Use a substring on the middle section of the string, and just use charAt to get the starting/ending characters for swapping (note: this code needs to be modified so it will check to see if the string has one or less characters, otherwise it will fail)
String flipped = str.charAt(str.length()) + str.subString(1,str.length() - 1) + str.charAt(0);
Substring works like this:
The first parameter is the index of the character to include. The second parameter is the index of the character to stop before (this character is not included).
ex:
String str = "0123456789";
System.out.println(str.substring(2,4)); // will printout "23"