Exceptions The Need for Exceptions Throwing Exceptions Exception Objects Object Construction Failure Catching Exceptions The Error Exception The Finally Clause OOP with Java, David J. Barnes Exceptions

The Need For Exceptions ''The World Outside'' is potentially hostile. A public interface allows external access. Possible exceptional conditions: Trying to move a ship that is moored. Trying to book a seat in a full theatre. Going faster than the local speed limit. The exceptional condition is based on the Ship's context, rather than its state. OOP with Java, David J. Barnes Exceptions

Possible Responses Print an error message. Terminate the program. Silently ignore the request. Implement the request anyway. Implement a modified or faked action. Request a correction interactively. Return the wrong answer. Return an error indication. OOP with Java, David J. Barnes Exceptions

Throwing an Exception Java's standard 'error indication' is an exception. Each is represented by an Exception object. ArithmeticException. ArrayIndexOutOfBoundsException. ClassCastException. NumberFormatException. OOP with Java, David J. Barnes Exceptions

The Throw Statement class UncheckedMain2 { public static void main(String[] args){ // Terminate the whole program. throw new RuntimeException( "I just can't cope!"); } Exception in thread "main" java.lang.RuntimeException: I just can't cope! at java.lang.Throwable.<init>(Throwable.java:78) at java.lang.Exception.<init>(Exception.java:42) at java.lang.RuntimeException.<init>(RuntimeException.java:47) at UncheckedMain2.main(UncheckedMain2.java:4) OOP with Java, David J. Barnes Exceptions

Exception Objects Exception objects have methods. public String getMessage(); public void printStackTrace(); public String toString(); These methods are used when an exception is caught, to provide diagnostic information. OOP with Java, David J. Barnes Exceptions

The Throws Clause public void setSpeed(double s) throws RuntimeException { // Check the validity of the request. if(Math.abs(s) > maximumSpeed){ // Too fast. Don't proceed. throw new RuntimeException("Speed "+s+ " is beyond the maximum of "+ maximumSpeed); } else{ speed = s; OOP with Java, David J. Barnes Exceptions

Object Construction Failure class Circle { public Circle(double r) throws RuntimeException { if(r <= 0){ throw new RuntimeException( "The radius must be positive: "+r); } radius = r; ... OOP with Java, David J. Barnes Exceptions

Checked Exceptions RuntimeException is an unchecked exception. Unchecked do not require a throws clause. Unchecked do not have to be 'caught'. Exception is a checked exception. Checked do require a throws clause. Checked do have to be dealt with in some way. OOP with Java, David J. Barnes Exceptions

Catching Exceptions An exception that is not caught or handled will result in immediate program failure. Catching an exception means we might be able to fix the problem. Try a slower speed for the ship. Java's try statement provides an exception handling (catching) mechanism. OOP with Java, David J. Barnes Exceptions

The Try Statement try{ // Things could go wrong within this block. ... } catch(Exception e){ // Something went wrong. finally{ OOP with Java, David J. Barnes Exceptions

Catching an Exception try{ // Construction could fail. Ship argo = new Ship(); double speed = ...; ... // Setting a new speed could fail. argo.setSpeed(speed); } catch(RuntimeException e){ System.err.println(e.getMessage()); OOP with Java, David J. Barnes Exceptions

Anticipating Multiple Exceptions try{ // The protected statements. ... } catch(IndexOutOfBoundsException e){ catch(ArithmeticException e){ catch(RuntimeException e){ OOP with Java, David J. Barnes Exceptions

Propagating Exceptions The rules for checked and unchecked exceptions differ. Unchecked exceptions. Propagated to the caller if there is no local caller. Checked exceptions. Must be handled locally, or propagated via an explicit throws clause. OOP with Java, David J. Barnes Exceptions

Rethrowing an Exception public void method() throws Exception { try{ // Protected statements ... } catch(Exception e){ // Partial recovery. // Rethrow the exception. throw e; OOP with Java, David J. Barnes Exceptions

The Error Exception The java.lang package defines Error as representing a particularly serious exception. One from which recovery is impossible. For instance, an out-of-memory problem. Use sparingly, and as a last resort. Anticipate exceptions, and plan for their recovery. OOP with Java, David J. Barnes Exceptions

The Finally Clause public void method(){ try{ // T2 might throw an exception. Statement-T1; Statement-T2; Statement-T3; } catch(Exception e){ Statement-C; finally{ Statement-F; OOP with Java, David J. Barnes Exceptions

Often Used to Close Files FileReader inFile = null; FileWriter outFile = null; try{ inFile = new FileReader(source); outFile = new FileWriter(destination,true); ... } finally{ if(inFile != null){ inFile.close(); if(outFile != null){ outFile.close(); OOP with Java, David J. Barnes Exceptions

Review An object might be asked to perform inappropriate actions. An object often does not know how to respond in such situations. Exceptions allow an object to leave the caller to sort out the problem. An exception is either checked or unchecked. OOP with Java, David J. Barnes Exceptions

Review (cont.) Exceptions may be caught and handled, or propagated. Uncaught (unhandled) exceptions result in premature program termination. The finally clause provides an always-executed route out of a try statement. Either the catch clause or finally clause may be omitted. OOP with Java, David J. Barnes Exceptions