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FinallyThere are times when you want some code to run regardless of an exception – you want it to run no matter what!

The finally block is where you put code that must run regardless of an exception.

A finally block lets you put all your important cleanup code in one place instead of duplicating it like this:

try {turnOvenOn( );x.bake( );turnOvenOff( );

} catch(BakingException ex) {

ex.printStackTrace();turnOvenOff( );

}

try {turnOvenOn( );x.bake( );

} catch(BakingException ex) {

ex.printStackTrace();} finally {

turnOvenOff( );}

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A Method Can Throw More Than One Exception

public class Laundry {public void doLaundry( ) throws PantsException, ShoeException {

// code that could throw either exception}

}

public class Foo {public void go( ) {

Laundry laundry = new Laundry( );try {

laundry.doLaundry( );} catch(PantsException ex) {

// recovery code} catch(ShoeException ex) {

// recovery code}

}}

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Exceptions Are Polymorphic

Don’t forget Exceptions are objects so a ClothingException can be extended into ShirtExceptions, PantsExceptions, and DressExceptions. Therefore:

You can DECLARE exceptions using a supertype of the exceptions you throw.

1

You can CATCH exceptions using a supertype of the exception thrown.

2

Look at page 330 for an example of polymorphic exceptions

Just because you CAN catch everything with one big super polymorphic catch, doesn’t mean you SHOULD.

Write a different catch block for each exception that you need to handle uniquely.

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Catching Multiple Exceptions

Multiple catch blocks must be ordered from smallest to biggest.

You can’t put bigger baskets above smaller baskets.

try {laundry.doLaundry( )

} catch (ClothingException ce) {

// recovery code goes here} catch (ShoeException se ) {

// recovery code goes here} catch (DressException de) {

// recovery code goes here}

Don’t do this!

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Paying It Forward

If you don’t want to handle an exception you can just throw it yourself so that whomever calls YOU will have to handle the exception.

If you call a risky method that does throw an exception, instead of you handling it, you can keep throwing it.

public void foo( ) throws PantsException, ShoeException {

// call risky method without a try/catch blocklaundry.doLaundry( );

}

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Ducking (by paying it forward) Only Delays the Inevitable

doLaundry( ) throws a ClothingException1

foo( ) ducks the exception2

main( ) ducks the exception3

The JVM shuts down4

public class Washer {Laundry laundry = new Laundry( );public void foo( ) throws ClothingException {

laundry.doLaundry( );}

public static void main(String[] args) throws ClothingException {Washer a = new Washer( );a.foo( );

}}

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Handle or Declare!So now we’ve seen both ways to satisfy the compiler when you call a risky (exception-throwing) method.

HANDLE – Wrap the risky call in a try/catch block1

DECLARE – duck it / pay it forward2

Let’s look at the sequencer to see how each method works.

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Exception Rules

You cannot have a catch or finally without a try

1

You cannot put code between the try and the catch

2

A try MUST be followed by either a catch or a finally

3

A try with only a finally (no catch) must still declare the exception

4

void go ( ) {Foo f = new

Foo( );f.foof( );

catch(fooException ex) { }}

try {x.doStuff( );

} finally {// cleanup

}

try {x.doStuff( );

} int y = 43;} catch (Exception ex ) { }

void go ( ) throws FooException {

try {x.doStuff(

);} finally { }

}

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Familiar Example

We now know what this means.

But, what about this?

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Input and Output

The java.io package includes a rich collection of different classes to support I/O.

Different classes provide different ways for programs to organize and retrieve data.

Java programs do not communicate directly with external devices, instead they create a stream object to connect the program to the device. Each stream functions as a conduit that establishes a path for the data to flow between the program and the I/O device.

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Streams

Java supports several different streams for different purposes.

Executing

Program

Output Stream

File (on disk)

Executing

ProgramInput Stream

File (on disk)

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Stream HierarchyOutput Stream

(abstract)

FileOutputStream

ObjectOutputStream

FilterOutputStream

DataOutputStream

to write raw bytes

to write whole objects

to write primitive values

Input Stream (abstract)

FileInputStream

ObjectInputStream

FilterInputStream

DataInputStream

to read raw bytes

to read whole objects

to read primitive values

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Using Streams

1. Open the file for input, instantiating associated stream objects.

2. Call read methods to retrieve part of or the entire stream’s content.

3. Close the file/stream.

1. Open the file for output, instantiating associated stream objects.

2. Call read methods to write data into the stream.

3. Close the file/stream.

Input Streams

Output Streams

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DataInputStreams & DataOutputStreams

FileOutputStream

Executing Program

DataOutputStream

File (on disk)

FileInputStream

Executing Program

DataInputStream

File (on disk)

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Text Files

Input StreamExecuting Program

Reader

File (on disk)

Input StreamExecuting Program

Reader

File (on disk)

BufferedRea

der

Output StreamExecuting Program

Writer

File (on disk)

BufferedWri

ter

Output StreamExecuting Program

Writer

File (on disk)

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Familiar Example

What if we didn’t “duck” the exceptions?

Now we know what this means

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Input and Output of Files

Use a FileReader/FileWriter objects as your “Reader” conduit.

Input StreamExecuting Program

FileReader

File (on disk)

Input StreamExecuting Program

FileReader

File (on disk)

BufferedRea

der

Output StreamExecuting Program

FileWriter

File (on disk)

BufferedWri

ter