public class ParseDouble
{
   public static void main(String[] args)
   {
      String a = "12.34";
      String b = "56.78";


      System.out.println( a + b );
                // a + b = "12.34" + "56.78" = "12.3456.78" 
                // Because + = concatenate when used on strings

      System.out.println( Double.parseDouble(a) + Double.parseDouble(b) );
                // Double.parseDouble(a) + Double.parseDouble(b)
                // = Double.parseDouble("12.34") + Double.parseDouble("56.78")
                // = 12.34 + 56.78 = 69.12
                // Because + = ADD when used on doubles
   }
}