Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of-CodePudding

I am trying to make a program that sees if a word is a palindrome or not. However, I am getting the above error. When I run the program it puts together the word backwards, up until there are no more letters left. That is when I get the error.

// Programmer: Your name
// Class, Section and Term: ...
// Purpose: Find the average of a list of integer numbers. The list is ended with a sentiel value SENTINEL.
// Topic: while-loop
// Import the Scanner Class for input operations.

      import java.util.Scanner;

      public class SimpleProgram {

      public static void main(String[] args) {

      // Create the Scanner object, scnr, to perform input operations.

      Scanner scnr = new Scanner(System.in);

      // Declare the identifiers
      String reverse;
      reverse = "";
      int index;
      int number;         
      String letter;     
      final String end = "Stop";

  
   System.out.println("Please type a word:");
   letter = scnr.next();
      index = letter.length()   0;

      
       while (!letter.equals(reverse) || (index >= 0)) {
        //  while (letter.equals(reverse.length()   0));
         
            index--;
         
        
      
     // index = letter.length()-1 - index--;
      // index   1;
     // System.out.println(index.CharAt(index--));
   
      reverse = reverse   letter.charAt(index);
      System.out.println(reverse);
      
          
      }
   if (letter.equals(reverse)) {
      System.out.println(letter   " is a palindrome!");
   }
  
   else {
      System.out.println(letter   " is not a palindrome, spelt backwards it is: "   reverse);
   
   }
   scnr.close();
   }
   
}

CodePudding user response：

You need just the change the while condition look at my edition it's works !!

public static void main(String[] args) {

        // Create the Scanner object, scnr, to perform input operations.

        Scanner scnr = new Scanner(System.in);

        // Declare the identifiers
        String reverse;
        reverse = "";
        int index;
        int number;         
        String letter;     
        final String end = "Stop";

        System.out.println("Please type a word:");
        letter = scnr.next();
        index = letter.length() - 1;


        while (!letter.equals(reverse) && (index >= 0)) {
            //  while (letter.equals(reverse.length()   0));
  
            // index = letter.length()-1 - index--;
            // index   1;
            // System.out.println(index.CharAt(index--));

            reverse = reverse   letter.charAt(index);
            System.out.println(reverse);
            index--;


        }
        if (letter.equals(reverse)) {
            System.out.println(letter   " is a palindrome!");
        }

        else {
            System.out.println(letter   " is not a palindrome, spelt backwards it is: "   reverse);

        }
        scnr.close();

result :

olaalo is a palindrome!

CodePudding user response：

A StringIndexOutOfBoundsException is a type of error that is thrown when either (as shown in this link) "an index is either negative or greater than the size of the string".

To prevent such cases, many times we will make the conditions of the while loop or for loop we are coding in specifically the size of the string (or zero if we are going downwards). You attempted to do this, but your error is still coming through so it must be a little bit deeper.

In your case, this is likely because the String letter never becomes equal to reverse, so the while loop goes on forever (hence the StringIndexOutOfBounds when index becomes negative).

To fix this you may want to rethink the conditions on your while loop, and instead use something directly based on the length of the string (as opposed to whether or not the reverse is ever true).