Exception Handling in Java
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.
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.
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 errors such as ClassNotFound, IOException, SQLException, Remote etc.
Advantage of Exception Handling
statement 1; statement 2;
statement 3;
statement 4;
statement 5;//exception occurs
statement 6;
statement 7;
statement 8;
statement 9;
statement 10;
Suppose there are 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 statement will be executed. That is why we use exception handling in java.
Types of Exception
There
are mainly three types of exceptions in java:
1. Checked
Exception
2. Unchecked
Exception
3. 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.
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.
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.
String s=null;
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.
String s="abc";
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:
int a[]=new int[5];
a[10]=50; //ArrayIndexOutOfBoundsException
Java Exception Handling Keywords
There are 5 keywords used in java exception handling.
1. try
2. catch
3. finally
4. throw
5. throws
1. try
2. catch
3. finally
4. throw
5. 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
Java try block must be followed by either catch or finally block.
Syntax of java try-catch
try{
//code that may throw exception
}catch(Exception_class_Name ref){}
Syntax
of try-finally block
try{
//code that may throw exception
}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.
public class Testtrycatch1{
public static void main(String args[]){
int data=50/0;//may throw exception
System.out.println("rest of the code...");
}
}
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.
public class Testtrycatch2{
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...
Now, as
displayed in the above example, rest of the code is executed i.e. rest of the
code... statement is printed.
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.
public class TestMultipleCatchBlock{
public static void main(String args[]){
try{
int a[]=new int[5];
a[5]=30/0;
}
catch(ArithmeticException e){System.out.println("task1 is completed");}
catch(ArrayIndexOutOfBoundsException e) {
System.out.println("task 2 completed");}
System.out.println("task 2 completed");}
catch(Exception e){System.out.println("common task completed");}
System.out.println("rest of the code...");
}
}
Output:task1 completed
rest of the code...
Note1: At a time only one Exception is occured and at a time only one catch block is executed.
Note2: All catch blocks must be ordered from most specific to most general i.e. catch for ArithmeticException must come before catch for Exception.
class TestMultipleCatchBlock1{
Note2: All catch blocks must be ordered from most specific to most general i.e. catch for ArithmeticException must come before catch for Exception.
class TestMultipleCatchBlock1{
public static void main(String args[]){
try{
int a[]=new int[5];
a[5]=30/0;
}
catch(Exception e){System.out.println("common task completed");}
catch(ArithmeticException e){System.out.println("task1 is completed");}
catch(ArrayIndexOutOfBoundsException e){
System.out.println("task 2 completed");
}
System.out.println("task 2 completed");
}
System.out.println("rest of the code...");
}
}
Output:
Compile-time error
Java Nested try block
The try
block within a try block is known as nested try block in java.
Why use nested try block
Sometimes
a situation may arise where a part of a block may cause one error and the
entire block itself may cause another error. In such cases, exception handlers
have to be nested.
Syntax:
....
try
{
statement 1;
statement 2;
try
{
statement 1;
statement 2;
}
catch(Exception e)
{
}
}
catch(Exception e)
{
}
....
Java nested try example
Let's
see a simple example of java nested try block.
class Excep6{
public static void main(String args[]){
try{
try{
System.out.println("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..");
}
}
Java 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 follows try or catch block.
Note: If you don't handle exception, before terminating the program, JVM executes finally block(if any).
Why use java finally
- Finally block in java can be used to put "cleanup" code such as closing a file, closing connection etc.
Usage of Java finally
Let's
see the different cases where java finally block can be used.
Case 1
Let's
see the java finally example where exception doesn't occur.
class TestFinallyBlock{
public static void main(String args[]){
try{
int data=25/5;
System.out.println(data);
}
catch(NullPointerException e){System.out.println(e);}
finally{System.out.println("finally block is always executed");}
System.out.println("rest of the code...");
}
}
Output:
finally block is always executed
rest of the code...
Case 2
Let's
see the java finally example where exception occurs and not handled.
class TestFinallyBlock1{
public static void main(String args[]){
try{
int data=25/0;
System.out.println(data);
}
catch(NullPointerException e){System.out.println(e);}
finally{System.out.println("finally block is always executed");}
System.out.println("rest of the code...");
}
}
Output:finally block is always executed
Exception in thread main java.lang.ArithmeticException:/ by zero
Case 3
Let's
see the java finally example where exception occurs and handled.
public class TestFinallyBlock2{
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...");
}
}
output:Exception in thread main java.lang.ArithmeticException:/ by zero
finally block is always executed
rest of the code...
Note1: For each try block there can be zero or more catch blocks, but only
one finally block.
Note2: The finally block will not be executed if program exits(either by calling
System.exit() or by causing a fatal error that causes the process to abort).
output:Exception in thread main java.lang.ArithmeticException:/ by zero
finally block is always executed
rest of the code...
Note1: For each try block there can be zero or more catch blocks, but only
one finally block.
