Stack overflow error in a recursive method

Question

My method is supposed to reverse the even indices of a given String.

EDITED WITH UPDATED CODE:

public static String revEven(String inString)
{
  String tempString = new String();

  if (inString.length() <= 2)
     return inString;
  if (inString.length() == 3)
  {
     tempString += inString.charAt(2);  
     tempString += inString.charAt(1);
     tempString += inString.charAt(0);

     return tempString;
  }
  if (inString.length() % 2 == 0)
  {
     return tempString += 
            revEven(inString.substring(0, inString.length() - 1)) +
            inString.charAt(inString.length() - 1);
  }
  else
  {
     return tempString += 
            inString.charAt(inString.length() - 1) + 
            inString.charAt(1) +
            revEven(inString.substring(2, inString.length() - 2)) +
            inString.charAt(inString.length() - 2) +
            inString.charAt(0);
  }
}

I ran through the recursion on paper, and it should be working correctly. When I enter abcde for a test I get 199cda printed out. It should print out ebcda .

It seems like it's printing out memory addresses or something, but I can't seem to figure out why.

Answer 1

Your edited code is returning exactly what it should.

The code below:

return tempString += 
        inString.charAt(inString.length() - 1) + 
        inString.charAt(1) +
        revEven(inString.substring(2, inString.length() - 2)) +
        inString.charAt(inString.length() - 2) +
        inString.charAt(0);

treats the first + as the addition operator , not the concatenation operator , adding the last char to the second char of the String. In Java, char values can be added together to produce numbers (in Java, 'a'+1 = 98 , the numeric value of the char b ).

The code below fixed the problem by concatenating to an empty string, therefore letting the Java compiler know that everything that follows is concatenation, not addition.

return tempString += "" 
    + inString.charAt(inString.length() - 1)
    + inString.charAt(1)
    + revEven2(inString.substring(2, inString.length() - 2))
    + inString.charAt(inString.length() - 2)
    + inString.charAt(0);
}

Answer 2

Best way to accomplish this is to have two method calls: one for your initial step (getting only the characters at even indices) and your recursive step (reversing a String).

The first step should look something like below:

public static String reverseEven(final String inString) {
    String tempString = "";
    for(int i = 0; i < inString.length(); i=i+2) {
        tempString += inString.charAt(i);
    }
    return revEven(tempString);
}

The above code takes only every second character from the original String and passes it your method.

The code you have so far almost performs this second step, bar a StackOverflowException.

To fix this, change the line:

return tempString += revEven(inString.substring(0, inString.length() - 1) + inString.charAt(inString.length() - 1));

to:

return tempString += inString.charAt(inString.length() - 1) + revEven(inString.substring(0, inString.length() - 1));

Your original code threw a StackOverflowException on Strings with an even number of characters since it would always the entirety of inString , because you would always add the last character (the one your substring call removes) back on to the end, resulting the in the same String over and over.

The suggested fix, instead, add this last character to the start of the String and reverses the rest as if it had an odd number of characters.

Answer 3

The problem is that you split the string from the start to one away from the end, but then add on the last character before you call the function, so you effectively call it with the same string over and over and over.

When you call inString.substring(0, inString.length() - 1) on the string "Hello" , for example, length() returns 5 - so that is the substring from indices 0 to 4, not including 4. That's all well and good, but then you add the character at index 4 on - or the last character.

Change the call revEven(inString.substring(0, inString.length() - 1) + inString.charAt(inString.length() - 1)) to just revEven(inString.substring(0, inString.length() - 1)) .