Exception-Handling Overview
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1 م.عبد الغني أبوجبل Exception Handling No matter how good a programmer you are, you cannot control everything. Things can go wrong. Very wrong. When you write a risky method, you need code to handle the bad things that might happen. Exception-Handling Overview An exception is a runtime error. A program that does not provide code for catching and handling exceptions will terminate abnormally, and may cause serious problems. For example, if your program attempts to transfer money from a savings account to a checking account, but because of a runtime error is terminated after the money is drawn from the savings account and before the money is deposited in the checking account, the customer will lose money. Exceptions occur for various reasons. The user may enter an invalid input, or the program may attempt to open a file that doesn't exist, or the network connection may hang up, or the program may attempt to access an outof-bounds array element. You have already been briefly introduced to exceptions in earlier chapters. you learned that an ArrayIndexOutOfBoundsException occurs if you access an element past the end of an array. you learned that a NullPointerException occurs when you invoke a method on a reference variable with null value. you learned that a StringIndexOutOfBoundsException occurs when you access a character in a string using an out-of-bound index. When an exception occurs, the normal execution flow of the program will be interrupted. Java provides programmers with the capability to handle runtime exceptions. With this capability, referred to as exception handling, you can develop robust programs for mission-critical computing. Example Page 1
2 م.عبد الغني أبوجبل The program above will terminate abnormally if you enter a floating-point value instead of an integer. Note that several lines of information are displayed above in response to this invalid input. This information, known as the stack trace, includes the name of the exception (java.util.inputmismatchexception) followed by the method call stack at the time the exception occurred. The stack trace helps in debugging a program. Starting from the last line of the stack trace, you see that the exception was detected in line 7 of the main method. Each line of the stack trace contains the class name and method (ExceptionDemo.main) along with the file name and line number (ExceptionDemo.java: 7). Moving up the stack trace, you see that the exception occurred in line 2000 in the nextint method of the Scanner class, the exception occurred in line 2040 in the overloaded nextint method of the Scanner class, the exception occurred in line 1431 in the next method of the Scanner class, the exception occurred in line 819 in the throwfor method of the Scanner class. The last method in the call chain actually threw an InputMismatchException. The try Block The first step in constructing an exception handler is to enclose the code that might throw an exception within a try block. In general, a try block looks like the following: try { code catch and finally blocks... To construct an exception handler for the main method from the ExceptionDemo class, enclose the exception-throwing statements of the main method within a try block. There is more than one way to do this. You can put each line of code that might throw an exception within its own try block and provide separate exception handlers for each. Or, you can put all the code within a single try block and associate multiple handlers with it. The following listing uses one try block for the entire method because the code in question is very short. Page 2
3 م.عبد الغني أبوجبل If an exception occurs within the try block, that exception is handled by an exception handler associated with it. To associate an exception handler with a try block, you must put a catch block after it. The catch Blocks You associate exception handlers with a try block by providing one or more catch blocks directly after the try block. No code can be between the end of the try block and the beginning of the first catch block. try { catch (ExceptionType name) { catch (ExceptionType name) { Each catch block is an exception handler and handles the type of exception indicated by its argument. The argument type, ExceptionType, declares the type of exception that the handler can handle and must be the name of a class that inherits from the Throwable class. The handler can refer to the exception with name. The catch block contains code that is executed if and when the exception handler is invoked. The runtime system invokes the exception handler when the handler is the first one in the call stack whose ExceptionType matches the type of the exception thrown. The system considers it a match if the thrown object can legally be assigned to the exception handler's argument. Page 3
4 م.عبد الغني أبوجبل The finally Block The finally block always executes when the try block exits. This ensures that the finally block is executed even if an unexpected exception occurs. Exception Types RuntimeException, Error, and their subclasses are known as unchecked exceptions. All other exceptions are known as checked exceptions, meaning that the compiler forces the programmer to check and deal with them. Declaring Exceptions Every method must state the types of checked exceptions it might throw. This is known as declaring exceptions. Because system errors and runtime errors can happen to any code, Java does not require that you declare Error and RuntimeException (unchecked exceptions) explicitly in the method. However, all other exceptions thrown by the method must be explicitly declared in the method declaration so that the caller of the method is informed of the exception. Page 4
5 م.عبد الغني أبوجبل To declare an exception in a method, use the throws keyword in the method declaration, as in this example: public void mymethod() throws IOException The throws keyword indicates that mymethod might throw an IOException. If the method might throw multiple exceptions, add a list of the exceptions, separated by commas, after throws: public void mymethod() throws Exception1, Exception2,..., ExceptionN If a method does not declare exceptions in the superclass, you cannot override it to declare exceptions in the subclass. Throwing Exceptions A program that detects an error can create an instance of an appropriate exception type and throw it. This is known as throwing an exception. Here is an example: Suppose the program detected that an argument passed to the method violates the method contract (e.g., the argument must be non-negative, but a negative argument is passed); the program can create an instance of IllegalArgumentException and throw it, as follows: IllegalArgumentException ex = new IllegalArgumentException("Wrong Argument"); throw ex; Or if you prefer, you can use the following: throw new IllegalArgumentException("Wrong Argument"); The keyword to declare an exception is throws, and the keyword to throw an exception is throw Catching Exceptions You now know how to declare an exception and how to throw an exception. When an exception is thrown, it can be caught and handled in a try-catch block, as follows: try { statements; // Statements that may throw exceptions catch (Exception1 exvar1) { handler for exception1; catch (Exception2 exvar2) { handler for exception2;... catch (ExceptionN exvar3) { handler for exceptionn; Page 5
