Midterm Examination

Handling Exceptions

The simplest way to handle Exceptions in java is to throw them and let the program stop executing. The more elegant way to handle Exceptions is to catch them and to explicitly tell the program what to do if something goes wrong. To do this, we use try-catch. In the try block we put the code where something exceptional might happen during execution, as well as any statements that depend on that code being successful (for example, we can't read from a file if we weren't able to open it). In the catch block we tell the program what to do if the try was unsuccessful.

For example : use try-catch with parseInt() in case the String doesn't represent a numerical value.

        // Ask the user for a number
           int number;
           System.out.println(" Enter a number.");
           System.out.print(" > ");

           try {
                 number = Integer.parseInt(kb.readLine());
             } // end try   
           catch (Exception e) {
                 System.out.print("Input Error: not an integer.");
             } // end catch

Another example: use try-catch when opening a file in case the file can't be found.

             try {
                 // Create an input link to the file
                    inFile = new BufferedReader(
                                 new InputStreamReader(
                                     new FileInputStream(
                                         new File (filename))));
                    int count = 0;
                    String s = inFile.readLine();

                    while (s != null) {
                          count = count + 1;
                          s = inFile.readLine();
                      }  // end while 

                    System.out.println("Lines in file = " + count);
               }  // end try
             catch (Exception e) {
                  System.out.println(" Couldn't open file.");
                  System.out.println(" Did you type the right name?");
               }  // end catch