How to find out the adjacent word pairs from a string in java?

I have a String like good and i want to find out the word pairs from that string such as oo and if String is success than out put should b开发者_运维百科e cc ss without using any String's built in functions in java.

Is it possible?

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Without any built-in method - no. But with just one or two - you can

char previous = 0;
char[] chars = str.toCharArray();
for (int i = 0; i < chars.length(); i++) {
   if (chars[i] == previous) {
       System.out.println(previous + "" + previous);
   }
   previous = chars[i];
}

I would prefer i < str.length() and str.charAt(i), but it uses more String methods.