Exception Handling in
Java
1.What
is Exception
2.Exception Handling
3.Advantage
of Exception Handling
4.Hierarchy of
Exception classes
5.Types of Exception
6.Scenarios where Exception may
occur
Exception
An exception
is a problem that arises during the execution of a program. An exception can
occur for many different reasons, including the following:
·
A user has entered invalid data.
·
A file that needs to be opened cannot be found.
·
A network connection has been lost in the middle of communications
or the JVM has run out of memory.
Some of these exceptions are
caused by user error, others by programmer error, and others by physical
resources that have failed in some manner.
To understand how exception
handling works in Java, you need to understand the three categories of
exceptions:
·
Checked
exceptions: A checked
exception is an exception that is typically a user error or a problem that
cannot be foreseen by the programmer. For example, if a file is to be opened,
but the file cannot be found, an exception occurs. These exceptions cannot
simply be ignored at the time of compilation.
·
Runtime
exceptions: A runtime
exception is an exception that occurs that probably could have been avoided by
the programmer. As opposed to checked exceptions, runtime exceptions are
ignored at the time of compilation.
·
Errors: These are not exceptions at all, but problems
that arise beyond the control of the user or the programmer. Errors are
typically ignored in your code because you can rarely do anything about an
error. For example, if a stack overflow occurs, an error will arise. They are
also ignored at the time of compilation.
The exception handling in java is
one of the powerful mechanism to handle the runtime errors so
that normal flow of the application can be maintained.
In this page, we will learn about java
exception, its type and the difference between checked and unchecked
exceptions.
What is exception
Dictionary Meaning: Exception
is an abnormal condition.
In java, exception is an event that disrupts the
normal flow of the program. It is an object which is thrown at runtime.
What is exception handling
Exception Handling is a mechanism to handle
runtime errors such as ClassNotFound, IO, SQL, Remote etc.
Advantage of Exception Handling
The core advantage of exception handling
is to maintain the normal flow of the application. Exception
normally disrupts the normal flow of the application that is why we use
exception handling. Let's take a scenario:
1.
statement 1;
2.
statement 2;
3.
statement 3;
4.
statement 4;
5.
statement 5;//exception occurs
6.
statement 6;
7.
statement 7;
8.
statement 8;
9.
statement 9;
10. statement 10;
Suppose there is 10 statements in your program
and there occurs an exception at statement 5, rest of the code will not be
executed i.e. statement 6 to 10 will not run. If we perform exception handling,
rest of the exception will be executed. That is why we use exception handling
in java.
Do You Know ?
|
·
What is the difference between checked and
unchecked exceptions ?
·
What happens behind the code int data=50/0; ?
·
Why use multiple catch block ?
·
Is there any possibility when finally block is
not executed ?
·
What is exception propagation ?
·
What is the difference between throw and
throws keyword ?
·
What are the 4 rules for using exception
handling with method overriding ?
|
Hierarchy of Java Exception classes
Types of Exception
There are mainly two types of exceptions:
checked and unchecked where error is considered as unchecked exception. The sun
microsystem says there are three types of exceptions:
- Checked
Exception
- Unchecked
Exception
- Error
Difference between checked and unchecked
exceptions
1) Checked Exception
The classes that extend Throwable class except
RuntimeException and Error are known as checked exceptions e.g.IOException,
SQLException etc. Checked exceptions are checked at compile-time.
2) Unchecked Exception
The classes that extend RuntimeException are
known as unchecked exceptions e.g. ArithmeticException, NullPointerException,
ArrayIndexOutOfBoundsException etc. Unchecked exceptions are not checked at
compile-time rather they are checked at runtime.
3) Error
Error is irrecoverable e.g. OutOfMemoryError,
VirtualMachineError, AssertionError etc.
Common scenarios where exceptions may occur
There are given some scenarios where unchecked
exceptions can occur. They are as follows:
1) Scenario where ArithmeticException occurs
If we divide any number by zero, there occurs an
ArithmeticException.
1. int a=50/0;//ArithmeticException
2) Scenario where NullPointerException occurs
If we have null value in any variable,
performing any operation by the variable occurs an NullPointerException.
1.
String s=null;
2. System.out.println(s.length());//NullPointerException
3) Scenario where NumberFormatException occurs
The wrong formatting of any value, may occur
NumberFormatException. Suppose I have a string variable that have characters,
converting this variable into digit will occur NumberFormatException.
1.
