Exception Handling in Java
- 1. BY K.LALITHAMBIGA II –Msc (CS&IT ) Department of CS&IT NS College of Arts and Science, Theni.
- 2. Exception Handling Exception are run time anomalies or unusual condition that a program may encounter during execution. Type of Program errors We distinguish between the following types of errors: • Syntax error: Errors due to the fact that the syntax of the language is not respected. • Semantic error: Errors due to an improper use of program statements. • Logical errors: Errors due to the fact that the specification is not respected. From the point of view of when errors are detected Compile time error: Syntax errors & Static semantic errors indicated by the compiler. Runtime errors: Dynamic semantic errors, that errors, that cannot be detected by the compiler(debugging).
- 3. Exception Handling Runtime Errors Division by zero Access to an array outside of its bounds Running out of memory Running out of disk space It was not a part of original C++ It is a new feature added to ANSI C++ Exception are Two kinds: Synchronous Exception Out of rage Over flow Asynchronous Exception Error that are caused by causes beyond the control of the program. Keyboard interrupts. In C++ only Synchronous exception can be handled.
- 4. Exception Handling Exception Handling Mechanism Find the problem (Hit the Exception) Inform that an error has occurred (Throw the Exception) Receive the error information (Catch the Exception) Take corrective action (Handle the Exception) Exception in Java can be categorized into two types: Checked Exception: These Exceptions are explicitly handled in the code itself with the help of try-catch blocks. Checked Exceptions are extended from the java.lang.Exceptionclass UnChecked Exception: These Exception are not essentially handled in the program code; instead the JVM handles such exceptions. Unchecked Exceptions are extended from the java.lang.RuntimeException class
- 5. Exception Handling Mechanism It is basically build upon three keywords. Try Throw Catch Throws Exception Object Catch Block Catch & handle the Exception Try Block Detects & throws and Exception
- 6. Exception Handling Mechanism The Keyword Try is used to preface a block of statements which may generate exceptions. When an exception is detected, it is Thrown using a throw statement in the try block. A Catch block defined by the Keyword ‘catch’ catches the exception and handles it appropriately. The catch block that catches an exception must immediately follow the try block that throws the exception.
- 7. Exception Handling Mechanism try { … //Block of statements throw exception; //which detect and … //throws an exception } catch(type arg) //catch exception { … //Block of statement … //that handles the exception }
- 8. Exception are objects used to transmit information about a problem. If the type of the object thrown matches the arg type in the catch statement, the catch block is executed. If they do not match, the program is aborted using the abort() function (default). Exception are thrown by functions that are invoked from within the try blocks. The point at which the throw is executed is called throw point. Once an exception is thrown to the catch block, control cannot return to the thrown point
- 9. Exception Handling Mechanism Throw Point •Function that cause an Exception Catch Block •Catch & handle the Exception Try Block •Invokes a function that contains an Exception Throws Exception Invoke function
- 10. Throwing Mechanism The throw statement can have one of the following 3 forms Throw(exception) Throw exception Throw //used to re-throw a exception • The operand object exception can be of any type, including constant. • It is also possible to throw an object not intended for error handling. • Throw point can be in a deeply nested scope within a try block or in a deeply nested function call. • In any case, control is transferred to the catch statement. Throw new Throwable subclass;
- 11. Catching Mechanism The type indicates the type of exception the catch block handles. The parameter arg is an optional parameter name. The catch statement catches an exception whose type matches with the type of the catch argument. If the parameter in the catch statement is name, then the parameter can be used in the exception handling code. If a catch statement does not match the exception it is skipped. More than one catch statement can be associated with a try block. Catch(type arg) { …. …. …. }
- 12. Catching Mechanism Multiple catch statement Try { statement; //generates an exception } Catch(type 1 arg) { statement; //catch block 1 } Catch(type 2 arg){ statement; //catch block 2 } … ... Catch(type N arg) { statement; //catch block N }
- 13. Catching Mechanism When an exception is thrown, the exception handlers are searched in order for a match. The first handler that yields a match is executed. If several catch statement matches the type of an exception the first handler that matches the exception type is executed. Catch all exception Catch(….) { //statement for processing all exception }
- 14. Using Finally statement Try { Try-block } Catch(Exception class 1 e) { catch-block } Catch(Exception class 2 e) { catch-block } … Finally{ Finally-block }
- 15. Using Finally statement Try-block: sequence of statements that will be executed under the control of the following catch clauses. Catch-block: Sequence of statements that will be executed if a statement in the try-block generates an exception of the type specified in the corresponding catch clause. Finally-block: Sequence of statements that will be always executed (both in the case where the try-block is executed without exceptions, and in the case where a catch-block is executed to catch an exception).
- 16. Common Java Exception Exception Cause of Exception Arithmetic Exception Math errors such as division by zero ArrayIndexOutofBoundsException Bad array indexes ArrayStoreException When a program tries to store the wrong type of data in an array FileNotFoundException Attempt to access a non existent file IOException General I/O failures, such as inability to read from a file NullPointerException Referencing a null object NumberFormatException When a conversion between strings & number fails OutOfMemoryException When there’s not enough memory to allocate a new object SecurityException When an applet tries to perform an action not allowed by the browser’s security setting StackOverFlowException When the system runs out of stack space StringIndexOutOfBoundException When a program attempts to access a nonexistent character position in a string