Exception

  • Exception is an abnormal condition that arises at run time
  • Event that disrupts the normal flow of the program.
  • It is an object which is thrown at runtime.

Exception Handling

  • Exception Handling is a mechanism to handle runtime errors.
  • Normal flow of the application can be maintained.
  • It is an object which is thrown at runtime.
  • Exception handling done with the exception object.

Types of Error

Syntax errors - arise because the rules of the language have not been followed. They are detected by the compiler

Runtime errors - occur while the program is running if the environment detects an operation that is impossible to carry out.

Logic errors - occur when a program doesn't perform the way it was intended to.

Types of Exception

There are mainly two types of exceptions:

  • Checked 
  • Unchecked – Eg. error

The sun microsystem says there are three types of exceptions:

  • Checked Exception - are checked at compile-time.
  • Unchecked Exception - are not checked at compile-time rather they are checked at runtime.
  • Error

Checked Exception - Classes that extend Throwable class except RuntimeException and Error are known as checked exceptions. Checked Exceptions means that compiler forces the programmer to check and deal with the exceptions. e.g.IOException, SQLException etc.

Unchecked Exception - Classes that extends RuntimeException, Error and their subclasses are known as unchecked exceptions e.g. ArithmeticException,NullPointerException, ArrayIndexOutOf Bounds Exception etc.

Error- is irrecoverable should not try to catch. e.g. OutOfMemoryError, VirtualMachineError, AssertionError etc

Exception Classes

Exception classes

System Error

System errors are thrown by JVM and represented in the Error class. The Error class describes internal system errors. Such errors rarely occur.

Exceptions

Exception describes errors caused by your program and external circumstances. These errors can be caught and handled by your program.

Runtime Exceptions

RuntimeException is caused by programming errors, such as bad casting, accessing an out-ofbounds array, and numeric errors.

Exception Handling Terms

try-used to enclose a segment of code that may produce a exception

throw – to generate an exception or to describe an instance of an exception

catch – placed directly after the try block to handle one or more exception types

finally – optional statement used after a try-catch block to run a segment of code regardless if a exception is generated

Exceptions –Syntax

 try { 
// Code which might throw an exception
 }
 catch(Exceptionclass object1) {
 // code to handle an exception
 }
 catch(Exceptionclass object2) {
 // code to handle an exception 
 }
 finally {
 // ALWAYS executed whether an exception was thrown or not
 }

Example

 class Simple {
 public static void main(String args[]) {
 int data=50/0; 
 System.out.println("rest of the code...");
 } }

Output

 Exception in thread main java.lang.ArithmeticException:/ by zero 
 Rest of the code is not executed (rest of the code..)statement is not printed

 

Notes-

  • JVM first checks whether the exception is handled or not.
  • If exception is not handled, JVM provides a default exception handler that performs the following tasks:

           -Prints out exception description.

           -Prints the stack trace (Hierarchy of methods where the exception occurred).

           -Causes the program to terminate.

  • if exception is handled by the application programmer, normal flow of the application is maintained i.e. rest of the code is executed.

Example

 class Simple1 {
 public static void main(String args[]) { 
 try { 
 int data=50/0; 
 }
 catch(ArithmeticException e) { 
 System.out.println(e); 
 }
 System.out.println("rest of the code...");
 }}

Output

 Exception in thread main java.lang.ArithmeticException:/ by zero rest of the code…

Multiple Catch Exceptions

Syntax

 try {
 // Code which might throw an exception 
 } catch(Exceptionclass object1) { 
 // code to handle an exception
 } catch(Exceptionclass object2) { 
 // code to handle an exception 
 }

Nested try Statements

  • 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

Nested Try Block

 try {
 statement 1; 
 statement 2; 
 try {
 statement 1;
 statement 2;
 }
 catch(Exception e) {
 } }
 catch(Exception e) { 
 }

Example

 class Excep6 { 
 public static void main(String args[]) {
 try {
 try {
 S.o.p("going to divide");
 int b =39/0;
 }
 catch(ArithmeticException e) {
 System.out.println(e);
 } try {
 int a[]=new int[5];
 a[5]=4; 
 }
 catch(ArrayIndexOutOfBoundsException e) { 
 System.out.println(e);
 }
 System.out.println("other statement"); 
 } catch(Exception e) {
 System.out.println("handeled"); 
 }
 System.out.println("normal flow.."); 
 } }

Finally Block

  • Finally is a block that is always executed.

  • To perform some important tasks such as closing connection, stream etc.

  • Used to put "cleanup" code such as closing a file, closing connection etc.

  • Finally creates a block of code that will be executed after a try/catch block has completed

  • Finally block will be executed whether or not an exception is thrown.

  • Each try clause requires at least one catch or finally clause

  • Note: Before terminating the program, JVM executes finally block(if any).

  • Note: finally must be followed by try or catch block.

Example

 class Simple {
 public static void main(String args[]) {
 try {
 int data=25/0;
 System.out.println(data);
 }
 catch(ArithmeticException e) {
 System.out.println(e);
 }
 finally { 
 System.out.println("finally block is always executed");
 } 
 System.out.println("rest of the code..."); 
 }} 

throw keyword

  • keyword is used to explictily throw an exception / custom exception.

           throw new ExceptionName("Error Message");

  • Throw either checked or uncheked exception.

           throw new ThrowableInstance

  • ThrowableInstance must be an object of type Throwable / subclass Throwable
  • There are two ways to obtain a Throwable objects:​​​​​​​

       -Using a parameter into a catch clause

       -Creating one with the new operator

Example 1

 public class bank {
 public static void main(String args[]) {
 int balance = 100, withdraw = 1000;
 if(balance < withdraw) {
 //ArithmeticException e = new ArithmeticException("No money please"); 
 //throw e;
 //throw new ArithmeticException(); 
 throw new ArithmeticException("No Money");
 }
 else {
 System.out.println("Draw & enjoy Sir, Best wishes of the day"); 
 } } }

Example 2

 import java.io.*;
 public class Example {
 public static void main(String args[]) throws IOException {
 DataInputStream dis=new DataInputStream(System.in);
 int x = Integer.parseInt(dis.readLine());
 if(x < 0) {
 throw new IllegalArgumentException();
 throw new IllegalArgumentException ("You have entered no"+" "+ x +" "+ "which is less than 0");
 }
 else {
 System.out.println("The no is"+x); 
 } } }

 

throws keyword

  • 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
throw throws
throw is used to explicitly throw an exception. throws is used to declare an exception.
checked exception can not be propagated without throws. checked exception can be propagated with throws.
throw is followed by an instance. throws is followed by class
throw is used within the method. throws is used with the method signature.
You cannot throw multiple exception You can declare multiple exception e.g. public void method()throws IOException,SQLException.

 

Create Your Own Exception

 class NumberRangeException extends Exception {
 String msg;
 NumberRangeException() {
 msg = new String("Enter a number between 20 and 100");
 } }
public class My_Exception {
public static void main (String args [ ]) {
try {
int x = 10; if (x < 20 || x >100)
throw new NumberRangeException( ); 
}
catch (NumberRangeException e) {
System.out.p