Exceptions

Chapter 15

15Throwing and Catching

ExceptionsWhen a program runs into a problem that it cannot handle, it throws an exception.

Exceptions are either caught (handled) or unhandled.

Unhandled exceptions cause the application to exit.

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Unhandled Exceptions

When Java encounters an unhandled exception (e.g., divide by zero), the compiler provides information that can help you find the source of the error.

15When an Exception is

ThrownProcessing stops at point of exception.

Java Virtual Machine (JVM) looks for a catch block to handle the exception.

JVM unwinds the stack, or retraces its steps searching for a catch block.

If no catch block is found, the program terminates with the default handler.

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Catching ExceptionsA try / catch block is the general approach to exception handling.

The try block contains the code that might throw an exception.

The catch block catches and handles the exception, if thrown.

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Throwing ExceptionsUse the throw keyword when throwing the exception.if ( divisor == 0 )

{

throw new ArithmeticException();

}

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Exception ClassificationsAll exceptions ultimately derive from Throwable class, which is divided into Error and Exception subclassesError exceptions are thrown by Java system internal errors (e.g., “out of memory”).

Error exceptions are never thrown by Java programmers.

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Exception ClassThe Exception class is divided into IOException and RunTimeException.

IOException problems are typically beyond your control (e.g., the disk is full).

RunTimeException usually indicates a programmer error (e.g., trying to write past the end of an array).

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Custom ExceptionsCreating your own exception classes allows you to throw more specific exceptions.

Exceptions can be used polymorphically.

Multiple exception handling must be coded in order from specific to more general.

An exception class derived from a more general exception must be caught first.

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The throws KeywordA method can declare that it throws a certain type of exception using the throws keyword.

“I am the readFile method and I might throw an IOException.”

public void readFile(String fileName) throws IOException

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The finally KeywordThe finally keyword works in conjunction with a try / catch block.finally “cleans up” after an exception

Important to include finally block to limit the chance of resource leaks due to open connections or files