Exception Handling in Java. An Object oriented Way for Handling Errors

Transcription

1 Exception Handling in Java An Object oriented Way for Handling Errors

2 An Exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's Instructions. This is one of the powerful feature of Java to handle run time error and maintain normal flow of java application. It is common to make mistakes while developing as well as typing a program. Such mistakes are categorised as: syntax errors - compilation errors. semantic errors leads to programs producing unexpected outputs. runtime errors most often lead to abnormal termination of programs or even cause the system to crash.

3 Dividing a number by zero. Accessing an element that is out of bounds of an array. Trying to store incompatible data elements. Using negative value as array size. Trying to convert from string data to a specific data value (e.g., converting string abc to integer value). File errors: opening a file in read mode that does not exist or no read permission Opening a file in write/update mode which has read only permission. Corrupting memory: - common with pointers 3

4 Exception describes errors caused by your program and external circumstances. These errors can be caught and handled by your program. Exception ClassNotFoundException IOException RuntimeException ArithmeticException NullPointerException IndexOutOfBoundsException Object Throwable Many more classes IllegalArgumentException Many more classes Error LinkageError VirtualMachineError Many more classes System errors are thrown by JVM and represented in the Error class. The Error class describes internal system errors. Such errors rarely occur.

5 Handling Exception means transferring execution of program to appropriate handler when an exception occurs. We can handle exception by using try catch block. try: try is used to define block of code where exception can occur. catch: catch is used to match specific type of exception. There could be more than one catch clause for one try block. finally: finally determine block of code which will always execute after try block. Even in case of Exception. throw: throw keyword is used to throw an exception explicitly. throws: is a keyword in java language which is used to throw the exception which is raised in the called method to it's calling method throws keyword always followed by method signature.

6 try Block Statements that causes an exception Throws exception Object catch Block Statements that handle the exception

7 Exception "thrown" here Exception handler Exception handler Thrown exception matched against first set of exception handlers If it fails to match, it is matched against next set of handlers, etc. If exception matches none of handlers, program is abandoned 7

8 Preceding step try block throw statement unmatched catch matching catch unmatched catch next step 8

9 ArithmeticException ArrayIndexOutOfBoundException ArrayStoreException FileNotFoundException IOException general I/O failure NullPointerException referencing a null object OutOfMemoryException SecurityException when applet tries to perform an action not allowed by the browser s security setting. StackOverflowException StringIndexOutOfBoundException 9

10 without Exception Handling class ExceptionDemo { public static void main(string[] args) { int a=10, ans=0; ans=a/0; System.out.println("Denominator not be zero"); Program does not reach here Abnormally terminate program and give a message like below, this error message is not understandable by user so we convert this error message into user friendly error message, like "denominator not be zero". Output: Exception Handling class ExceptionDemo { public static void main(string[] args) { int a=10, ans=0; try { Program reaches here ans=a/0; catch (Exception e) { System.out.println("Denominator not be zero"); Output: Denominator not be zero

11 When the JVM encounters an error such as divide by zero, it creates an exception object and throws it as a notification that an error has occurred. If the exception object is not caught and handled properly, the interpreter will display an error and terminate the program. If we want the program to continue with execution of the remaining code, then we should try to catch the exception object thrown by the error condition and then take appropriate corrective actions. This task is known as exception handling.

12 try and catch block try block Inside try block we write the block of statements which causes executions at run time in other words try block always contains problematic statements. Each and every try block must contains at least one catch block. But it is highly recommended to write multiple catch blocks for generating multiple user friendly error messages. One try block can contains another try block that is nested or inner try block can be possible. catch block Inside catch block we write the block of statements which will generates user friendly error messages. Catch block will execute exception occurs in try block. You can write multiple catch blocks At a time only one catch block will execute out of multiple catch blocks.

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14 Checked Exception are the exception which checked at compile-time. These exception are directly subclass of java.lang.exception class. Checked exception are checked by the compiler at compile-time. e.g. FileNotFoundException, NumberNotFoundException etc. Un-Checked Exception are the exception both identifies or raised at run time. These exception are directly sub-class of java.lang.runtimeexception class. In real time application mostly we can handle un-checked exception. Un-checked means not checked by compiler and are checked at run-time not compile time. e.g.arithmeticexception, NullPointerException, ArrayIndexOutOfBoundsException etc.

