Recursive method to reverse a word, StackOverFlow error

Problem. Before we get to the answer, let's look at this call you're making:

invertirPalabra(palabra.substring(1)+ palabra.charAt(0));

If the word had 10 characters, it would be recursively calling the function with a long string ........

Can you see the problem now?

Yes, 10 characters too!!!... In other words, the recursive calls for invertirPalabra("abc")would be:

invertirPalabra("bca")
invertirPalabra("cab")
invertirPalabra("abc")
invertirPalabra("bca")
...

Clearly the condition will never be met if(palabra.length()==0){.

We did not reduce any characters in the recursive call. That should be the idea, right?

Well, that's the problem.

Solution. What if we recursively call the function with only what remains to be reversed ?

res = invertirPalabra(palabra.substring(1)) + palabra.charAt(0);
//                                          ^^^^^^^^^^^^^^^^^^^
//                                Esto no se está pasando como parámetro
//                                Sólo se está devolviendo como resultado de la función

Thus, to invertirPalabra("abc")achieve:

invertirPalabra("bc") + "a"
   ^-- invertirPalabra("c") + "b"
          ^-- invertirPalabra("") + "c"
                 ^-- ""

Code:

private static String invertirPalabra(String palabra) {
    String res = "";

    if(palabra.length() == 0){
        res = palabra;
    }else{
        res = invertirPalabra(palabra.substring(1)) + palabra.charAt(0);
    }
    return res;
}

demonstration:

http://ideone.com/iXj5az

Extra. If you are interested in shortening the code a bit more:

public static String invertirPalabra(String palabra) {
    if (palabra.length() <= 1) {
        return palabra;
    }
    return invertirPalabra(palabra.substring(1)) + palabra.charAt(0);
}

if it is about recursion, then we must consider the base case and recursive case well:

I better explain it with an image

in code it would be:

public String invertirPalabra(String palabra){
    if (palabra.length() == 1){
        return palabra;
    }
    return invertirPalabra(palabra.substring(1)) + palabra.charAt(0);
}

Recursive Solution:

    public static String invertirPalabra(String palabra)
    {
      if (palabra.length() <= 1) 
      {
        return palabra;
      }
      return invertirPalabra(palabra.substring(1)) + palabra.charAt(0);
    }

Non-recursive solution using the StringBuilder Class:

    public static String invertirPalabra(String palabra)
    {
      return new StringBuilder(palabra).reverse().toString();
    }