Java Exception Handling

Java Exceptions:

Errors:

There are three types of errors.

1. Compile Time Errors:
         Syntactical error found in the program due to which program fails to compile.
   Ex: Unable to compile if semicolon is missing end of the line.
                 public static void main(String args[]) throws Excetpion{
                 System.out.println(“Executed”)
                }

2. Runtime Errors:

                   These errors represent inefficiency of computer system to execute a particular program.

  3. Logical Errors:

These errors depict flaws in the logic .Program might be using the wrong formula.

Exceptions:

        An exception is a runtime error. Compile time errors are not exceptions. These are all errors.

         All exception occurs at runtime but some exceptions are detected at compile time and some others at runtime. The Exceptions that are checked by the compiler at compilation time are called Checked Exceptions. The Exceptions are checked by JVM at the runtime are called unchecked Exceptions.

                           +-------+

                    | Object |
                    +--------+
                        |
                        |
                   +-----------+
                   | Throwable |
                   +-----------+
                    /         \
                   /           \
          +-------+          +-----------+
          | Error |          | Exception |
          +-------+          +-----------+
           /  |  \           / |        \
         \________/       \______/       \
                                        +------------------+
        unchecked        checked        | RuntimeException |
                                        +------------------+
                                          /   |    |      \
                                         \_________________/
                                           
                                           unchecked

        Unchecked exceptions and errors are considered as unrecoverable and the programmer cannot do anything whey they occur.

The Exceptions occur at runtime but in case of checked exceptions whether you are handling it or not it is detected at compilation time.

                  public static void main(String args[]){

                            calFileReader();

               }

             public void calFileReader() throws IOException{

                     BufferedReader in = new BufferedReader(new FileReader("file.xml"));

                }

Exception Handling: 

               1. We can handle the exceptions using try, catch and finally.

               2. Handle multiple exceptions in a method.

               3. Only one exceptions can handle at a same time.

Types of Exceptions:

         There are two types of exceptions.

              1. Built in exceptions

                           - NullpointerException

                           - ArithmaticException

                           -ArrayIndexoutboundException

                           - NumberFormatException

             2. User defined exceptions

                      - User defined exceptions are done by extending the Exception class since Exception is the super class for Runtime exceptions/Unchecked Exceptions.

                public class MyException extends Exception {

                   public MyException() {

                    super();

                   }

                 public MyException(final String msg) {

                  super(msg);

                 }

             }