Exception Handling in Java Topics: Introduction Errors and Error handling Exceptions Types of Exceptions Coding Exceptions Summary

Introduction Users have high expectations for the code we produce. Users will use our programs in unexpected ways. Due to design errors or coding errors, our programs may fail in unexpected ways during execution It is our responsibility to produce quality code that does not fail unexpectedly. Consequently, we must design error handling into our programs.

Errors and Error Handling An Error is any unexpected result obtained from a program during execution. Unhandled errors may manifest themselves as incorrect results or behavior, or as abnormal program termination. Errors should be handled by the programmer, to prevent them from reaching the user. Some typical causes of errors: Memory errors (i.e. memory incorrectly allocated, memory leaks, “null pointer”) File system errors (i.e. disk is full, disk has been removed) Network errors (i.e. network is down, URL does not exist) Calculation errors (i.e. divide by 0)

Errors and Error Handling More typical causes of errors: Array errors (i.e. accessing element –1) Conversion errors (i.e. convert ‘q’ to a number) Can you think of some others? Traditional Error Handling 1. Every method returns a value (flag) indicating either success, failure, or some error condition. The calling method checks the return flag and takes appropriate action. Downside: programmer must remember to always check the return value and take appropriate action. This requires much code (methods are harder to read) and something may get overlooked.

Errors and Error Handling Traditional Error Handling Where used: traditional programming languages (i.e. C) use this method for almost all library functions (i.e. fopen() returns a valid file or else null) 2. Create a global error handling routine, and use some form of “jump” instruction to call this routine when an error occurs. Downside: “jump” instruction (GoTo) are considered “bad programming practice” and are discouraged. Once you jump to the error routine, you cannot return to the point of origin and so must (probably) exit the program.

Errors and Error Handling Traditional Error Handling Where used: many older programming texts (C, FORTRAN) recommended this method to programmers. Those who use this method will frequently adapt it to new languages (C++, Java).

Errors and Error Handling Exceptions – a better error handling Exceptions are a mechanism that provides the best of both worlds. Exceptions act similar to method return flags in that any method may raise and exception should it encounter an error. Exceptions act like global error methods in that the exception mechanism is built into Java; exceptions are handled at many levels in a program, locally and/or globally.

Exceptions What are they? An exception is a representation of an error condition or a situation that is not the expected result of a method. Exceptions are built into the Java language and are available to all program code. Exceptions isolate the code that deals with the error condition from regular program logic.

Exceptions How are they used? Exceptions fall into two categories: Checked Exceptions Unchecked Exceptions Checked exceptions are inherited from the core Java class Exception. They represent exceptions that are frequently considered “non fatal” to program execution Checked exceptions must be handled in your code, or passed to parent classes for handling.

Exceptions How are they used? Unchecked exceptions represent error conditions that are considered “fatal” to program execution. You do not have to do anything with an unchecked exception. Your program will terminate with an appropriate error message. Examples: Checked exceptions include errors such as “array index out of bounds”, “file not found” and “number format conversion”. Unchecked exceptions include errors such as “null pointer”.

Exceptions How do you handle exceptions? Exception handling is accomplished through the “try – catch” mechanism, or by a “throws” clause in the method declaration. For any code that throws a checked exception, you can decide to handle the exception yourself, or pass the exception “up the chain” (to a parent class).

Exceptions How do you handle exceptions? To handle the exception, you write a “try-catch” block. To pass the exception “up the chain”, you declare a throws clause in your method or class declaration. If the method contains code that may cause a checked exception, you MUST handle the exception OR pass the exception to the parent class (remember, every class has Object as the ultimate parent)

Coding Exceptions Try-Catch Mechanism Wherever your code may trigger an exception, the normal code logic is placed inside a block of code starting with the “try” keyword: After the try block, the code to handle the exception should it arise is placed in a block of code starting with the “catch” keyword.