String s="abc";
2. int i=Integer.parseInt(s);//NumberFormatException
4) Scenario where
ArrayIndexOutOfBoundsException occurs
If you are inserting any value in the wrong
index, it would result ArrayIndexOutOfBoundsException as shown below:
1.
int a[]=new int[5];
2. a[10]=50; //ArrayIndexOutOfBoundsException
Java Exception Handling Keywords
There are 5 keywords used in java exception
handling.
- try
- catch
- finally
- throw
- throws
Java try-catch
Java try block
Java try block is used to
enclose the code that might throw an exception. It must be used within the
method.
Java try block must be followed
by either catch or finally block.
Syntax of java try-catch
1.
try{
2.
//code that may throw exception
3.
}catch(Exception_class_Name ref){}
Syntax of try-finally block
1.
try{
2.
//code that may throw exception
3.
}finally{}
Java catch block
Java catch block is used to
handle the Exception. It must be used after the try block only.
You can use multiple catch
block with a single try.
Problem without exception handling
Let's try to understand the
problem if we don't use try-catch block.
1.
public class Testtrycatch1{
2.
public static void main(String args[]){
3.
int data=50/0;//may throw exception
4.
System.out.println("rest of the code...");
5.
}
6.
}
Output:
Exception in thread main java.lang.ArithmeticException:/ by zero
As displayed in the above
example, rest of the code is not executed (in such case, rest of the code...
statement is not printed).
There can be 100 lines of code
after exception. So all the code after exception will not be executed.
Solution by exception handling
Let's see the solution of above
problem by java try-catch block.
1.
public class Testtrycatch2{
2.
public static void main(String args[]){
3.
try{
4.
int data=50/0;
5.
}catch(ArithmeticException e){System.out.println(e);}
6.
System.out.println("rest of the code...");
7.
}
8.
}
Output:
Exception in thread main java.lang.ArithmeticException:/ by zero
rest of the code...
Now, as displayed in the above
example, rest of the code is executed i.e. rest of the code... statement is
printed.
Internal working of java try-catch block
The JVM firstly 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.
But if exception is handled by
the application programmer, normal flow of the application is maintained i.e.
rest of the code is executed.
Java catch multiple exceptions
Java Multi catch block
If you have to perform
different tasks at the occurrence of different Exceptions, use java multi catch
block.
Let's see a simple example of
java multi-catch block.
1.
public class TestMultipleCatchBlock{
2.
public static void main(String args[]){
3.
try{
4.
int a[]=new int[5];
5.
a[5]=30/0;
6.
}
7.
catch(ArithmeticException e){System.out.println("task1 is completed");}
8.
catch(ArrayIndexOutOfBoundsException e){System.out.println("task 2 completed");}
9.
catch(Exception e){System.out.println("common task completed");}
10.
11. System.out.println("rest of the code...");
12. }
13. }
Output:task1 completed
rest of the code...
Rule: At
a time only one Exception is occured and at a time only one catch block is
executed.
Rule: All
catch blocks must be ordered from most specific to most general i.e. catch for
ArithmeticException must come before catch for Exception .
1.
class TestMultipleCatchBlock1{
2.
public static void main(String args[]){
3.
try{
4.
int a[]=new int[5];
5.
a[5]=30/0;
6.
}
7.
catch(Exception e){System.out.println("common task completed");}
8.
catch(ArithmeticException e){System.out.println("task1 is completed");}
9.
catch(ArrayIndexOutOfBoundsException e){System.out.println("task 2 completed");}
10. System.out.println("rest of the code...");
11. }
12. }
Output:
Compile-time error
Exceptions Methods:
Following is the list of
important medthods available in the Throwable class.
|
SN
|
Methods
with Description
|
|
1
|
public String getMessage()
Returns
a detailed message about the exception that has occurred. This message is
initialized in the Throwable constructor.
|
|
2
|
public
Throwable getCause()
Returns
the cause of the exception as represented by a Throwable object.
|
|
3
|
public
String toString()
Returns
the name of the class concatenated with the result of getMessage()
|
|
4
|
public
void printStackTrace()
Prints
the result of toString() along with the stack trace to System.err, the error
output stream.
|
|
5
|
public
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.
|
|
6
|
public
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 Exceptions:
A method catches an exception
using a combination of the try and catchkeywords.
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:
try {
//Protected
code
}catch(ExceptionName e1)
{
//Catch
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.
