve, and manipulate ordered collections of data while allowing duplicates.