ECE 122 Engineering Problem Solving with Java Lecture 24 Exceptions

1. ECE 122Engineering Problem Solving with JavaLecture 24Exceptions

2. Overview • Problem: Can we detect run-time errors and take corrective action? • Try-catch • Test for a variety of different program situations • Java provides a series of test cases • Allows for graceful exit of program or treatment of special cases

3. Errors • Errors do occur in programming. • Problems opening a file, dividing by zero, accessing an out-of-bounds array element, hardware errors, and many more • The question becomes: What do we do when an error occurs? • How is the error handled? • Who handles it? • Should the program terminate? • Can the program recover from the error? Should it? • Java uses exceptions to provide the error-handling capabilities for its programs.

4. Exceptions • An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions. • Represented as an object in Java • Throwing an exception • An error occurs within a method. An exception object is created and handed off to the runtime system. The runtime system must find the code to handle the error. • Catching an exception • The system searches for code to handle the thrown exception. It can be in the same method or in some method in the call stack.

5. Exceptions • An exception is an object that describes an unusual or erroneous situation • Exceptions are thrown by a program • May be caught and 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 • Usually represents a unrecoverable situation and should not be caught

6. Handling Exceptions • General form: try { statement(s); } catch (ExceptionTypename) { statement(s); } finally { statement(s); }

7. 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

8. Exception Handling • If exception is ignored by the program • The program will terminate abnormally and produce an appropriate message • The message includes a call stack trace that: • indicates the line on which the exception occurred • shows the method call trail that lead to the attempted execution of the offending line

9. Exception (derived from Throwable) RunTimeException ClassNotFoundException IOException ArithmeticException This is just a small part of the Exception hierarchy NullPointerException IndexOutOfBoundsException NumberFormatException Exception Class Hierarchy • Java has a predefined set of exceptions and errors that can occur during execution.

10. The try Statement • Handling 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 exception occurs: • Processing continues at the first catch clause that matches the exception type

11. Handling Exceptions • Three statements help define how exceptions are handled: • try- identifies a block of statements within which an exception might be thrown • catch - must be associated with a try statement and identifies a block of statements that can handle a particular type of exception. • The statements are executed if an exception of a particular type occurs within the try block. • A try statement can have multiple catch statements associated with it. • finally - must be associated with a try statement • identifies a block of statements that are executed regardless of whether or not an error occurs within the try block. • Even if the try and catch block have a return statement in them, finally will still run.

12. 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

13. Summary • Exceptions form an important part of Java • Graceful error checking and handling allow for better designed code • Try-catch evaluates a number of possibilities • Often error messages are the result