Note2: The finally block will not be executed if program exits(either by calling
System.exit() or by causing a fatal error that causes the process to abort).
Java throw exception
Java throw keyword
The Java
throw keyword is used to explicitly throw an exception.
We can
throw either checked or uncheked exception in java by throw keyword. The throw
keyword is mainly used to throw custom exception. We will see custom exceptions
later.
The
syntax of java throw keyword is given below.
throw exception;
Let's
see the example of throw IOException.
throw new IOException("sorry device error);
java throw keyword example
In this
example, we have created the validate method that takes integer value as a
parameter. If the age is less than 18, we are throwing the ArithmeticException
otherwise print a message welcome to vote.
public class TestThrow1{
static void validate(int age){
if(age<18)
throw new ArithmeticException("not valid");
else
System.out.println("welcome to vote");
}
public static void main(String args[]){
validate(13);
System.out.println("rest of the code...");
}
}
Output:
Exception in thread main java.lang.ArithmeticException:not valid
Java Exception propagation
An exception is first thrown from the top of the stack and if it
is not caught, it drops down the call stack to the previous method,If not
caught there, the exception again drops down to the previous method, and so
on until they are caught or until they reach the very bottom of the call
stack.This is called exception propagation.
Note: By default Unchecked Exceptions are forwarded in calling chain (propagated). |
Program of Exception Propagation
class TestExceptionPropagation1{
void m(){
int data=50/0;
}
void n(){
m();
}
void p(){
try{
n();
}catch(Exception e){System.out.println("exception handled");}
}
public static void main(String args[]){
TestExceptionPropagation1 obj=new TestExceptionPropagation1();
obj.p();
System.out.println("normal flow...");
}
}
output:exception
handled
normal flow...
In the
above example exception occurs in m() method where it is not handled, so it is
propagated to previous n() method where it is not handled, again it is
propagated to p() method where exception is handled.
Exception
can be handled in any method in call stack either in main() method, p() method,
n() method or m() method.
Note: By default, Checked Exceptions are not forwarded in calling chain (propagated).
Note: By default, Checked Exceptions are not forwarded in calling chain (propagated).
Program which describes that checked exceptions are
not propagated
class TestExceptionPropagation2{
void m(){
throw new java.io.IOException("device error");//checked exception
}
void n(){
m();
}
void p(){
try{
n();
}catch(Exception e){System.out.println("exception handeled");}
}
public static void main(String args[]){
TestExceptionPropagation2 obj=new TestExceptionPropagation2();
obj.p();
System.out.println("normal flow");
}
}
Test
Output:Compile
Time Error
Java throws keyword
The Java throws keyword is used to declare an exception. It
gives an information to the programmer that there may occur an exception so it
is better for the programmer to provide the exception handling code so that
normal flow can be maintained.
Exception
Handling is mainly used to handle the checked exceptions. If there occurs any
unchecked exception such as NullPointerException, it is programmers fault that
he is not performing check up before the code being used.
Syntax of java throws
return_type method_name() throws exception_class_name{
//method code
}
Which exception should be declared
Ans) checked exception only, because:
- unchecked Exception: under your
control so correct your code.
- error: beyond your control e.g. you are unable to do anything if there occurs VirtualMachineError or StackOverflowError.
Advantage of Java throws keyword
Now
Checked Exception can be propagated (forwarded in call stack).
It
provides information to the caller of the method about the exception.
Java throws example
Let's see the example of java throws clause which describes that checked exceptions can be propagated by throws keyword.
import java.io.IOException;
class Testthrows1{
void m()throws IOException{
throw new IOException("device error");//checked exception
}
void n()throws IOException{
m();
}
void p(){
try{
n();
}catch(Exception e){System.out.println("exception handled");}
}
public static void main(String args[]){
Testthrows1 obj=new Testthrows1();
obj.p();
System.out.println("normal flow...");
}
}
Output:
exception handled
normal flow...
Note: If you are calling a method that declares an exception, you must either
caught or declare the exception.
Output:
exception handled
normal flow...
Note: If you are calling a method that declares an exception, you must either
caught or declare the exception.
There are two cases:
Case1:You caught the exception i.e. handle the
exception using try/catch.
Case2:You declare the exception i.e. specifying
throws with the method.
Case1: You handle the exception
- In case you handle the exception, the
code will be executed fine whether exception occurs during the program or
not.
import java.io.*;
class M{
void method()throws IOException{
throw new IOException("device error");
}
}
public class Testthrows2{
public static void main(String args[]){
try{
M m=new M();
m.method();
}catch(Exception e){System.out.println("exception handled");}
System.out.println("normal flow...");
}
}
Output:exception handled
normal flow...
Case2: You declare the exception
- A)In case you declare the exception,
if exception does not occur, the code will be executed fine.
- B)In case you declare the exception if
exception occures, an exception will be thrown at runtime because throws
does not handle the exception.
