Exception Handling in Java

1. 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 ?

The exception handling is one of the powerful mechanism provided in java. It provides the mechanism to handle the runtime errors so that normal flow of the application can be maintained.

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

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.

Exception Handling

Exception Handling is a mechanism to handle runtime errors.

Advantage of Exception Handling

The core advantage of exception handling is that normal flow of the application is maintained. 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;  
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.

Hierarchy of Exception classes

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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

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What is the 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 of Exception Handling 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  

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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  

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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  

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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                                                                                 

Use of try-catch block in Exception handling:

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Five keywords used in Exception handling:

try catch finally throw throws

try block

Enclose the code that might throw an exception in try block. It must be used within the method and must be followed by either catch or finally block.

Syntax of try with catch block

1. try{  
2. ...  
3. }catch(Exception_class_Name reference){}  

Syntax of try with finally block

1. try{  
2. ...  
3. }finally{}  

catch block

Catch block is used to handle the Exception. It must be used after the try block.

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Problem without exception handling

1. class Simple{  
2.   public static void main(String args[]){  
3.       int data=50/0;  
4.     
5.       System.out.println("rest of the code...");  
6. }  
7. }  

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

As displayed in the above example, rest of the code is not executed i.e. rest of the code... statement is not printed. Let's see what happens behind the scene:

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What happens behind the code int a=50/0;

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.

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Solution by exception handling

1. class Simple{  
2.   public static void main(String args[]){  
3.    try{  
4.       int data=50/0;  
5.     
6.    }catch(ArithmeticException e){System.out.println(e);}  
7.       
8.    System.out.println("rest of the code...");  
9. }  
10. }  

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.

Multiple catch block:

If you have to perform different tasks at the occrence of different Exceptions, use multple catch block.

1. <b><i>Example of multiple catch block</i></b>  
2.   
3. class Excep4{  
4.   public static void main(String args[]){  
5.    try{  
6.     int a[]=new int[5];  
7.     a[5]=30/0;  
8.    }  
9.    catch(ArithmeticException e){System.out.println("task1 is completed");}  
10.    catch(ArrayIndexOutOfBoundsException e){System.out.println("task 2 completed");}  
11.    catch(Exception e){System.out.println("common task completed");}  
12.   
13.    System.out.println("rest of the code...");  
14.  }  
15. }  

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 Excep4{  
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.      
11.    System.out.println("rest of the code...");  
12.  }  
13. }  

Output:Compile-time error

Nested try block:

try block within a try block is known as nested try block.

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. ....  

Example:

1. <b><i>Example of nested try block</i></b>  
2.   
3. class Excep6{  
4.  public static void main(String args[]){  
5.   try{  
6.     try{  
7.      System.out.println("going to divide");  
8.      int b =39/0;  
9.     }catch(ArithmeticException e){System.out.println(e);}  
10.    
11.     try{  
12.     int a[]=new int[5];  
13.     a[5]=4;  
14.     }catch(ArrayIndexOutOfBoundsException e){System.out.println(e);}  
15.      
16.     System.out.println("other statement);  
17.   }catch(Exception e){System.out.println("handeled");}  
18.   
19.   System.out.println("normal flow..");  
20.  }  
21. }  

finally block

The finally block is a block that is always executed. It is mainly used to perform some important tasks such as closing connection, stream etc.

Note:Before terminating the program, JVM executes finally block(if any).

Note:finally must be followed by try or catch block.

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Why use finally block?

finally block can be used to put "cleanup" code such as closing a file,closing connection etc.

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case 1

Program in case exception does not occur

1. class Simple{  
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.    
9.   finally{System.out.println("finally block is always executed");}  
10.    
11.   System.out.println("rest of the code...");  
12.   }  
13. }  

Output:5
       finally block is always executed
       rest of the code...

case 2

Program in case exception occured but not handled

1. class Simple{  
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.    
9.   finally{System.out.println("finally block is always executed");}  
10.    
11.   System.out.println("rest of the code...");  
12.   }  
13. }  

Output:finally block is always executed
       Exception in thread main java.lang.ArithmeticException:/ by zero

case 3

Program in case exception occured and handled

1. class Simple{  
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.    
9.   finally{System.out.println("finally block is always executed");}  
10.    
11.   System.out.println("rest of the code...");  
12.   }  
13. }  

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).

throw keyword

The throw keyword is used to explictily throw an exception.

We can throw either checked or uncheked exception. The throw keyword is mainly used to throw custom exception. We will see custom exceptions later.

Example of throw keyword

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. class Excep13{  
2.   
3.    static void validate(int age){  
4.      if(age<18)  
5.       throw new ArithmeticException("not valid");  
6.      else  
7.       System.out.println("welcome to vote");  
8.    }  
9.      
10.    public static void main(String args[]){  
11.       validate(13);  
12.       System.out.println("rest of the code...");  
13.   }  
14. }  

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

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.

Rule: By default Unchecked Exceptions are forwarded in calling chain (propagated).

