Exceptions Handling in Java

Transcription

1 Exceptions Handling in Exceptions in Compilers catch errors that occur at compilation time but not runtime errors. tries to deal with runtime error as well by using exception handlers. Exception = error in the program that occurs during its execution and disrupts the normal flow of instructions. Exception is a shorthand for exceptional event. Examples: division by zero, trying to access an out-of-bounds array elements, trying to open a file that does not exists, etc. tries to deal with errors in a more unified manner. When such an error occurs an exception object is created. This object contains information about the type of the error and the state of the program when the error occurred. The exception object is then passed to the runtime system and it is responsible to find some code to handle the error. Why use exceptions instead of return values? Forces error checking Cleans up your code by separating the normal case from the exceptional case. (The code isn't littered with a lot of if-else blocks checking return values.) Low overhead for non-exceptional case Separating Error Handling Code from "Regular" Code Propagating Errors Up the Call Stack Grouping Error Types and Error Differentiation Throwing an exception - in terms - creating an exception object and handling it to the runtime system to process it. Look at this later Exceptions An exception is an object that describes an unusual or erroneous situation Exceptions are thrown by a program, and may be caught and handled by another part of the program A program can therefore be separated into a normal execution flow and an exception execution flow An error is also represented as an object in, but usually represents a unrecoverable situation and should not be caught A program can deal with an exception in one of three ways: o ignore it o handle it where it occurs o handle it an another place in the program The manner in which an exception is processed is an important design consideration Louis Henry Page 1 22/04/2014

2 Exception Handling If an exception is ignored by the program, the program will terminate and produce an appropriate message The message includes a call stack trace that indicates on which line the exception occurred The call stack trace also shows the method call trail that lead to the execution of the offending line Categories of Exceptions Checked Exceptions versus Unchecked Checked exception: Must either be caught or declared within the method where it is thrown. Monitored by the compiler & Run-time. Example: IOException Unchecked exceptions: belong to a subclass of RuntimeException. Not monitored by the compiler. are subclasses of the RuntimeException class. do NOT need to be handled in order for the program to compile. do need to be handled if you don't want your program to die when an exception is thrown. Example: This program below will throw an ArithmeticException when the divide() method is called with zero as the denominator. Since ArithmeticException is a subclass of RuntimeException, it is an unchecked exception. This program below will compile, but when it runs an ArithmeticException will be thrown.and since this rithmeticexception is not handled, the program will die. Louis Henry Page 2 22/04/2014

3 public class UncheckedOne{ public static void main(string[] args) { int qa = divide(5, 2); System.out.println(qa); int qb = divide(5, 0); System.out.println(qb); public static int divide(int numerator, int denominator) { return numerator/denominator; The try Statement To process an exception when it occurs, the line that throws the exception is executed within a try block A try block is followed by one or more catch clauses, which contain code to process an exception Each catch clause has an associated exception type When an exception occurs, processing continues at the first catch clause that matches the exception type An exception is either checked or unchecked A checked exception can only be thrown within a try block or within a method that is designated to throw that exception The compiler will complain if a checked exception is not handled appropriately An unchecked exception does not require explicit handling, though it could be processed that way Catching an exception One of the advantages of exception handling is that it allows you to concentrate on the problem you are trying to solve in one place and then dealing with the errors from that code in another place. This is done by using try blocks and catch blocks in code. try { // code that might generate exceptions catch ( Exception1 ex1 ) { Louis Henry Page 3 22/04/2014

4 catch ( Exception2 ex2 ) { catch ( Exception3 ex3 ) { // etc. Exceptions can be handled in 2 ways: Key to Remember 1. Catch the exception in the method in which it is thrown 2. Append a clause to the method signature that notifies users that the method may throw an exception This program demonstrates the first way: catch the exception in the method in which it is thrown. public class UncheckedTwo { public static void main(string[] args) { int qa = divide(5, 2); System.out.println(qa); int qb = divide(5, 0); System.out.println(qb); public static int divide(int numerator, int denominator) { int result = 0; try { result = numerator/denominator; catch(arithmeticexception ex) { System.out.println(ex); result = 0; return result; Louis Henry Page 4 22/04/2014

5 In the try block you "try" your various method calls here. In the catch block you code the exception handlers. The handlers must appear directly after the try block. If an exception is thrown, the exception handling mechanism goes hunting for the first handler with an argument that matches the type of the exception. Then it enters that catch clause, and the exception is considered handled. The search for handlers stops once the catch clause is finished. Only the matching catch clause executes. Within the try block, a number of different method calls might generate the same exception, but you need only the handler. Finally Block The finally block is actually a catch block and it is optional; if it is present it is placed after the last of a try block's catch blocks: try { // code that might generate exceptions catch ( Exception1 ex1 ) { catch ( Exception2 ex2 ) { catch ( Exception3 ex3 ) { finally { // free the resources guarantees that a finally block (if one is present) will be executed regardless of whether or not any exception is thrown in a try block or any of tits corresponding catch blocks. also guarantees that a finally block will be executed if a try block is exited via a return, break or continue statement. Resource-release code is placed in a finally block. Suppose a resource is allocated in a try block. If no exception occurs, the catch handlers are skipped and control proceeds to the finally block, which frees the resource and control proceeds to the first statement after the finally block. Standard Exceptions contains a class called Throwable that describes everything that can be thrown as an exception. exceptions are packages, libraries and methods specific. Louis Henry Page 5 22/04/2014

6 Some java.lang package exceptions: Exception ClassNotFoundException IllegalAccessException NoSuchFieldException NoSuchMethodException RuntimeException ArithmeticException IllegalArgumentException IllegalThreadStateException NumberFormatException IllegalStateException IndexOutOfBoundsException ArrayIndexOutOfBoundsException StringIndexOutOfBoundsException SecurityException Some java.io package exceptions: Exception IOException CharConversionException EOFException FileNotFoundException InterruptedIOException ObjectStreamException UnsupportedCodingException UTFDataFormatException Exception guidelines - use exceptions to: 1. Fix the problem and call the method (which causes the exception) again. 2. Continue without retrying to execute the method again. 3. Do something else (e.g. calculate some alternative result) instead of what the method was supposed to do. 4. Do whatever you can in the current context and rethrow the same exception to a higher contest. 5. Do whatever you can in the current context and throw a different exception to a higher contest. 6. Terminate the program. Louis Henry Page 6 22/04/2014