Exception Handling in Java

In this page, we will learn about java exception, its type and the difference between checked and unchecked exceptions.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.

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.

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.

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

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...");
}
}

....

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: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.

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...

.....

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...

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