为什么会收到NoSuchElementException？

Question

This method is essentially a method that reads through a given input file and recursively populates a binary tree with the given information.

The input file is of a very particular format. One line that contains either Q: or A: to indicate whether or not the following line is a question or an answer. It is assumed that all files used with this method will follow this format.

Since every file follows the same format, and there should never be an odd number lines, the data shouldn't be fully consumed before reaching one of the nextLine() calls. Nonetheless, the program consistently throws a NoSuchElementException .

Is there something I'm missing?

private QuestionNode readHelper(Scanner input){
    // Base case: If the given input has no more lines to read.
    if (input.hasNextLine()) {
        String category = input.nextLine();
        String text = input.nextLine();
        QuestionNode root  = new QuestionNode(text);
        if (category.startsWith("Q")) {
            // Recursive case: If there are still questions available to ask
            // more input is read, which replaces the currently stored data.
            root.left = readHelper(input);
            root.right = readHelper(input);
        } else {
            return root;
        }
    }
    return null;
}

Answer 1

The second call to nextLine() within the if statement. There is no guaranty there is a nextLine() after String category = input.nextLine().