Exceptions

1. Exceptions

2. Exceptions • Many problems in code are handled when the code is compiled, but not all • Some are impossible to catch before the program is run • Must run the program to actually determine the values of variables, get user input, etc. • These errors that occur when the program is running are “exceptions”

3. Exceptions • An exception is an object that describes an unusual or erroneous situation • Exceptions are generated by part of a program, and may be handled by another part of the program • A program can be separated into a normal execution flow and an exception execution flow • An error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught

4. Exception Handling • Java has a predefined set of exceptions and errors that can occur during execution • A program can deal with an exception in one of three ways: • ignore it • handle it where it occurs • handle it an another place in the program • The manner in which an exception is processed is an important design consideration

5. Exception Handling • If an exception isn’t handled by the program, it halts and prints an error: Exception in thread “main” java.lang.NullPointerException at SomeClass.method(SomeClass.java:54) at Except.e2(Except.java:12) at Except.main(Except.java:22) • The error message gives the type of exception that was thrown (NullPointerException) • and the “stack trace”…

6. The Call Stack Trace • Indicates the line on which the exception occurred • (SomeClass.java:54) • Shows the method call trail that lead to the attempted execution of the offending line • main→ e2 → method

7. Tracing Problems • The call stack can be very long, and call methods in many libraries Exception in thread “main” java.util.UnknownFormatConversionException: Conversion = ‘i’ at java.util.Formatter$FormatSpecifier.conversion(Formatter.java:2603) at java.util.Formatter$FormatSpecifier.<init>(Formatter.java:2631) at java.util.Formatter.parse(Formatter.java:2477) at java.util.Formatter.format(Formatter.java:2411) at java.io.PrintStream.format(PrintStream.java:899) at java.io.PrintStream.printf(PrintStream.java:800) at Except.e3(Except.java:17) at Except.main(Except.java:22) • It can be tricky to find the “real” cause • should be the innermost code that isn’t fully tested

8. The Exception Class • In Java, exceptions are objects • There is a hierarchy of exception types, with Exception at the top • you don’t generally instantiate Exception; one is automatically created when there is an error • There are many subclasses of Exception that are used for particular types of errors

9. Exception Subclasses • The subclasses of Exception are types that more specifically identify the problem, e.g.: • ArithmeticException • most often caused by a divide-by-zero • IndexOutOfBoundsException • array/List/String index doesn’t exist • NoClassDefFoundError • .class file was there when compiling, but not there now • Tend to have informative names: useful errors

10. The Exception Class Hierarchy • All error and exception classes are descendents of the Throwable class • A programmer can define an exception by extending the Exception class or one of its descendants • The parent class used depends on how the new exception will be used

11. Catching Exceptions • We say that a certain block of code “throws” an exception • think: when the exception occurs, an exception object is generated and tossed at the interpreter • How do you handle something that is thrown at the interpreter? • you “catch” it • Uncaught exceptions stop the program… caught exceptions do not

12. The try Statement • To handle an exception in a program, the line that throws the exception is executed within a try block • A try block is followed by one or more catch clauses • Each catch clause has an associated exception type and is called an exception handler • When an exception occurs, processing continues at the first catch clause that matches the exception type

13. A try/catch sequence try { \\ execute the code that you find here } catch(Exception e) { \\ if the there is an exception \\ then you do the stuff in this part }

14. An Example try { int x; x = 10/0; } catch(Exception e) { System.out.println(“error occurred”); }

15. The Exception Parameter • The catch is a little like an else, and a little like a function definition • try {…} catch(Exception e) {…} • The argument (“exception parameter”) is assigned to an appropriate Exception object • In the divide-by-zero example, e will be an instance of ArithmeticException • The exception parameter’s value can be ignored if you don’t need to use it

16. Another Example try { int x; x = 10/0; } catch(ArithmeticException e) { System.out.println(“divided by 0”); }

17. Being Specific • Can have multiple catch blocks • The exception parameter type allows different types of exceptions to be handled differently

18. Being Specific try { int x = userin.nextInt(); System.out.println(10/x); } catch(ArithmeticException e) { System.out.println(“no zeroes”); } catch(InputMismatchException e) { System.out.println(“enter a number”); } Most likely causes

19. Being Specific • The exception parameter is matched like arguments in an overloaded function • The type of exception thrown is matched against the exception parameters • That class… or any subclass… will match • Examples: • Exception matches all exceptions • IndexOutOfBounds matches both array and string exceptions

20. The Catch All try { … a bunch of code, involving an integer x… System.out.println(10/x); } catch(ArithmeticException e) { System.out.println(“no zeroes”); } catch(Exception e) { System.out.println(“some other problem”); }

21. However… • In general, use a type as specific as possible • only catch the exceptions you want to catch and handle properly • Don’t incorrectly handle other problems • the catch-all hides problems you might want to know about • Other exceptions are propogated • they can be caught farther up the call stack

22. Exception Propagation • An exception can be handled at a higher level if it is not appropriate to handle it where it occurs • Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the level of the main method • A try block that contains a call to a method in which an exception is thrown can be used to catch that exception

23. Checked versus Unchecked • An exception is either checked or unchecked • An unchecked exception does not require explicit handling, though it could be processed that way • The only unchecked exceptions in Java are objects of type RuntimeException or any of its descendants • Errors are similar to RuntimeException and its descendants in that: • Errors should not be caught • Errors do not require a throws clause

24. Checked Exceptions • Checked exceptions (other subclasses of Exception) must be handled explicitly • must be caught by a method in a catch block • must be listed in the throws clause of any method that might throw it • The compiler will issue an error if a checked exception is not caught or asserted in a throws clause

25. The throws Clause • Used to indicate that the method will “handle” the checked exception by passing it off to the calling code • Similarly, the calling code must either catch it or be declared indicating that it throws the exception public int myMethod() throws Exception1, Exception2 • Any uncaught checked exceptions must be listed in the throws clause

26. The throw Statement • When your code gets to a situation it can’t handle, it can throw an exception • e.g. constructor/setter given an illegal value • Create an instance of the appropriate exception class • new SomeException(“Error message”); • Throw the exception with a throw statement • throw new SomeException(“message”);

27. The throw Statement • The throw statement appears in the body of a method, with an appropriate throws clause • Usually a throw statement is executed inside an if statement that evaluates a condition to see if the exception should be thrown if(x<0) throw new NegativeInputException;

28. Example • From the Student class: public void setFirstName(String name) { if(name.length()>0) firstName = name; else throw new IllegalArgumentException (“Name must have length >0”); }

29. The finally Clause • A try statement can have an optional clause following the catch clauses, designated by the reserved word finally • The statements in the finally clause always are executed • If no exception is generated, the statements in the finally clause are executed after the statements in the try block complete • If an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete

30. Why use finally ? • To make sure that a certain part of an algorithm are executed • To leave objects in a stable state • e.g. An object representing a database connection might be disconnected when an error occurs • … there are good security reasons for doing this