Exception handling

Exception Handling Presentation on Presented By : Sumit Lole

Agenda What is Exception? Exception Hierarchy Exception vs. Error Types of Exception Default Exception Handler Exception Handling Creating Your own Exception

Exception Definition: “ An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions” An exception occurs when our code asks the JVM to perform the technically impossible and unanticipated (by the compiler)!

Exception Hierarchy All exception types are subclasses of the built-in class Throwable Throwable has two subclasses : Exception Error

Throwable Error Exception Runtime Exception IO Exception The Java Exception Hierarchy

Error Error hierarchy describes internal errors and resource exhaustion. Don’t throw an object of this type! Relatively rare Mostly beyond programmers control

Types of Exceptions in Java Checked exceptions All exception inherited from Exception Class Unchecked exceptions All exception inherited from RuntimeException Class

Default Exception Handler ? Default handler prints a stack trace from the point at which the exception occurred, and terminates the program Example: BasicDemo.java

Exception Handling Anticipate the error by the user/system. Return the program to a safe state that enables the user to execute other commands. Inform the user of the error’s cause. Allow the user to save work and terminate the program gracefully.

Ways to throw an Exception Calling a method that throws a checked exception Code detects an error and generates checked exception with throw statement Programming error e.g. a[-1] =0; generates an unchecked exception ArrayIndexOutOfBoundsException 4. JVM or runtime library internal error

Keywords for Java Exceptions throws throw try catch finally

Catching Exceptions “ What goes up must come down” Catching exceptions is a bit trickier An uncaught exception will terminate the program … … print the exception type to screen … … print the stack trace.

Using try and catch Placed the code (which is suppose to throw an exception) inside a block of code starting with the “ try ” keyword The catch clause should follow immediately the try block. Once the catch statement has executed, program control continues with the next line in the program following the entire try/catch mechanism Example : InitialDemo.java

Nested Try Statement A try statement can be inside the block of another try Each time a try statement is entered, the context of that exception is pushed on the stack If an inner try statement does not have a catch, then the next try statement’s catch handlers are inspected for a match If a method call within a try block has try block within it, then then it is still nested try Example : ExceptionChaining.java

What information does java.lang.Exception contain The type of exception -- the exception class Where the exception occurred -- the stack trace Context and explanatory information -- the error message, and other state information Example : StackTrace.java

finally It is used to handle premature execution of a method (i.e. a method open a file upon entry and closes it upon exit) finally creates a block of code that will be executed after try/catch block has completed and before the code following the try/catch block finally clause will execute whether or not an exception is thrown

Some Important Exception that generally occurs… IOException ClassNotFoundException RuntimeException Most occurring exception Caused by programming error “ If it’s a RuntimeException it’s your fault!” Example : ArrayIndexOutOfBoundException IndexOutOfBound ArithmeticException NullPointerException Example : ExceptionExample.java

throw It is possible for your program to throw an exception explicitly throw ExceptionInstance Here, ExceptionInstance must be an object of type Throwable or a subclass Throwable There are two ways to obtain a Throwable objects: Using a parameter into a catch clause Creating one with the new operator

Creating Your Own Exceptions Java provides over 40 categories of Exceptions Java allows you to create your own Exceptions Extend a subclass of Throwable Exception class constructors Exception() Exception(String message) Exception(String message, Throwable cause) Exception(Throwable cause)

Example throw statement : throw <object reference expression>; Class DivisionByZeroException extends Exception{ DivisionByZeroException(String msg) { super(msg); } } public void division(){ int num1 = 10; int num2 = 0; if(num2 == 0) throw new DivisionByZeroException(); System.out.println(num1/num2); }

throws 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 type method-name parameter-list) throws exception-list { // body of method } It is not applicable for Error or RuntimeException, or any of their subclasses

Rule for Method Overriding While overriding, the new method definition in the subclass can only specify all or a subset of the exception classes specified in the throws clause of the overridden method in the superclass

Some Methods Inherited from Throwable fillStackTrace() returns Throwable getCause() returns Throwable getMessage() returns String getStackTrace() return StackTraceElement[] - Throwable initCause(Throwable) printSatckTrace() return void setStackTrace(Throwable) toString() return String

Exception handling

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Exception handling

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