15 Checked Exception Classes FileNotFoundException ClassNotFoundException IOException InterruptedException Un-Checked Exception Classes ArithmeticException ArrayIndexOutOfBoundsException StringIndexOutOfBoundsException NumberFormateException NullPointerException NoSuchMethodException NoSuchFieldException

16 Exception ArithmeticException ArrayIndexOutOfBoundsException Example Divide by zero exception StringIndexOutOfBoundsException NumberFormatException NullPointerException NoSuchMethodException This exception will be raised whenever calling method is not existing in the program. FileNotFoundException In case the file is missing, the following output is produced:

17 Programs can throw an exception explicitly. throw ThrowableInstance; The exception must be of type java.lang.throwable class or it s sub classes. Primitive types, such as int or char, as well as non-throwable classes, such as String and Object, cannot be used as exceptions. There are two ways you can obtain a Throwable object: using a parameter in a catch clause, or creating one with the new operator

18 The flow of execution stops immediately after the throw statement; any subsequent statements are not executed. The nearest enclosing try block is inspected to see if it has a catch statement that matches the type of exception. If no matching catch is found, then the default exception handler halts the program and prints the stack trace.

19 How to throw an already defined exception using throw keyword class AgeValidator { static void validatestudent(int age){ try { if(age<5) throw new ArithmeticException("age to play more"); else System.out.println("welcome to school"); catch (ArithmeticException e) { System.out.println(e); public static void main(string args[]) { validatestudent(4);

20 class ThrowDemo { static void demoproc() { try { throw new NullPointerException("demo"); catch(nullpointerexception e) { System.out.println("Caught inside demoproc."); public static void main(string args[]) { demoproc();

21 class Exception2{ static int sum(int num1, int num2){ try { if (num1 == 0) throw new ArithmeticException("First parameter is not valid"); else System.out.println("Both parameters are correct!!"); catch (ArithmeticException e) { System.out.println(e); return num1+num2; public static void main(string args[]){ int res=sum(3,12); System.out.println(res); System.out.println("Continue Next statements");

22 If a method is capable of causing an exception that it does not handle, it must specify this behavior so that callers of the method can guard themselves against that exception. A throws clause lists the types of exceptions that a method might throw. This is necessary for all exceptions, except those of type Error or RuntimeException, or any of their subclasses. type method(args-list) throws exception-list { // body of method exception-list is a comma-separated list of the exceptions that a method can throw.

23 class ThrowsDemo { static void throwone() throws IllegalAccessException { System.out.println("Inside throwone."); throw new IllegalAccessException("demo"); public static void main(string args[]) { try { throwone(); catch (IllegalAccessException e) { System.out.println("Caught " + e); O/P: Inside throwone Caught java.lang.illegalaccessexception: demo

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25 class ThrowExample1 { void mymethod(int num) throws ArithmeticException, ClassNotFoundException{ if(num==1) throw new ArithmeticException("Number is one"); else throw new ClassNotFoundException("Number is not one"); class ThrowExample{ public static void main(string args[]){ try{ ThrowExample1 obj=new ThrowExample1(); //obj.mymethod(2); obj.mymethod(1); catch(exception ex) { System.out.println(ex);

26 Built-in exception classes handle some generic errors. For application-specific errors define your own exception classes. Define a subclass of Exception: class MyException extends Exception { MyException need not implement anything its mere existence in the type system allows to use its objects as exceptions. If any exception is design by the user known as user defined or Custom Exception

27 User defined exception-example

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29 Defining Your Own Exceptions To define your own exception you must do the following: Create an exception class to hold the exception data. Your exception class must subclass "Exception" or another exception class Note: to create unchecked exceptions, subclass the RuntimeException class. Minimally, your exception class should provide a constructor which takes the exception description as its argument. To throw your own exceptions: If your exception is checked, any method which is going to throw the exception must define it using the throws keyword When an exceptional condition occurs, create a new instance of the exception and throw it.

30 finally block Java finally block is a block that is used to execute important code such as closing connection, stream etc. Java finally block is always executed whether exception is handled or not. Java finally block must be followed by try or catch block. Writing finally block is optional.

31 Write a program for finding a Sum of Valid Integer Values Passed as Command Line Parameters by leaving Invalid data from the Command Line. (Hint: use NumberFormatException). Create a custom exception class named LowGpaException extended from the class Exception. This class should contain a constructor with no parameter to call the Exception class constructor with the message Your GPA is not sufficient to apply for this job (2.5). Write a program to implement the following ArithmeticException StringIndexOutOfBoundsException ArrayIndexOutOfBoundsException