java - how to change an index in a StringBuilder based on what the user enters -

i want change index on stringbuilder based on string user enters example made array have change string used stringbuilder so. created string have user input , replace index user entered. here have:

    arrays inventory = new arrays();     string[] words = inventory.getwords();     string[] category = inventory.getcategory();     string board = "";     stringbuilder builder = new stringbuilder();     for(string s : words) {         builder.append(s+"\n");     } 

this part of code. part want change index error saying method setcharat undefined string type. tried other methods replace() same error. have use strings this:

 string word1 = builder.substring(0, 7);      board.equals(word1);      scanner in = new scanner(system.in);      system.out.println("enter letter guess word");      string input = in.nextline();      char achar = builder.charat(0);      if (input.equals(achar)){          board.setcharat(0, input);          system.out.println(board);      } 

setup stringbuilder *'s each letter:

  string theword = "enter";   stringbuilder board = new stringbuilder();   (int = 0; < theword.length(); i++) {      board.append("*");   } 

gather users input (not included brevity)

  char usersinput = 'e';  // example 

find characters match ones in original word , replace * in stringbuilder.

  (int = 0; < theword.length(); i++) {      if (theword.charat(i) == usersinput) {         board.setcharat(i, usersinput);      }   }   system.out.println(board.tostring()); 

this produce "e**e*"