Question

I am creating a program that takes a series of numbers and adds the minimal pair of these numbers. The failing code follows:

import java.util.*;

public class Library {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        String answer;
        int count;
        int books;
        int writers;
        List<Integer> booksList = new LinkedList<>();
        System.out.printf("Numbers: ");

        answer = input.nextLine();
        String[] arr = answer.split(" ");

        for (String num : arr) {
            booksList.add(Integer.parseInt(num));
        }

        books = booksList.remove(0);
        writers = booksList.remove(0);

        while (booksList.size() > writers) {
            mergeMinimalPair(booksList);
        }
    }

    public static void mergeMinimalPair(List<Integer> books) {  
        int index = 0;
        int minValue = books.get(0) + books.get(1);

        for (int i = 1; i <= books.size() - 1; i++){
            if ((books.get(i) + books.get(i + 1)) < minValue){
                index = i;
                minValue = books.get(i) + books.get(i + 1);
            }
        }
        //combine(books, index, index + 1);
    }

The combine method is not yet implemented. I checked with the debugger and when it is about to execute the mergeMinimalPair method, it throws the following exception:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 7, Size: 7
    at java.util.LinkedList.checkElementIndex(LinkedList.java:553)
    at java.util.LinkedList.get(LinkedList.java:474)
    at Library.mergeMinimalPair(Library.java:40)
    at Library.main(Library.java:29)
Java Result: 1

How do I avoid this exception?

Answer 1

The problem is here:

for (int i = 1; i <= books.size() - 1; i++){
    if ((books.get(i) + books.get(i + 1)) < minValue){
        index = i;
        minValue = books.get(i) + books.get(i + 1);
    }
}

You are iterating up to books.size() - 1. When i is exactly equal to books.size() - 1, i + 1 is equal to books.size(), which is considered out of bounds when you do books.get(i + 1). Fix:

for (int i = 1; i < books.size() - 1; i++){
    if ((books.get(i) + books.get(i + 1)) < minValue){
        index = i;
        minValue = books.get(i) + books.get(i + 1);
    }
}