java - I need this calculator to redirect the user for input incase he enters the wrong one -

in third input, asks number entered specific operation. when example "5" entered, doesn't automatically send default section display message "the value doesn't exist.". in addition that, need display input prompt again , ask number 1 through 4 entered. thx

import java.io.bufferedreader; import java.io.inputstreamreader;   public class specialized {      public static void main(string[] args) {         string s1 = getinput("enter number: ");         string s2 = getinput("enter number: ");         string op = getinput("enter: 1 = add, 2 = subtract, 3 = multiply, 4    = divide ");           int opint = integer.parseint(op);          double result= 0;          switch (opint) {         case 1:             result = addvalues(s1,s2);             break;         case 2:             result = subtractvalues(s1,s2);             break;         case 3:             result = multiplyvalues(s1,s2);             break;         case 4:             result = dividevalue(s1,s2);             break;         default:             system.out.println("the value doesn't exist.");             break;         }         system.out.println(result);     }      private static string getinput(string prompt) {         bufferedreader stdin = new bufferedreader(                 new inputstreamreader(system.in));         system.out.print(prompt);         system.out.flush();          try {             return stdin.readline();         } catch (exception e) {             return "error: " + e.getmessage();         }     }      private static double addvalues(string s1, string s2)           throws numberformatexception {         double d1 = double.parsedouble(s1);         double d2 = double.parsedouble(s2);         double result = d1 + d2;         return result;     }      private static double subtractvalues(string s1, string s2) {         double d1 = double.parsedouble(s1);         double d2 = double.parsedouble(s2);         double result = d1 - d2;         return result;     }      private static double multiplyvalues(string s1, string s2) {         double d1 = double.parsedouble(s1);         double d2 = double.parsedouble(s2);         double result = d1 * d2;         return result;     }      private static double dividevalue(string s1, string s2) {         double d1 = double.parsedouble(s1);         double d2 = double.parsedouble(s2);         double result = d1 / d2;         return result;     }  } 

typically, surround "getting of input user" while loop until desired input. this.

public static void main(string[] args) {         string s1 = getinput("enter number: ");         string s2 = getinput("enter number: ");          //notice declare outside can compute new value inside loop         int opint = 0;         double result = 0;          //you can change condition later if add more options         while(opint < 1 || opint > 4) {             string op = getinput("enter: 1 = add, 2 = subtract, 3 = multiply, 4    = divide ");             //recalculate choice             opint = integer.parseint(op);              switch (opint) {                 case 1:                     result = addvalues(s1, s2);                     break;                 case 2:                     result = subtractvalues(s1, s2);                     break;                 case 3:                     result = multiplyvalues(s1, s2);                     break;                 case 4:                     result = dividevalue(s1, s2);                     break;                 default:                     system.out.println("the value doesn't exist.");                     break;             }         }         system.out.println(result);     }