Question

I’m trying to understand if this code below creates 12 objects for a string like “stephan”

public String reverse(String str) {
            if ((null == str) || (str.length()  <= 1)) {
                return str;
            }
            return reverse(str.substring(1)) + str.charAt(0);
        }

This recursively reverses a string. I understand how it works. But I was thinking if there is a relationship in this case between the length of the strings and number of string objects created through concatenation?

Answer 1

Yes, it will create tons of string objects.

Every recursive call to “reverse()” will create 2:

1. str.substring(1) will create a new String object

2. reverse() call will create a new string for its return value, but we will NOT count that since that’s counted when analyzing that recursive call (e.g. it will be the string from bullet point #3 from the next reverse() call).

3. And since Java Strings are immutable, adding a char via “+” will create a second String object.

Therefore, for a string of length N, it will create (N-1)*2 objects (since a reverse of 1-char string does NOT create new strings); so for “stephan”‘s 7 characters, it will create 6*2=12 string objects.