Exception Handling in Java

Exception Handling
Advantage of Exception Handling
Hierarchy of Exception classes
Types of Exception
Scenarios where exception may occur

The exception handling in java is one of the powerfulmechanism 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. }

Test it Now

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. }

Test it Now

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. }

Test it Now

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. }

Test it Now

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:

1. ....

2. try

3. {

4. statement 1;

5. statement 2;

6. try

7. {

8. statement 1;

9. statement 2;

10. }

11. catch(Exception e)

12. {

13. }

14. }

15. catch(Exception e)

16. {

17. }

18. ....

Java nested try example

Let's see a simple example of java nested try block.

1. class Excep6{

2. public static void main(String args[]){

3. try{

4. try{

5. System.out.println("going to divide");

6. int b =39/0;

7. }catch(ArithmeticException e){System.out.println(e);}

9. try{

10. int a[]=new int[5];

11. a[5]=4;

12. }catch(ArrayIndexOutOfBoundsException e){System.out.println(e);}

13.

14. System.out.println("other statement);

15. }catch(Exception e){System.out.println("handeled");}

16.

17. System.out.println("normal flow..");

18. }

19. }

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 must be followed by 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.

1. class TestFinallyBlock{

2. public static void main(String args[]){

3. try{

4. int data=25/5;

5. System.out.println(data);

6. }

7. catch(NullPointerException e){System.out.println(e);}

8. finally{System.out.println("finally block is always executed");}

9. System.out.println("rest of the code...");

10. }

11. }

Test it Now

Output:5

       finally block is always executed

       rest of the code...

Case 2

Let's see the java finally example where exception occurs and not handled.

1. class TestFinallyBlock1{

2. public static void main(String args[]){

3. try{

4. int data=25/0;

5. System.out.println(data);

6. }

7. catch(NullPointerException e){System.out.println(e);}

8. finally{System.out.println("finally block is always executed");}

9. System.out.println("rest of the code...");

10. }

11. }

Test it Now

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.

1. public class TestFinallyBlock2{

2. public static void main(String args[]){

3. try{

4. int data=25/0;

5. System.out.println(data);

6. }

7. catch(ArithmeticException e){System.out.println(e);}

8. finally{System.out.println("finally block is always executed");}

9. System.out.println("rest of the code...");

10. }

11. }

Test it Now

Output:Exception in thread main java.lang.ArithmeticException:/ by zero

       finally block is always executed

       rest of the code...

Rule: For each try block there can be zero or more catch blocks, but only one finally block.

Note: 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.

1. throw exception;

Let's see the example of throw IOException.

1. 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.

1. public class TestThrow1{

2. static void validate(int age){

3. if(age<18)

4. throw new ArithmeticException("not valid");

5. else

6. System.out.println("welcome to vote");

7. }

8. public static void main(String args[]){

9. validate(13);

10. System.out.println("rest of the code...");

11. }

12. }

Test it Now

Output:

Exception in thread main java.lang.ArithmeticException:not valid

Monday, 13 April 2015