CharAT Throws StringIndexOutOfBoundsException.

Question

The program works fine until the user has finished entering the Swell Heights then it returns to the menu and then Throws this silly exception.
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import java.util.Scanner; //import the Scanner Class
import java.lang.reflect.Array; //import the Array Class
@Program Name: SwellHeights.java
@Purpose: To get swell information and the display
           proccessed information about the swells.
//Start of the Program
public class SwellHeights
  //creates a Global Array so that the functions can get access to it
  public static float[] arraySwellHeight = new float[7];
  //Start of the MAIN function
  public static void main(String args[])
    float day = 0; //Init var to get data from dayLargeSwell()
    float smallest = 0; //Init var to get data from smallSwell()
    float largest = 0; //Init var to get data from largeSwell()
    float average = 0; //Init var to get data from averageSwell()
    Scanner in = new Scanner (System.in); //Creates a scanner to capture input   
    char choice; //declare var for use in Case Statement
    do //Starts do while loop
       //Program heading
       System.out.println("SWELL HEIGHT ANALYSIS PROGRAM");
       System.out.println("*****************************");
       System.out.println("");
       System.out.println("a)    Enter maximum daily swell heights");
       System.out.println("b)    Analyse swell height figures");
       System.out.println("c)    Exit program");
       System.out.println("");
       System.out.println("Enter Option a, b or c");
       System.out.println("Please use lowercase for your answer.");
       System.out.println("");
       String s = in.nextLine(); //Sets user input as s
       choice = s.charAt(0); //charAt only reads the 1st char in s
       //Switch statment to define where the program will go to
       switch(choice)
           //If a in input form user it runs this section
           case 'a' :
                 int count; //declares a counter to use in for loop
                 System.out.println("");
                 System.out.println("ENTER MAXIMUM SWELL HEIGHTS FOR THE LAST 7 DAYS");
                 System.out.println("===============================================");
                 System.out.println("Note: Input swell height figures in metres");
                 System.out.println("");
                 System.out.println("Enter 7 daily Swell Hieghts\n");
                 for (count = 0; count < 7; count++)
                 //This loop will run 7 times
                      System.out.print(" Enter swell heights for day " + (count+1)+" $: ");
                      SwellHeights.arraySwellHeight [count] = in.nextFloat();
                      System.out.println(SwellHeights.arraySwellHeight [count]);
                      //This is creating an Array with 7 elements
                System.out.println("");
                //Stops the program from carrying on to case b
                break;
           //If b is input from user it runs this section
           case 'b' :
                System.out.println("");
                System.out.println("MAXIMUN DAILY SWELL HEIGHTS FOR THE WEEK"); 
                System.out.println("****************************************");
                System.out.println("");
                System.out.println("   Day  Metres");
                System.out.println("   ---  ------");
                // + SwellHeights.arraySwellHeight[#] displays what is inside that element
                System.out.println("     1  " + SwellHeights.arraySwellHeight[0]);
                System.out.println("     2  " + SwellHeights.arraySwellHeight[1]);
                System.out.println("     3  " + SwellHeights.arraySwellHeight[2]);
                System.out.println("     4  " + SwellHeights.arraySwellHeight[3]);
                System.out.println("     5  " + SwellHeights.arraySwellHeight[4]);
                System.out.println("     6  " + SwellHeights.arraySwellHeight[5]);
                System.out.println("     7  " + SwellHeights.arraySwellHeight[6]);
                System.out.println("");
                System.out.println("============================================================================");
                System.out.print("Average swell height    : " );
                average = averageSwell(); //calls the averageSwell() function
                System.out.println(average); //prints out the result of that functions process
                System.out.print("Smallest swell height   : " );
                smallest = smallSwell(); //calles the  smallSwell() function
                System.out.println(smallest); //prints out the result of that funtions process
                System.out.print("Largest swell height    : " );
                largest = largeSwell(); //calles the largeSwell() function
                System.out.print(largest); //prints out the result of the functions process
                System.out.print(" metres occured on day ");
                day = dayLargeSwell(); //calles the daylargeSwell() function
