java - How to write a calculation using a String and convert the result to a new Double correctly? -

i new java , have been asked write program males 3 different types of calculations using 3 different methods. purpose of question provide 2 examples of comparison. method stuck on, following rules must apply:

1. must test x("12 [ 3") returns null because [ not valid operator.

2. must written using following parameters:

       double x;     /*     * chops input on ' ' decides whether add or multiply.     * if string not contain valid format returns null.     */      public double x(string x){             return new double(0);     } 

here have far along example of calculation works fine:

testcalculator class

public class testcalculator {         double x;   string string = "b";  double doubleobject = 1.0;  double doubleprimitive = 2;          public void testparsing() {           if (multiplyx(12.0) == 60) {             system.out.println("multiplying success");}             else {                     system.out.println("multiplying fail");                     }          if (x("12") == null) {             system.out.println("ovalid operator success");}             else {                     system.out.println("invalid operator fail");                     }         }          /*         * chops input on ' ' decides whether add or multiply.         * if string not contain valid format returns null.         */         public double x(string x){                 return new double(x) + ("[ 3");         }         /*         * multiplies parameter x instance variable x , return value double.         */         public double multiplyx(double x){                 system.out.println("== multiplying ==");                 this.x = x;                 return new double(x * 5);         } } 

main class

public class main {  public static void main(string[] args) {  testcalculator call = new testcalculator();  call.testparsing();   } } 

my main queries are:

• how make method uses string parameter return new double value?
• since can put strings long within quotation marks, make java recognize "[" invalid operator?

any on these issues appreciated, thanks.

you errors because try add double string.

please @ example:

public static void main(string[] args) {     system.out.println(x("12 + 3"));     system.out.println(x("12 * 3"));     system.out.println(x("12 [ 3")); }  // static example purposes public static double x(string x){      string[] parsed;     if (x.contains("*")) {         // * special character in regex         parsed = x.split("\\*");          return double.parsedouble(parsed[0]) * double.parsedouble(parsed[1]);     }     else if (x.contains("+")) {         // + again special character in regex         parsed = x.split("\\+");          return double.parsedouble(parsed[0]) + double.parsedouble(parsed[1]);     }      return null; }