Example:
The following is an
array is 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 would produce the
following result:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3
Out of the block
Multiple catch Blocks:
A try block can be
followed by multiple catch blocks. The syntax for multiple catch blocks looks
like the following:
try
{
//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.
Example:
Here 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;
}
The throws/throw
Keywords:
If 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
different in throws and throw keywords.
The following method
declares that it throws a RemoteException:
import java.io.*;
public class className
{
public void deposit(double amount) throws RemoteException
{
//
Method implementation
throw new RemoteException();
}
//Remainder
of class definition
}
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:
import java.io.*;
public class className
{
public void withdraw(double amount) throws RemoteException,
InsufficientFundsException
{
//
Method implementation
}
//Remainder
of class definition
}
The finally Keyword
The finally keyword is
used to create a block of code that follows a try block. A finally block of
code always executes, whether or not an exception has occurred.
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:
try
{
//Protected
code
}catch(ExceptionType1 e1)
{
//Catch
block
}catch(ExceptionType2 e2)
{
//Catch
block
}catch(ExceptionType3 e3)
{
//Catch
block
}finally
{
//The
finally block always executes.
}
Example:
public class ExcepTest{
public static void main(String args[]){
int a[] = new int[2];
try{
System.out.println("Access element
three :" + a[3]);
}catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception
thrown :" + e);
}
finally{
a[0] = 6;
System.out.println("First element value: " +a[0]);
System.out.println("The finally
statement is executed");
}
}
}
This would produce the
following result:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3
First element value: 6
The finally statement is executed
Note the following:
·
A catch clause cannot
exist without a try statement.
·
It is not compulsory to
have finally clauses when ever a try/catch block is present.
·
The try block cannot be
present without either catch clause or finally clause.
·
Any code cannot be
present in between the try, catch, finally blocks.
Declaring you own
Exception:
You can create your own
exceptions in Java. Keep the following points in mind when writing your own
exception classes:
·
All exceptions must be a
child of Throwable.
·
If you want to write a
checked exception that is automatically enforced by the Handle or Declare Rule,
you need to extend the Exception class.
·
If you want to write a
runtime exception, you need to extend the RuntimeException class.
We can define our own
Exception class as below:
class MyException extends Exception{
}
You just need to extend
the Exception class to create your own Exception class. 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.
Example:
//
File Name InsufficientFundsException.java
import java.io.*;
public class InsufficientFundsException extends Exception
{
private double amount;
public InsufficientFundsException(double amount)
{
this.amount = amount;
}
public double getAmount()
{
return amount;
}
}
To demonstrate using our
user-defined exception, the following CheckingAccount class contains a
withdraw() method that throws an InsufficientFundsException.
//
File Name CheckingAccount.java
import java.io.*;
public class CheckingAccount
{
private double balance;
private int number;
public CheckingAccount(int number)
{
this.number = number;
}
public void deposit(double amount)
{
balance += amount;
}
public void withdraw(double amount) throws
InsufficientFundsException
{
if(amount <= balance)
{
balance -= amount;
}
else
{
double needs = amount - balance;
throw new InsufficientFundsException(needs);
}
}
public double getBalance()
{
return balance;
}
public int getNumber()
{
return number;
}
}
The following BankDemo
program demonstrates invoking the deposit() and withdraw() methods of
CheckingAccount.
// File Name
BankDemo.java
public class BankDemo
{
public static void main(String [] args)
{
CheckingAccount c = new CheckingAccount(101);
System.out.println("Depositing
$500...");
c.deposit(500.00);
try
{
System.out.println("\nWithdrawing
$100...");
c.withdraw(100.00);
System.out.println("\nWithdrawing
$600...");
c.withdraw(600.00);
}catch(InsufficientFundsException
e)
{
System.out.println("Sorry, but you
are short $"
+ e.getAmount());
e.printStackTrace();
}
}
}
Compile all the above
three files and run BankDemo, this would produce the following result:
Depositing $500...
Withdrawing $100...
Withdrawing $600...
Sorry, but you are short $200.0
InsufficientFundsException
at CheckingAccount.withdraw(CheckingAccount.java:25)
at BankDemo.main(BankDemo.java:13)
Common Exceptions:
In Java, it is possible
to define two catergories of Exceptions and Errors.
·
JVM
Exceptions: - These are
exceptions/errors that are exclusively or logically thrown by the JVM. Examples
: NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException,
·
Programmatic
exceptions: - These exceptions
are thrown explicitly by the application or the API programmers Examples:
IllegalArgumentException, IllegalStateException.
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