Question

I’m trying to read a math expression like 3+9-2*10/5 from JOptionPane and get its result — taking into account order of operations, of course. I split up the string into just digits and just operands using String.split() and created a for loop that is looking for either multiplication signs or division signs — in this case it is detecting the string “*” since it comes up first in the string.

public static void main(String[] args)
{
    String mathString = JOptionPane.showInputDialog("Please type a simple math expression (i.e., without parentheses).");

    String[] parsedIntegers = mathString.split("\\D");
    String[] parsedOperands = mathString.split("\\d+");
    parsedOperands[0] = null;
    System.out.println(Arrays.toString(parsedIntegers));
    System.out.println(Arrays.toString(parsedOperands));

      for (int index = 1; index <= parsedOperands.length; index = index + 1)
      {

          if (parsedOperands[index].equals("*"))
           {
                 System.out.println("The multiplication sign is at index " + index + ".");
                 int multResult = Character.getNumericValue(parsedIntegers[index - 1].charAt(index - 1)) * Character.getNumericValue(parsedIntegers[index].charAt(index));
                 System.out.println(multResult);
           }
      }
}

The string array parsedOperands looks like this: [null, +, -, *, /].
The string array parsedIntegers looks like this: [3, 9, 2, 10, 5].

However, when I look for “*” which is at index 3 in parsedOperands and then try to multiply what is in (index – 1) and (index) in parsedIntegers, Java returns an IndexOutOfBoundsException. Why does this happen? Am I missing something?

Here is the error:

[3, 9, 2, 10, 5]

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 2

[null, +, -, *, /]

The multiplication sign is at index 3.
    at java.lang.String.charAt(String.java:658)
    at programmingpractice.SolveMathExpression.main(SolveMathExpression.java:49)
Java Result: 1

Answer 1

Each element in your array parsedIntegers is a single-character string, so when you use charAt, it should just be charAt(0):

int multResult = Character.getNumericValue(parsedIntegers[index - 1].charAt(0)) *
                 Character.getNumericValue(parsedIntegers[index].charAt(0));

Using charAt(index) or charAt(index - 1) there tries to read past the end of the one-character string and throws the StringIndexOutOfBoundsException you got.

However, a more robust way to do this is probably to use Integer.parseInt so you can have multi-digit integers:

int multResult = Integer.parseInt(parsedIntegers[index - 1]) *
                 Integer.parseInt(parsedIntegers[index]);