Exceptions

Exceptional Cases

Exceptional Cases: events that cannot be controlled.

• ie. when a file doesn’t exist
• ie. when a server goes down.
• Java exception handling facilities are used when the invocation of a method may cause something exceptional to occur.

try-throw-catch Mechanism

• Java has build in exception handling which is achieved through a system of throwing and catching exceptions.
• Java has exceptions which are predefined, however the user can define their own as well.
• Exceptions can either be caught or passed on.
• Errors are similar to Exceptions in that they also are ‘throwable’ but should not be confused with exceptions.

Typical try-throw-catch Block

try {
    throw new ExceptionClass(Arguments);
} catch(Exception e) {
    //ExceptionHandlingCode
}

finally

• You can append a finally block at the end of a try-catch-block.
• The finally block will always be run after a try block (whether an exception is thrown or not).
• The finally block will even be executed after a return statement within a try or catch block.

Catching Method

• Exceptions can be caught within a code block by placing code that throws exceptions in a try block.
• The argument to the throw operator is always an object of some exception class.
• When catching multiple exceptions, the order of the catch block is important.
  • They are examined in order and the first one that matches is executed.
  • Throw more specific exceptions first; ie. IOException before Exception.

Declare Method

• As an alternative to handling an exception at the block level, exceptions can be passed at the method level.
• A throw statement is similar to a method call, but instead of calling a method, it calls a catch block.
• Eventually every thrown exception should be caught.

User-Defined Exceptions

• Must include a constructor which passes a string argument to super.
• Must include a no-argument constructor which passes a default string to super.
• example: public class BadNumberException extends Exception{...}.

Checked Exceptions

If a method throws a checked exception then it should be handled by either:

• Using a try-catch-block: try{}catch(Exception ex ){}.
• Including the throws keyword in its definition.

Predefined Checked Exception Classes

Unchecked Exceptions (Runtime)

• Anything which extends the RuntimeException class will (should) be an unchecked exception.
• Unchecked exceptions can be caught, however not doing so will not cause a runtime or compilation error.

Predefined RuntimeExceptions