What is an exception How are exceptions handled in Java explain with example?
An exception (or exceptional event) is a problem that arises during the execution of a program. When an Exception occurs the normal flow of the program is disrupted and the program/Application terminates abnormally, which is not recommended, therefore, these exceptions are to be handled. Show
An exception can occur for many different reasons. Following are some scenarios where an exception occurs.
Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner. Based on these, we have three categories of Exceptions. You need to understand them to know how exception handling works in Java.
For example, if you use FileReader class in your program to read data from a file, if the file specified in its constructor doesn't exist, then a FileNotFoundException occurs, and the compiler prompts the programmer to handle the exception. Exampleimport java.io.File; import java.io.FileReader; public class FilenotFound_Demo { public static void main(String args[]) { File file = new File("E://file.txt"); FileReader fr = new FileReader(file); } } If you try to compile the above program, you will get the following exceptions. OutputC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error Note − Since the methods read() and close() of FileReader class throws IOException, you can observe that the compiler notifies to handle IOException, along with FileNotFoundException.
For example, if you have declared an array of size 5 in your program, and trying to call the 6th element of the array then an ArrayIndexOutOfBoundsExceptionexception occurs. Examplepublic class Unchecked_Demo { public static void main(String args[]) { int num[] = {1, 2, 3, 4}; System.out.println(num[5]); } } If you compile and execute the above program, you will get the following exception. OutputException in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5 at Exceptions.Unchecked_Demo.main(Unchecked_Demo.java:8)
Exception HierarchyAll exception classes are subtypes of the java.lang.Exception class. The exception class is a subclass of the Throwable class. Other than the exception class there is another subclass called Error which is derived from the Throwable class. Errors are abnormal conditions that happen in case of severe failures, these are not handled by the Java programs. Errors are generated to indicate errors generated by the runtime environment. Example: JVM is out of memory. Normally, programs cannot recover from errors. The Exception class has two main subclasses: IOException class and RuntimeException Class. Following is a list of most common checked and unchecked Java's Built-in Exceptions. Exceptions MethodsFollowing is the list of important methods available in the Throwable class. Sr.No.Method & Description1public String getMessage() Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor. 2public Throwable getCause() Returns the cause of the exception as represented by a Throwable object. 3public String toString() Returns the name of the class concatenated with the result of getMessage(). 4public void printStackTrace() Prints the result of toString() along with the stack trace to System.err, the error output stream. 5public StackTraceElement [] getStackTrace() Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack, and the last element in the array represents the method at the bottom of the call stack. 6public Throwable fillInStackTrace() Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stack trace. Catching ExceptionsA method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following − Syntaxtry { // Protected code } catch (ExceptionName e1) { // Catch block } The code which is prone to exceptions is placed in the try block. When an exception occurs, that exception occurred is handled by catch block associated with it. Every try block should be immediately followed either by a catch block or finally block. A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed into a method parameter. ExampleThe following is an array declared with 2 elements. Then the code tries to access the 3rd element of the array which throws an exception. // File Name : ExcepTest.java import java.io.*; public class ExcepTest { public static void main(String args[]) { try { int a[] = new int[2]; System.out.println("Access element three :" + a[3]); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Exception thrown :" + e); } System.out.println("Out of the block"); } } This will produce the following result − OutputException thrown :java.lang.ArrayIndexOutOfBoundsException: 3 Out of the block Multiple Catch BlocksA try block can be followed by multiple catch blocks. The syntax for multiple catch blocks looks like the following − Syntaxtry { // Protected code } catch (ExceptionType1 e1) { // Catch block } catch (ExceptionType2 e2) { // Catch block } catch (ExceptionType3 e3) { // Catch block } The previous statements demonstrate three catch blocks, but you can have any number of them after a single try. If an exception occurs in the protected code, the exception is thrown to the first catch block in the list. If the data type of the exception thrown matches ExceptionType1, it gets caught there. If not, the exception passes down to the second catch statement. This continues until the exception either is caught or falls through all catches, in which case the current method stops execution and the exception is thrown down to the previous method on the call stack. ExampleHere is code segment showing how to use multiple try/catch statements. try { file = new FileInputStream(fileName); x = (byte) file.read(); } catch (IOException i) { i.printStackTrace(); return -1; } catch (FileNotFoundException f) // Not valid! { f.printStackTrace(); return -1; } Catching Multiple Type of ExceptionsSince Java 7, you can handle more than one exception using a single catch block, this feature simplifies the code. Here is how you would do it − catch (IOException|FileNotFoundException ex) { logger.log(ex); throw ex; The Throws/Throw KeywordsIf a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature. You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword. Try to understand the difference between throws and throw keywords, throws is used to postpone the handling of a checked exception and throw is used to invoke an exception explicitly. The following method declares that it throws a RemoteException − ExampleC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error0 A method can declare that it throws more than one exception, in which case the exceptions are declared in a list separated by commas. For example, the following method declares that it throws a RemoteException and an InsufficientFundsException − ExampleC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error1 The Finally BlockThe finally block follows a try block or a catch block. A finally block of code always executes, irrespective of occurrence of an Exception. Using a finally block allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code. A finally block appears at the end of the catch blocks and has the following syntax − SyntaxC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error2 ExampleC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error3 This will produce the following result − OutputC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error4 Note the following −
The try-with-resourcesGenerally, when we use any resources like streams, connections, etc. we have to close them explicitly using finally block. In the following program, we are reading data from a file using FileReader and we are closing it using finally block. ExampleC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error5 try-with-resources, also referred as automatic resource management, is a new exception handling mechanism that was introduced in Java 7, which automatically closes the resources used within the try catch block. To use this statement, you simply need to declare the required resources within the parenthesis, and the created resource will be closed automatically at the end of the block. Following is the syntax of try-with-resources statement. SyntaxC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error6 Following is the program that reads the data in a file using try-with-resources statement. ExampleC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error7 Following points are to be kept in mind while working with try-with-resources statement.
User-defined ExceptionsYou can create your own exceptions in Java. Keep the following points in mind when writing your own exception classes −
We can define our own Exception class as below − C:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error8 You just need to extend the predefined Exception class to create your own Exception. These are considered to be checked exceptions. The following InsufficientFundsException class is a user-defined exception that extends the Exception class, making it a checked exception. An exception class is like any other class, containing useful fields and methods. ExampleC:\>javac FilenotFound_Demo.java FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error9 To demonstrate using our user-defined exception, the following CheckingAccount class contains a withdraw() method that throws an InsufficientFundsException. public class Unchecked_Demo { public static void main(String args[]) { int num[] = {1, 2, 3, 4}; System.out.println(num[5]); } }0 The following BankDemo program demonstrates invoking the deposit() and withdraw() methods of CheckingAccount. public class Unchecked_Demo { public static void main(String args[]) { int num[] = {1, 2, 3, 4}; System.out.println(num[5]); } }1 Compile all the above three files and run BankDemo. This will produce the following result − Outputpublic class Unchecked_Demo { public static void main(String args[]) { int num[] = {1, 2, 3, 4}; System.out.println(num[5]); } }2 Common ExceptionsIn Java, it is possible to define two catergories of Exceptions and Errors.
|