Editing JavaLanguage (section)

==Exception Handling==
An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions. When an error occurs within a method, the method creates an object and hands it off to the runtime system. The object, called an '''exception object''', contains information about the error, including its type and the state of the program when the error occurred. Creating an exception object and handing it to the runtime system is called '''throwing an exception'''. The runtime system searches the call stack for a method that contains a block of code that can handle the exception. This block of code is called an '''exception handler'''. The exception handler chosen is said to '''catch the exception'''. 

'''Checked''' exceptions have to be catched. Checked exceptions are exceptions that must be declared in the throws clause of a method. They extend Exception and are intended to be an "in your face" type of exceptions. A checked exception indicates an expected problem that can occur during normal system operation. Some examples are problems communicating with external systems, and problems with user input. 

'''Unchecked''' exceptions are exceptions that do not need to be declared in a throws clause. They extend RuntimeException. An unchecked exception indicates an unexpected problem that is probably due to a bug in the code. The most common example is a NullPointerException. '''Errors''' are serious problems that are almost certainly not recoverable. Some examples are OutOfMemoryError, LinkageError, and StackOverflowError.

Since JDK 1.4 there is a new ctor for Throwable with a cause, another Throwable object. This feature is also known as exception chaining.

 try
 {
   throw ... // explicit throwing or
   x.method ... // method is throwing exception
 }
 catch (<ExceptionType> e)
 {
   ...
 }
 finally
 {
   ... // always executed
 }

The code after the '''throw''' statement is not executed. The finally code is always executed no matter what happens inside the try block and it is a good place for general clean-up. If the clean-up itself produces exceptions they have to be handled inside finally or thrown.