Tradional way to handle runtime error

  • When a method execution encounters an error, the method would return an error code

  • Example: 

       public static void main(String[] args)
   {
       double area = areaOfCircle( -4 );

       if ( area == -1 )  // Check error code
       {
           System.out.println("Error");
       }
   }

   public static double areaOfCircle( double radius )
   {
      if ( radius < 0 )
         return -1;      // Error code
      else
         return 3.1415 * radius * radius;
   }

     

DEMO: demo/02-review/ErrorCode.java

Modern way to handle runtime error

  • In newer programming languages, the method will "throw an exception" when it encounters an error:

  • Example: 

       public static void main(String[] args)
   {
       double area = areaOfCircle( -4 );

       
       
       // No need to test error code return value...   
      
   }

   public static double areaOfCircle( double radius )
   {
      if ( radius < 0 )
         throw new RuntimeException("***neg radius***");
      else
         return 3.1415 * radius * radius;
   }

    This is more fancy way to return an error code object

DEMO: demo/02-review/ThrowException.java

Modern way to handle runtime error

  • You can test (= catch) for the "error code" (exception type) and execute a block when the "error code" is detected:

  • Example: 

       public static void main(String[] args)
   {
       try   // Indicates that statements may return an "error code"
       {
          double area = areaOfCircle( -4 ); 
       }
       catch (RuntimeException e)  // Test if "error code" = RunTime..
       {
          System.out.println("Error: " + e);
          System.out.println("... continue...");
       }

       System.out.println("DONE");
   }

   public static double areaOfCircle( double radius )
   {
      if ( radius < 0 )
         throw new RuntimeException("***neg radius***");
      else
         return 3.1415 * radius * radius;
   }

DEMO: demo/02-review/CatchException.java