java - Explain how this permutation works -

i searched code permutation in java:

public class mainclass {    public static void main(string args[]) {       permutestring("", "string");    }      public static void permutestring(string beginningstring, string endingstring) {      if (endingstring.length() <= 1)         system.out.println(beginningstring + endingstring);      else         (int = 0; < endingstring.length(); i++) {           try {              string newstring = endingstring.substring(0, i) + endingstring.substring(i + 1);               permutestring(beginningstring + endingstring.charat(i), newstring);           } catch (stringindexoutofboundsexception exception) {       exception.printstacktrace();     }   }  } } 

i can't understand though know basic code. want explain me make clearer. thank guys

one can construct permutation, picking items bag repeatedly , constructing sequence. string, bag collection of characters. can use string represent this.

if want construct random permutated string, first if bag empty. in above code, bag endingstring , emptiness check done with:

if (endingstring.length() <= 1)     system.out.println(beginningstring + endingstring); 

as can see check not whether bag empty: moment string has 1 character (one element), evidently pick one. pick , print after sequence we've constructed.

problem: problem approach if want list permutations of empty string (there one: empty string), 1 errors.

now need iterative case. remember beginningstring stores sequence we've constructed till , endingstring stores list of characters still can pick from. way pick select valid index i in endingstring. character @ index picked.

we update sequence (beginningstring appending character placed @ i, thus:

beginningstring + endingstring.charat(i) 

in order update bag, means bag contains characters before index, , ones after index. formalized as:

string newstring = endingstring.substring(0, i) + endingstring.substring(i + 1); 

newstring here new bag. can recursive call pick next item bag. given index i, in order pick , call recursively, code reads:

string newstring = endingstring.substring(0, i) + endingstring.substring(i + 1); permutestring(beginningstring + endingstring.charat(i), newstring); 

now since wish enumerate on all possible permutations, loop on possible indices i. since recursively consequence, enumerate permutations.