Passing a string that could several values(not an array) :        public class ArithmeticTester
       {
          public static void main(String[] args)
          {
      call(3, "+", 4, "7");
      call(3, "-", 4, "-1");
      call(3, "*", 4, "12");
      call(3, "@", 4, "java.lang.IllegalArgumentException");
      call(13, "/", 4, "3");
      call(13, "/", 0, "java.lang.IllegalArgumentException");
          }

          public static void call(int a, String op, int b, String expected)
          {
      try
      {
         System.out.println(Arithmetic.compute(a, op, b));
      }
      catch (Throwable ex)
      {
         System.out.println(ex.getClass().getName());
      }
      System.out.println("Expected: " + expected);
          }
       }

This is provided by the book as the testing class

    public class Arithmetic
    {
       /**
      Computes the value of an arithmetic expression
      @param value1 the first operand
      @param operator a string that should contain an operator + - * or /
      @param value2 the second operand
      @return the result of the operation      
       */
       public static int compute(int value1, String operator, int value2)
       {
      
       int a;
       a = 1;
       
       String b;
       b = "-";
       
       int c;
       c = 3;
       
       return (a, b, c);
       
       
     
    
      
      
       }
    }

I dont even really know where to begin, i am completely lost at what to even do the book does a shit job of explaining what to do and my teacher is useless at helping students. 

Am i supposed to make an if statement that changes operator ever time it loops? Please help.