A)Program if exception does not occur
import java.io.*;
class M{
void method()throws IOException{
System.out.println("device operation performed");
}
}
class Testthrows3{
public static void main(String args[])throws IOException{//declare exception
M m=new M();
m.method();
System.out.println("normal flow...");
}
}
Output:device operation performed
normal flow...
B)Program if exception occurs
import java.io.*;
class M{
void method()throws IOException{
throw new IOException("device error");
}
}
class Testthrows4{
public static void main(String args[])throws IOException{//declare exception
M m=new M();
m.method();
System.out.println("normal flow...");
}
}
Output:Runtime
Exception
Difference between throw and throws in Java
There
are many differences between throw and throws keywords. A list of differences
between throw and throws are given below:
Throw
|
Throws
|
Java throw keyword is used to
explicitly throw an exception.
|
Java throws keyword is used to declare
an exception.
|
Checked exception cannot be propagated
using throw only.
|
UnChecked 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 exceptions.
|
You can declare multiple exceptions
e.g. public void method()throws
IOException,SQLException.
|
Java throw example
void m(){
throw new ArithmeticException("sorry");
}
Java throws example
Java throw and throws example
void m()throws ArithmeticException{
//method code
}
void m()throws ArithmeticException{
throw new ArithmeticException("sorry");
}
}
Java throws example
Java throw and throws example
void m()throws ArithmeticException{
//method code
}
void m()throws ArithmeticException{
throw new ArithmeticException("sorry");
}
ExceptionHandling with MethodOverriding in Java
There
are many rules if we talk about methodoverriding with exception handling. The
Rules are as follows:
o If the superclass
method does not declare an exception
o If the superclass
method does not declare an exception, subclass overridden method cannot
declare the checked exception but it can declare unchecked exception.
o If the superclass
method declares an exception
o If the superclass method
declares an exception, subclass overridden method can declare same, subclass
exception or no exception but cannot declare parent exception.
|
If the superclass method does not declare an exception
Note 1: If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception.
import java.io.*;
class Parent{
void msg(){System.out.println("parent");}
}
class TestExceptionChild extends Parent{
void msg()throws IOException{
System.out.println("TestExceptionChild");
}
public static void main(String args[]){
Parent p=new TestExceptionChild();
p.msg();
}
}
Output:Compile
Time Error
Rule2: If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception but can declare unchecked exception.
import java.io.*;
class Parent{
void msg(){System.out.println("parent");
}
}
}
class TestExceptionChild1 extends Parent{
void msg()throws ArithmeticException{
System.out.println("child");
}
public static void main(String args[]){
Parent p=new TestExceptionChild1();
p.msg();
}
}
Output:child
Note3: If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.
Example in case subclass overridden method declares parent
exception
import java.io.*;
class Parent{
void msg()throws ArithmeticException{System.out.println("parent");}
}
class TestExceptionChild2 extends Parent{
void msg()throws Exception{System.out.println("child");}
public static void main(String args[]){
Parent p=new TestExceptionChild2();
try{
p.msg();
}catch(Exception e){}
}
}
Output:Compile
Time Error
Example in case subclass overridden method declares same exception
import java.io.*;
class Parent{
void msg()throws Exception{System.out.println("parent");}
}
class TestExceptionChild3 extends Parent{
void msg()throws Exception{System.out.println("child");}
public static void main(String args[]){
Parent p=new TestExceptionChild3();
try{
p.msg();
}catch(Exception e){}
}
}
Output:child
Example in case subclass overridden method declares subclass exception
import java.io.*;
class Parent{
void msg()throws Exception{System.out.println("parent");}
}
class TestExceptionChild4 extends Parent{
void msg()throws ArithmeticException{System.out.println("child");}
public static void main(String args[]){
Parent p=new TestExceptionChild4();
try{
p.msg();
}catch(Exception e){}
}
}
Output:child
Example in case subclass overridden method declares no exception
import java.io.*;
class Parent{
void msg()throws Exception{System.out.println("parent");}
}
class TestExceptionChild5 extends Parent{
void msg(){System.out.println("child");}
public static void main(String args[]){
Parent p=new TestExceptionChild5();
try{
p.msg();
}catch(Exception e){}
}
}
Output:child
Java Custom Exception
If you
are creating your own Exception that is known as custom exception or
user-defined exception. Java custom exceptions are used to customize the
exception according to user need.
By the
help of custom exception, you can have your own exception and message.
Let's
see a simple example of java custom exception.
class InvalidAgeException extends Exception{
InvalidAgeException(String s){
super(s);
}
}
class TestCustomException1{
static void validate(int age)throws InvalidAgeException{
if(age<18)
throw new InvalidAgeException("not valid");
else
System.out.println("welcome to vote");
}
public static void main(String args[]){
try{
validate(13);
}catch(Exception m){System.out.println("Exception occured: "+m);}
System.out.println("rest of the code...");
}
}
Output:Exception
occured: InvalidAgeException:not valid
rest of the code...
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