deleteCharAt() not deleting character in stringbuilder all of sudden in loop -

i need delete chars stringbuilder couple not deleted reason. have far: have method string , arraylist paramaters. string i'm passing following: "beabeefeab" , arraylist contains following 6 character combinations of length 2: ab, ae, af, be, bf, ef. name of arraylist combos arraylistcombos , walk through arraylist in outside loop , individual chars of string combo can compare individual chars string char in stringbuilder. inside loop walk through stringbuilder , compare modstring.charat(x) firstchar taken combo. @ first combo ab example, firstchar=a , secondchar=b. able delete first e string "beabeefeab"but 2 consecutive e's don't deleted , following string: "babeeab" , should "babab" a's , b's deleted. can - appreciate.

    for(int w=0;w<arraylistcombos.size();w++){         string tempcombotokeep = arraylistcombos.get(w);         //split char in combo , store independantly in order compare         char firstchar = tempcombotokeep.charat(0);         system.out.println("first char: "+firstchar);         char secondchar = tempcombotokeep.charat(1);         system.out.println("second char: "+secondchar);         stringbuilder modstring = new stringbuilder(s);         //system.out.println("here stringbuilder before modify: "+modstring.tostring());         //walk through stringbuilder find individual chars , remove rest         for(int x=0;x<modstring.length();x++){             //if first char not equal 1 of combos, delete             if (modstring.charat(x) != firstchar){                 system.out.println("char not equal firstchar: "+modstring.charat(x)+"  "+firstchar);                 //the char inside stringbuilder not equal either of combo chars need removed                 if (modstring.charat(x) != secondchar){                     system.out.println("char not equal secondchar either!!!  " +                             "delete char string builder: "+modstring.charat(x));                     modstring.deletecharat(x);                 }             }         } 

i figured out problem. looping through stringbuilder, deleting characters stringbuilder , trying use modified stringbuilder in loop go through characters. fix created new stringbuilder , used append method , appended each time needed keep character , allowed me continue looping original stringbuilder don't miss characters.