Chapter-Four

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.

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
statement will be executed. That is why we use exception handling in java.

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 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:

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.

3) Error

Error is irrecoverable e.g. OutOfMemoryError, VirtualMachineError, AssertionError etc.

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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.
1. try
2. catch
3. finally
4. throw
5. throws

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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. }

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.

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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. } }

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.

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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. System.out.println("rest of the code...");
11. }
12. }

Rule: At a time only one Exception is occurred 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. }

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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);}
8. try{
9. int a[]=new int[5];
10. a[5]=4;
11. }catch(ArrayIndexOutOfBoundsException e){System.out.println(e);}
12. System.out.println("other statement);
13. }catch(Exception e){System.out.println("handeled");}
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 14. System.out.println("normal flow..");
15. }
16. }

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.

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 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. }

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. }

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. }

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).

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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. }

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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

1. return_type method_name() throws exception_class_name{

2. //method code
3. }

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.

1. import java.io.IOException;
2. class Testthrows1{
3. void m()throws IOException{
4. throw new IOException("device error");//checked exception
5. }
6. void n()throws IOException{
7. m();
8. }
9. void p(){
10. try{
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 11. n();
12. }catch(Exception e){System.out.println("exception handled");}
13. }
14. public static void main(String args[]){
15. Testthrows1 obj=new Testthrows1();
16. obj.p();
17. System.out.println("normal flow...");
18. }
19. }

Rule: If you are calling a method that declares an exception, you must either caught or declare the
exception.
There are two cases:

1. Case1:You caught the exception i.e. handle the exception using try/catch.
2. 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.

1. import java.io.*;
2. class M{
3. void method()throws IOException{
4. throw new IOException("device error");
5. }
6. }
7. public class Testthrows2{
8. public static void main(String args[]){
9. try{
10. M m=new M();
11. m.method();
12. }catch(Exception e){System.out.println("exception handled");}
13.
14. System.out.println("normal flow...");
15. }
16. }

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 occurs, an exception will be thrown at runtime because
throws does not handle the exception.

A) Program if exception does not occur

1. import java.io.*;

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 2. class M{
3. void method()throws IOException{
4. System.out.println("device operation performed");
5. }
6. }
7. class Testthrows3{
8. public static void main(String args[])throws IOException{//declare exception
9. M m=new M();
10. m.method();
11.
12. System.out.println("normal flow...");
13. }
14. }

B) Program if exception occurs

1. import java.io.*;
2. class M{
3. void method()throws IOException{
4. throw new IOException("device error");
5. }
6. }
7. class Testthrows4{
8. public static void main(String args[])throws IOException{//declare exception
9. M m=new M();
10. m.method();
11. System.out.println("normal flow...");
12. }
13. }

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:

No. Throw throws

1) Java throw keyword is used to explicitly throw Java throws keyword is used to declare an
an exception. exception.
2) Checked exception cannot be propagated using Checked exception can be propagated with
throw only. throws.
3) Throw is followed by an instance. Throws is followed by class.
4) Throw is used within the method. Throws is used with the method signature.
5) You cannot throw multiple exceptions. You can declare multiple exceptions e.g.
public void method()throws
IOException,SQLException.

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Java throw example
1. void m(){
2. throw new ArithmeticException("sorry");
3. }
Java throws example
1. void m()throws ArithmeticException{
2. //method code
3. }
4.

Java throw and throws example

1. void m()throws ArithmeticException{
2. throw new ArithmeticException("sorry");
3. }

Difference between final, finally and finalize

There are many differences between final, finally and finalize. A list of differences between final, finally
and finalize are given below:

No. Final finally finalize

1) Final is used to apply restrictions on class, Finally is used to place Finalize is used to
method and variable. Final class can't be important code, it will be perform clean up
inherited, final method can't be overridden executed whether processing just before
and final variable value can't be changed. exception is handled or object is garbage
not. collected.
2) Final is a keyword. Finally is a block. Finalize is a method.

Java final example

1. class FinalExample{
2. public static void main(String[] args){
3. final int x=100;
4. x=200;//run Time Error
5. }}

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Java finally example
1. class FinallyExample{
2. public static void main(String[] args){
3. try{
4. int x=300;
5. }catch(Exception e){System.out.println(e);}
6. finally{System.out.println("finally block is executed");}
7. }}

Java finalize example

1. class FinalizeExample{
2. public void finalize(){System.out.println("finalize called");}
3. public static void main(String[] args){
4. FinalizeExample f1=new FinalizeExample();
5. FinalizeExample f2=new FinalizeExample();
6. f1=null;
7. f2=null;
8. System.gc();
9. }}

ExceptionHandling with MethodOverriding in Java

There are many rules if we talk about method overriding with exception handling. The Rules are as follows:

 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.
 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

1) Rule: If the superclass method does not declare an exception, subclass overridden method
cannot declare the checked exception.

1. import java.io.*;
2. class Parent{
3. void msg(){System.out.println("parent");}
4. }

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 5. class TestExceptionChild extends Parent{
6. void msg()throws IOException{
7. System.out.println("TestExceptionChild");
8. }
9. public static void main(String args[]){
10. Parent p=new TestExceptionChild();
11. p.msg();
12. } }

2) Rule: If the superclass method does not declare an exception, subclass overridden method
cannot declare the checked exception but can declare unchecked exception.

1. import java.io.*;
2. class Parent{
3. void msg(){System.out.println("parent");}
4. }
5. class TestExceptionChild1 extends Parent{
6. void msg()throws ArithmeticException{
7. System.out.println("child");
8. }
9. public static void main(String args[]){
10. Parent p=new TestExceptionChild1();
11. p.msg();
12. } }

If the superclass method declares an exception

1) Rule: 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

1. import java.io.*;
2. class Parent{
3. void msg()throws ArithmeticException{System.out.println("parent");}
4. }
5. class TestExceptionChild2 extends Parent{
6. void msg()throws Exception{System.out.println("child");}
7. public static void main(String args[]){
8. Parent p=new TestExceptionChild2();
9. try{
10. p.msg();
11. }catch(Exception e){}
12. } }

Example in case subclass overridden method declares same exception

1. import java.io.*;

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 2. class Parent{
3. void msg()throws Exception{System.out.println("parent");}
4. }
5. class TestExceptionChild3 extends Parent{
6. void msg()throws Exception{System.out.println("child");}
7. public static void main(String args[]){
8. Parent p=new TestExceptionChild3();
9. try{
10. p.msg();
11. }catch(Exception e){}
12. } }

Example in case subclass overridden method declares subclass exception

1. import java.io.*;
2. class Parent{
3. void msg()throws Exception{System.out.println("parent");}
4. }
5. class TestExceptionChild4 extends Parent{
6. void msg()throws ArithmeticException{System.out.println("child");}
7. public static void main(String args[]){
8. Parent p=new TestExceptionChild4();
9. try{
10. p.msg();
11. }catch(Exception e){}
12. } }

Example in case subclass overridden method declares no exception

1. import java.io.*;
2. class Parent{
3. void msg()throws Exception{System.out.println("parent");}
4. }
5. class TestExceptionChild5 extends Parent{
6. void msg(){System.out.println("child");}
7. public static void main(String args[]){
8. Parent p=new TestExceptionChild5();
9. try{
10. p.msg();
11. }catch(Exception e){}
12. }
13. }

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