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CPSC150

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Exceptions

When things go TRAGICALLY AWRY

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How things go TRAGICALLY AWRY

• User enters bad data

• Programmer makes a mistake

• Another method/program passes bad data to a method

• System problems happen

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When others use your program

• Check values being returned are reasonable. Don't do actions that will cause problems

• Create return value that indicates a problem so clients can:– Attempt recovery on error– Avoid program failure– Ignore the return value

• Cannot be prevented• Likely to lead to program failure

• Create something client cannot ignore: – an exception

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Exception-throwing principles

• Exceptions are part of many languages, central to Java.

• A special language feature.

• No ‘special’ return value needed.

• Errors cannot be ignored in the client.– The normal flow-of-control is interrupted.

• Specific recovery actions are encouraged.

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Exceptions

• Exceptions are “thrown” by the method finding the problem

• Exceptions are “caught” by the method dealing with the problem

• maintains integrity and decoupling

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The effect of an exception

• The throwing method finishes prematurely.

• No return value is returned.

• Control does not return to the client’s point of call.– So the client cannot carry on regardless.

• A client may ‘catch’ an exception.

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Throwing an exception

public void setDenominator(int den){ if(den == 0) { throw new Exception(“Divide by 0"); } denominator = den;}

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Throwing an exception

• An exception object is constructed:– new ExceptionType("...");– Most often, use existing Exception types

• The exception object is thrown:– throw ...– Most often, exception is thrown by java

classes, not programmer-defined

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Error recovery

• Clients should take note of error notifications.– Check return values.– Don’t ‘ignore’ exceptions.

• Include code to attempt recovery.– Will often require a loop.

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Handling Exceptions:The try-catch statement

• Clients catching an exception must protect the call with a try statement:

try {// put statements here that might throw an // exception

}catch(Exception e) { // Report and recover from the

// exception here.}

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The try-catch statement

1. Exception thrown from here

2. Control transfers to here

Fraction f;

try {

f.setDenominator(n); // n is an already defined with a value

}

catch (ArithmeticException ex) {

System.out.println("you dummy, use something other than 0");

}

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try { // code that might throw an exception }catch (TypeOfException ex) { // what to do when there is a problem }// what to do afterwards regardless of // whether there was an exception

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1. if method call, do method

try { // code that might throw an exception }catch (TypeOfException ex) { // what to do when there is a problem }// what to do afterwards regardless of // whether there was an exception

Order of statement execution

2a. continue with rest of try

3a. skip catch, do code after try-catch

Case a: no exception thrown

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try

{

// code that might throw an exception

}

catch (TypeOfException ex)

{

// what to do when there is a problem

}

// what to do afterwards regardless of

// whether there was an exception



2b. skip rest of try; do code in catch

3b. if catch code does not abort program, continue after catch block

Case b: TypeOfException thrown

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try

{

// code that might throw an exception

{

catch (TypeOfException ex)

{

// what to do when there is a problem

}

// what to do afterwards regardless of

// whether there was an exception



2c. skip rest of try; do not do catch; exit from this method at the code in try; do not do any other code

3c. No other code in this method is executed. catch code and code afterwards is skipped

Case c: DifferentException thrown

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public class George{private int x;

public George() { x = 0; }

public int divideByY(int y) {

if (y == 0) { throw new ArithmeticException( "in divideByY, y, a denominator, is 0"); }

y= x /y; return y; } }

An Exception Example

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• Write a main method that calls divideByY

• Write a main method that has try-catch block

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public static void main(String[] args)

{

George m = new George();

System.out.println("m with 4 is " + m.divideByY(4));

System.out.println("m with 0 is " + m.divideByY(0));

System.out.println("all done");

} // end main

} // end class

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public static void main(String args[ ]) {George m = new George();

try {System.out.println("m with 4 is " +

m.divideByY(4)); System.out.println("m with 0 is " +

m.divideByY(0)); } catch (ArithmeticException e) { System.out.println("don't call method with 0 as parameter");

e.printStackTrace(); } System.out.println("all done"); }

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Try-Catch second chance

• Can put try-catch blocks in a loop.– if catch block not entered, exit loop– if catch block entered, print a message; ask

the user to fix the problem– if catch block entered too many times, give up

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public class Test { public static void main(String args[]) { int n = 0; Scanner kbd = new Scanner(System.in); int mynbr; String s = null; boolean valid = false; while (!valid && n<3) { try { System.out.print(“Enter an integer: ”); mynbr = kbd.nextInt( ); valid = true; } catch (Exception georgette) { System.out.print(“You must enter a valid integer-> ”);

n++; } }// end of loop if (valid) System.out.println("The number is " + mynbr ); }}

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Catching multiple exceptionstried in order that they appear

try { ... ref.process(); ...}catch(EOFException e) { // Take action on an end-of-file exception. ...}catch(IOException e) { // Take action on any other IOexception. ...}

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The finally clause

try { Protect one or more statements here.}catch(Exception e) { Report and recover from the exception here.}finally { Perform any actions here common to whether or not an exception is thrown.}

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The finally clause: why

• A finally clause is executed even if a return statement is executed in the try or catch clauses.

• A uncaught or propagated exception still exits via the finally clause.

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The exception class hierarchy

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Exception categories

• Checked exceptions: extra syntax required– Subclass of Exception– Use for anticipated failures.– Where recovery may be possible.– Often not a programming problem (e.g., file not found)

• Unchecked exceptions: extra syntax NOT required– Subclass of RuntimeException– Use for unanticipated failures.– Where recovery is unlikely.– Often, a programming problem (e.g., index out of bounds)

• Third category: errors

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Checked Exceptions• Previous examples all unchecked

exceptions• Checked exceptions are meant to be

caught.• The compiler ensures that their use is

tightly controlled.– In both server and client.

• Used properly, failures may be recoverable.

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Some Checked Exceptions

– ClassNotFoundException– IOException– NoSuchFieldException– ServerNotActiveException– UnsupportedFlavorException

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The throws clause

• Methods throwing a checked exception must handle the exception or include a throws clause:

public void saveToFile(String destinationFile) throws IOException

• (NOT throw) throw is when the exception is thrown

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throws

• must include throws in methods that throw checked exceptions

• how do I know?

• javadoc/syntax errors

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Text input-output:tie to exceptions

•java.io.IOException is a checked exception.

• cannot do files without exceptions

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