Program of Exception Propagation

1. class Simple{  
2.   void m(){  
3.     int data=50/0;  
4.   }  
5.   void n(){  
6.     m();  
7.   }  
8.   void p(){  
9.    try{  
10.     n();  
11.    }catch(Exception e){System.out.println("exception handled");}  
12.   }  
13.   public static void main(String args[]){  
14.    Simple obj=new Simple();  
15.    obj.p();  
16.    System.out.println("normal flow...");  
17.   }  
18. }  

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.

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Rule: By default, Checked Exceptions are not forwarded in calling chain (propagated).

Program which describes that checked exceptions are not propagated

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

Output:Compile Time Error

throws keyword

The 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 throws keyword:

1. void method_name() throws exception_class_name{  
2.  ...   
3. }  

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Que) Which exception should we declare?

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.

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Advantage of throws keyword:

Now Checked Exception can be propagated (forwarded in call stack).

Program which describes that checked exceptions can be propagated by throws keyword.

1. import java.io.IOException;  
2. class Simple{  
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{  
11.     n();  
12.    }catch(Exception e){System.out.println("exception handled");}  
13.   }  
14.   public static void main(String args[]){  
15.    Simple obj=new Simple();  
16.    obj.p();  
17.    System.out.println("normal flow...");  
18.   }  
19. }  

Output:exception handled
       normal flow...

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Rule: 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.

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

Output:exception handled
       normal flow...

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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

1. import java.io.*;  
2. class M{  
3.  void method()throws IOException{  
4.   System.out.println("device operation performed");  
5.  }  
6. }  
7.   
8.   
9. class Test{  
10.    public static void main(String args[])throws IOException{//declare exception  
11.     Test t=new Test();  
12.     t.method();     
13.   
14.     System.out.println("normal flow...");  
15.   }  
16. }  

Output:device operation performed
       normal flow...

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.   
8.   
9. class Test{  
10.    public static void main(String args[])throws IOException{//declare exception  
11.     Test t=new Test();  
12.     t.method();     
13.   
14.     System.out.println("normal flow...");  
15.   }  
16. }  

Output:Runtime Exception

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Difference between throw and throws:

throw keyword	throws keyword
1)throw is used to explicitly throw an exception.	throws is used to declare an exception.
2)checked exception can not be propagated without throws.	checked exception can be propagated with 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 exception	You can declare multiple exception e.g. public void method()throws IOException,SQLException.

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Que) Can we rethrow an exception?

Yes by throwing same exception in catch block.

ExceptionHandling with MethodOverriding

There are many rules if we talk about methodoverriding with exception handling. The Rules are as follows: If the superclass method does not declare an exception 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 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. }  
5.   
6. class Child extends Parent{  
7.   void msg()throws IOException{  
8.     System.out.println("child");  
9.   }  
10.   public static void main(String args[]){  
11.    Parent p=new Child();  
12.    p.msg();  
13.   }  
14. }  

Output:Compile Time Error

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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.   
6. class Child extends Parent{  
7.   void msg()throws ArithmeticException{  
8.     System.out.println("child");  
9.   }  
10.   public static void main(String args[]){  
11.    Parent p=new Child();  
12.    p.msg();  
13.   }  
14. }  

Output:child

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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.   
6. class Child extends Parent{  
7.   void msg()throws Exception{System.out.println("child");}  
8.   
9.   public static void main(String args[]){  
10.    Parent p=new Child();  
11.    try{  
12.    p.msg();  
13.    }catch(Exception e){}  
14.   }  
15. }  

Output:Compile Time Error

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Example in case subclass overridden method declares same exception

1. import java.io.*;  
2. class Parent{  
3.   void msg()throws Exception{System.out.println("parent");}  
4. }  
5.   
6. class Child extends Parent{  
7.   void msg()throws Exception{System.out.println("child");}  
8.   
9.   public static void main(String args[]){  
10.    Parent p=new Child();  
11.    try{  
12.    p.msg();  
13.    }catch(Exception e){}  
14.   }  
15. }  

Output:child

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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.   
6. class Child extends Parent{  
7.   void msg()throws ArithmeticException{System.out.println("child");}  
8.   
9.   public static void main(String args[]){  
10.    Parent p=new Child();  
11.    try{  
12.    p.msg();  
13.    }catch(Exception e){}  
14.   }  
15. }  

Output:child

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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.   
6. class Child extends Parent{  
7.   void msg(){System.out.println("child");}  
8.   
9.   public static void main(String args[]){  
10.    Parent p=new Child();  
11.    try{  
12.    p.msg();  
13.    }catch(Exception e){}  
14.   }  
15. }  

Output:child

Custom Exception

If you are creating your own Exception that is known as custom exception or user-defined exception.

1. class InvalidAgeException extends Exception{  
2.  InvalidAgeException(String s){  
3.   super(s);  
4.  }  
5. }  

1. class Excep13{  
2.   
3.    static void validate(int age)throws InvalidAgeException{  
4.      if(age<18)  
5.       throw new InvalidAgeException("not valid");  
6.      else  
7.       System.out.println("welcome to vote");  
8.    }  
9.      
10.    public static void main(String args[]){  
11.       try{  
12.       validate(13);  
13.       }catch(Exception m){System.out.println("Exception occured: "+m);}  
14.   
15.       System.out.println("rest of the code...");  
16.   }  
17. }  

Output:Exception occured: InvalidAgeException:not valid
       rest of the code...