6 م.عبد الغني أبوجبل If no exceptions arise during the execution of the try block, the catch blocks are skipped. If one of the statements inside the try block throws an exception, Java skips the remaining statements in the try block and starts the process of finding the code to handle the exception. The code that handles the exception is called the exception handler; it is found by propagating the exception backward through a chain of method calls, starting from the current method. Each catch block is examined in turn, from first to last, to see whether the type of the exception object is an instance of the exception class in the catch block. If so, the exception object is assigned to the variable declared and the code in the catch block is executed. If no handler is found, Java exits this method, passes the exception to the method that invoked the method, and continues the same process to find a handler. If no handler is found in the chain of methods being invoked, the program terminates and prints an error message on the console. The process of finding a handler is called catching an exception. Suppose the main method invokes method1, method1 invokes method2, method2 invokes method3, and an exception occurs in method3, as shown in figure below Consider the following scenario: If method3 cannot handle the exception, method3 is aborted and the control is returned to method2. If the exception type is Exception3, it is caught by the catch block for handling exception ex3 in method2. statement5 is skipped, and statement6 is executed. If the exception type is Exception2, method2 is aborted, the control is returned to method1, and the exception is caught by the catch block for handling exception ex2 in method1. statement3 is skipped, and statement4 is executed. If the exception type is Exception1, method1 is aborted, the control is returned to the main method, and the exception is caught by the catch block for handling exception ex1 in the main method. statement1 is skipped, and statement2 is executed. If the exception type is not Exception1, Exception2, or Exception3, the exception is not caught and the program terminates. statement1 and statement2 are not executed. Various exception classes can be derived from a common superclass. If a catch block catches exception objects of a superclass, it can catch all the exception objects of the subclasses of that superclass. Page 6
7 م.عبد الغني أبوجبل The order in which exceptions are specified in catch blocks is important. A compilation error will result if a catch block for a superclass type appears before a catch block for a subclass type. For example, the ordering in (a) is erroneous, because RuntimeException is a subclass of Exception. The correct ordering should be as shown in (b). Java forces you to deal with checked exceptions. If a method declares a checked exception (i.e., an exception other than Error or RuntimeException), you must invoke it in a try-catch block or declare to throw the exception in the calling method. For example, suppose that method p1 invokes method p2, and p2 may throw a checked exception (e.g., IOException), you have to write the code as shown in (a) or (b). Getting Information from Exceptions An exception object contains valuable information about the exception. You may use the following instance methods in the java.lang.throwable class to get information regarding the exception, as shown below.the printstacktrace() method prints stack trace information on the console. The getstacktrace() method provides programmatic access to the stack trace information printed by printstacktrace(). Page 7
8 م.عبد الغني أبوجبل Example public class CircleWithException { /** The radius of the circle */ private double radius; /** The number of the objects created */ private static int numberofobjects = 0; /** Construct a circle with radius 1 */ public CircleWithException() { this(1.0); /** Construct a circle with a specified radius */ public CircleWithException(double newradius) { setradius(newradius); numberofobjects++; /** Return radius */ public double getradius() { return radius; /** Set a new radius */ public void setradius(double newradius) throws IllegalArgumentException { if (newradius >= 0 ) radius = newradius; else throw new IllegalArgumentException("Radius cannot be negative"); public static int getnumberofobjects() { return numberofobjects; Test Class: public class TestCircleWithException { /** Main method */ public static void main(string[] args) { try { CircleWithException c1 = new CircleWithException(5); CircleWithException c2 = new CircleWithException(-5); CircleWithException c3 = new CircleWithException(0); catch (IllegalArgumentException ex) { System.out.println(ex); System.out.println("Number of objects created: " + CircleWithException.getNumberOfObjects()); Page 8
9 م.عبد الغني أبوجبل True or False 1) A try block must be followed by a catch and a finally block( ). 2) If you write a method that might cause a compiler-checked exception, you must wrap that risky code in a try-catch block( ). 3) Catch blocks can be polymorphic( ) 4) Only compiler-checked exception can be caught(). 5) If you define a try-catch block, a matching finally block is optional( ). 6) If you define a try block, you can pair it with a matching catch or finally block, or both( ). 7) If you write a method that declares that it can throw a compiler-checked exception, you must also wrap the exception throwing code in a try-catch block( ). 8) The main() method in your program must handle all unhandled exceptions thrown to it( ). 9) A single try block can have many different catch blocks( ). 10) A method can only throw one kind of exception( ). 11) A finally block will run regardless of whether an exception is thrown(). 12) A finally block can exists without a try block( ). 13) A try block can exist by itself, without a catch block or a finally block( ). 14) The order of catch blocks never mattes( ). 15) A method with a try block and a finally block, can optionally declare the exception( ). 16) Runtime exceptions must be handled or declared( ). F,F,T,F,T,T,F,F,T,F,T,F,F,F,F,F Exercise Write a method to test whether a certain Arabic word contains then remove it. consider that they are saved in.txt file.(this exercise is a part from the final project). Page 9
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