如何解决此“ java.lang.IndexOutOfBoundsException”错误？

Question

Currently I am writing the code for a library application which stores manuals and allows users to borrow said manuals out for a certain period of time.

I am almost complete with the application however I have run into a confusing problem which I am unable to solve.

When the user selects to borrow a manual from the library, they are asked to enter an index number which states which book in the library to borrow, each manual can be borrowed successfully and if an incorrect index number is entered they are displayed with an "error" message.

However, if the user enters an index number which is equal to the amount of manuals present in the library, the application cuts off and displays the following error:

This error occurred once 2 manuals had been stored in the library, the user then entered "2" as the index number.

Here is the code I am currently working with, if anyone can tell me what is causing this error:

public static void borrowManual(){
    displayManualList();

    //register user's Manual choice.
    ManualChoice = (Console.readInteger(Messages.enterManualIndexMessage, Messages.ManualIndexNotInListMessage, 0, Library.ManualList.size()));

    borrowLoop:
    while(Menu.menuChoice == 3){
        //Check if the Manual to be borrowed is available.
        //ManualChoice = (Console.readInteger(Messages.enterManualIndexMessage, Messages.ManualIndexNotInListMessage, 1, Library.ManualList.size()));

        if ((ManualList.get(ManualChoice).status.equalsIgnoreCase(status1)) && (ManualList.size() >= ManualChoice)){
            //Print the borrowed Manual information and change the Manual status to borrowed.
            ManualList.get(ManualChoice).status = "Borrowed";
            ManualList.get(ManualChoice).borrower = User.userName;
            ManualList.get(ManualChoice).borrowDate = "Today.";
            ManualList.get(ManualChoice).returnDate = "In two weeks.";
            //Add the borrowed Manual to the borrowedManuals arraylist:
            borrowedManuals.add(ManualList.get(ManualChoice));

            System.out.printf("\n==========================================================================\n");
            System.out.printf("\n\nYou have chosen the following Manual:\n\n %s\n\n", ManualList.get(ManualChoice).displayManual());
            System.out.println("Please return the Manual within two weeks!\n");
            System.out.println("\n--------------------------------------------------------------------------");
            System.out.println("\n                             Manual borrowed!\n");
            System.out.println("--------------------------------------------------------------------------\n");
            break borrowLoop;

        }else if(ManualList.get(ManualChoice).status.equalsIgnoreCase(status2) && ManualList.size() >= ManualChoice){
            System.out.println("\n\n--------------------------------------------------------------------------");
            System.out.println("\nError! The manual you wish to borrow is already on loan.");
            System.out.println("\n--------------------------------------------------------------------------\n");
            break borrowLoop;

        }else if(ManualChoice > ManualList.size()-1){
            System.out.println(Messages.noSuchManualMessage);
            break borrowLoop;
        }
    }
    Menu.displayMenu();
}

If I need to include more of my code from other classes please let me know as I am relatively new to programming.

Answer 1

When the ArrayList has 2 elements (ie its size() is 2), the valid indices are 0 and 1 . 2 is out of bounds.

ManualChoice must be < ManualList.size() . It can't be equal to it.

You should probably change

ManualChoice = (Console.readInteger(Messages.enterManualIndexMessage, Messages.ManualIndexNotInListMessage, 0, Library.ManualList.size()));

to

ManualChoice = (Console.readInteger(Messages.enterManualIndexMessage, Messages.ManualIndexNotInListMessage, 0, Library.ManualList.size() - 1));

Answer 2

When using such things as ArrayLists and tables, one must remember that computers start counting at 0, not 1. So if an array list has a size of 4, its indexes go 0,1,2,3. What you can do is check to see if the index the user is imputing is a valid input before you let them check out the manual.