Coding Exceptions Try-Catch Mechanism You may also write an optional “finally” block. This block contains code that is ALWAYS executed, either after the “try” block code, or after the “catch” block code. Finally blocks can be used for operations that must happen no matter what (i.e. cleanup operations such as closing a file)

Coding Exceptions Example try { … normal program code } catch(Exception e) { … exception handling code }

Coding Exceptions Passing the exception In any method that might throw an exception, you may declare the method as “throws” that exception, and thus avoid handling the exception yourself Example public void myMethod throws IOException { … normal code with some I/O }

Coding Exceptions Types of Exceptions All checked exceptions have class “Exception” as the parent class. You can use the actual exception class or the parent class when referring to an exception Where do you find the exception classes? Reference books such as “Java in a Nutshell” (O’Reilly, 2001), or the Java Documentation.

Coding Exceptions Types of Exceptions Examples: public void myMethod throws Exception { public void myMethod throws IOException { try { … } catch (Exception e) { … } try { … } catch (IOException ioe) { … } 1. Demonstration of an unchecked exception (NullPointerException) 2. Demonstration of checked exceptions: Passing a DivideByZeroException Handling a DivideByZeroException

Summary Exceptions are a powerful error handling mechanism. Exceptions in Java are built into the language. Exceptions can be handled by the programmer (try-catch), or handled by the Java environment (throws).

Nested try Statements The try statement can be nested. That is, a try statement can be inside the block of another try.

class NestTry { public static void main(String args[]) { try { int a = args.length; int b = 42 / a; System.out.println("a = " + a); try { if(a==1) a = a/(a-a); // division by zero if(a==2) { int c[ ] = { 1 }; c[42] = 99; } catch(ArrayIndexOutOfBoundsException e) { System.out.println("Array index out-of-bounds: " + e); } catch(ArithmeticException e) { System.out.println("Divide by 0: " + e); }

Normal program execution is immediately branched when an exception is thrown. Checked exceptions must be caught or forwarded. This can be done in a try... catch statement or by defining the exception in the method definition. The exception is caught by the first catch block whose associated exception class matches the class or a superclass of the thrown exception. If no matching catch block is found in the exception chain, the thread containing the thrown exception is terminated. The finally block after a try... catch statement is executed regardless whether an exception is caught or not. Returning within a finally block breaks the exception chain to the invoker even for uncaught exceptions.

Exceptions in Java Exceptions are nothing but some anomalous conditions that occur during the execution of the program. Exceptions are the conditions or typically an event which may interrupt the normal flow of the program's instructions. Exceptions in Java Exception Classes The hierarchy of exception classes commence from Throwable class which is the base class in java.lang. This class can be instantiated and thrown by the program. The Throwable class is further divided into two subclasses :- Exception Classes Catching and Handling Exceptions The three exception handler components are used to catch and handle the exceptions. These are try, catch and finally clause. The mechanism to catch an exception in Java is to use try and catch block. Catching and Handling Exceptions

How to Throw Exceptions Before catching an exception it is must to be thrown first. This means that there should be a code somewhere in the program that could catch the exception. How to Throw Exceptions Handling Multiple Catch Clauses In java when we handle the exceptions then we can have multiple catch blocks for a particular try block to handle many different kind of exceptions that may be generated while running the program. Handling Multiple Catch Clauses Nested Try-Catch Blocks In Java we can have nested tryand catch blocks. It means that, a try statement can be inside the block of another try. Nested Try-Catch Blocks Catching Normal Exceptions The exceptions that are generated by methods are referred to as normal exceptions. We have already learned that to catch an exception we use try and catch block. Catching Normal Exceptions

Making Custom (User Defined) Exceptions If you encounter a situation where none of those exception describe your exception accurately or if you can't find the appropriate exception in the Java API, you can code a class that defines an exception that is more appropriate Making Custom (User Defined) Exceptions What are Chained Exceptions Chained exceptions are the exceptions which occur one after another i.e. most of the time to response to an exception is given by an application by throwing another exception. What are Chained Exceptions How to Print a Stack Trace Message Instead of this this method you can get more information about the error process if you print a stack trace from the exception using the printStackTrace() method that prints the stack trace to the console How to Print a Stack Trace Message