                System.out.println(day); //prints out the result of that functions process
                System.out.println("============================================================================");
                System.out.println("");
                //Stops the program from carrying on to case c
                break;
           //If c in input form user it runs this section  
           case 'c' :
                System.out.println("");
                System.out.println("Thank you and Goodbye");
                //Stops the program from carrying on to default
                break;
           default :
                System.out.println(choice+ " Is An Invalid Selection" );
                System.out.println("");
                //Stops the program from exiting the case statement
                break;
          //End of Case Statement
    //End of Do-While Loop, when user enters c
    while(choice != 'c');
  //End of main()
  Purpose: To calculate the average swell
           height from the values in the
           Array.
  @return: Returns the average to be
           displayed in main()   
  public static float averageSwell()
    int count = 0; //Init var for Do-While Loop
    float total = 0.0f; //Init var for the total
    float average = 0.0f; //Init var to hold the average
    //Start of Do-While loop
    do
      //Accumulate Total
       total = total + SwellHeights.arraySwellHeight[count];
       count = count + 1;
    //Finishes when count is equal to 7
    while (count < 7);
    //Average is total / number of elements
    average = total/count;
    //Returns the average var to main()
    return average;
   }//end of averageSwell()
  Purpose: To calculate the minimum swell
           height from the values in the
           Array.
  @return: Returns the minimum to be
           displayed in main()   
  public static float smallSwell()
    int count = 0; //Init var for Do-While Loop
    float minimum; // Declare float to hold minimum
    //minimum is then assigned the first element
    minimum = SwellHeights.arraySwellHeight[0];
    //Start of Do-While loop
    do
      //Check for new minimum
       if (SwellHeights.arraySwellHeight[count] < minimum)
         //New minimum
         minimum = SwellHeights.arraySwellHeight[count];
      //Incrementing count
      count = count + 1;
    //Finishes when count is equal to 7
    while (count < 7);
    //Returns the minimum var to main()
    return minimum;
   } //End of smallSwell()
  Purpose: To calculate the maximum swell
           height from the values in the
           Array.
  @return: Returns the maximum to be
           displayed in main()   
  public static float largeSwell()
    int count = 0; //Init var for Do-While Loop
    float maximum; // Declare float to hold maximum
    //maximum is then assigned the first element
    maximum = SwellHeights.arraySwellHeight[0];
    //Start of Do-While loop
    do
      //Check for new maximum
       if (SwellHeights.arraySwellHeight[count] > maximum)
        //New maximum
        maximum = SwellHeights.arraySwellHeight[count];
       //Incrementing count
       count = count + 1;
    //Finishes when count is equal to 7
    while (count < 7);
    //Returns the maximum var to main()
    return maximum;  
   } //End of largeSwell()
  Purpose: To calculate the day of the
           largest swell height from the
           values in the Array.
  @return: Returns the day of the largest
           swell to be displayed in main()   
  public static float dayLargeSwell()
    int count = 0; //Init var for Do-While Loop
    int day = 0; //Init var to hold days
    float maximum; //Declare float to hold maximum
    //maximum is then assigned the first element
    maximum = SwellHeights.arraySwellHeight[0];
    //Start of Do-While loop
    do
       //Check for new maximum
       if (SwellHeights.arraySwellHeight[count] > maximum)
        //New maximum
        maximum = SwellHeights.arraySwellHeight[count];
        //Maximum element number caputured
        day = count + 1;
       //Incrementing count
       count = count + 1;
    //Finishes when count is equal to 7
    while (count < 7);
    //Returns the day var to main()
    return day;  
  } //End of dayLargeSwell
} //End of Program

Answer 1

charAt throws IndexOutOfBoundsException if the index argument is negative or not less than the length of this string.
A quick look at your code shows that you are using charAt only once:String s = in.nextLine(); //Sets user input as s
choice = s.charAt(0); //charAt only reads the 1st char in sThe conclusion is that s has a length of zero.
I guess this could happen when user hits the return key without entering a character (or might come from Scanner behaviour when using nextFloat() followed by nextLine(), I don't know much about Scanner as I'm still stuck to 1.4 .)
You could therefore add a check on user input before trying to extract the character, something like:String s = null;
while((s = in.nextLine()).length() < 1); //read user input until it is not empty
choice = s.charAt(0); //charAt only reads the 1st char in sHope it helps.