Java implementation of simple arithmetic expressions (2)

On the internet to see a very strong method of implementation, the use of recursion.

      /*** Ideas * 1, exclude empty string * 2, get index of the first symbol * 3, if there is only a number, direct conversion and return * 4, if there is an opening parenthesis, then find the right parenthesis, and the parentheses inside the string is calculated recursively, and for Change, and then the overall recursion, remove all brackets * 5, the first recursive decomposition plus minus, to ensure that the priority of multiplication * 6, and then recursive decomposition multiplication, multiplication calculated after the calculation will automatically add and subtract *@paramS *@return       * @throwsException*/        Public Static floatOpt (String s)throwsexception{//1         if(s = =NULL|| "". Equals (S.trim ())) {             return0f; }         //2         intA1=s.indexof (' + ')); intA2=s.indexof ('-'); intA3=s.indexof (' * ')); intA4=s.indexof ('/'); intA5=s.indexof (' ('); //3 Calculating the individual numbers         if(A1==-1&&a2==-1&&a3==-1&&a4==-1){             if(S.trim () = =NULL||"". Equals (S.trim ())) {                 Throw NewException ("Operate error"); }             returnfloat.parsefloat (S.trim ()); }          //4 parentheses are calculated first         if(A5!=-1){         intA6=s.lastindexof (') '); if(A6==-1){                 Throw NewException ("Parentheses do not match"); }Else{                 floatF=opt (S.substring (a5+1, A6). Trim ()); S=s.replace (S.substring (a5,a6+1), string.valueof (f)); returnopt (s); }         }                   //5 first add and subtract to ensure the priority of multiplication         if(A1!=-1){             returnOpt (s.substring (0,A1)) +opt (S.substring (a1+1, S.length ())); }         if(A2!=-1){             returnOpt (s.substring (0,A2))-opt (S.substring (a2+1, S.length ())); }         //6         if(A3!=-1){             returnOpt (s.substring (0,A3)) *opt (S.substring (a3+1, S.length ())); }         if(A4!=-1){             returnOpt (s.substring (0,A4))/opt (S.substring (a4+1, S.length ())); }         returnInteger.parseint (S.trim ()); }

There is also a more convenient way to take advantage of the built-in JS engine

         Public Static Object jscalculate (String script) {          new  Scriptenginemanager ();           = Manager.getenginebyname ("javascript");           Try {            return  (Object) engine.eval (script);           Catch (Scriptexception ex) {            ex.printstacktrace ();          }           return NULL ;        }

Java implementation of simple arithmetic expressions (2)