index.stream().reduce((first, second) -> second)

This line of code uses the reduce() method to get the last element in the index stream.

The reduce() method takes a BinaryOperator as an argument, which in this case is a lambda expression (first, second) -> second. This lambda expression takes two elements from the stream and returns the second element.

As reduce() processes the stream, it applies the lambda expression to the first and second elements, then applies it again to the result and the next element, and so on until there are no more elements in the stream.

Since the lambda expression always returns the second element, reduce() will eventually return the last element in the stream.