Java Custom Exception

What is custom exception (user defined exception)?

• Java supports CustomException to create your own exception types to handle situations specific to your applications.
• CustomException class defines a subclass of Exception class(which is a subclass of Throwable class).
• CustomException class overrides toString method to display the decription for the exception.

How to define custom exception?

class CustomException extends Exception {

 public String toString() {
  //return message
 }
}

//Custom Exception class creation.
class CustomException extends Exception {                                       
 private String details;                                                        
                                                                                
 CustomException(String msg) {                                                  
  details = msg;                                                                
 }                                                                              
 public String toString() {                                                     
  return "CustomException: "+ details;                                          
 }                                                                              
}  

How to handle the custom exception?

// throw and handle custom exception in this main class.                                                                             
class CustomExceptionTest {                                                     
 static int division(int val) throws CustomException {                          
  int result = 0;                                                               
  if(val > 20) {                                                                
   throw new CustomException("Invalid value");                                  
  }                                                                             
  return result;                                                                
 }                                                                              
 public static void main(String args[]) {                                       
  int val = 0;                                                                  
  try {                                                                         
    val = CustomExceptionTest.division(25);                                     
    System.out.println(val);                                                    
  } catch(CustomException ex) {                                                 
    System.out.println("Exception: " + ex);                                     
  }                                                                             
 }                                                                              
}      

$ javac CustomExceptionTest.java 
$ java CustomExceptionTest
Exception: CustomException: Invalid value

Digit and Alphabet Custom Exception Java Program

Raises digit exception when user enters digits in alphabet exception option, similarly raises alphabet exception when enters digits in alphabet exception option.

import java.util.*; 
class digit_exception extends Exception 
{  
  digit_exception()  
  {  
    super("bad key:digits are not allowed");  
  } 
} 
class alphabet_exception extends Exception 
{  
  alphabet_exception(String s)  
  {  
    super(s);  
  } 
} 
class CustomException
{ 
  public static void main(String args[])    
  { 
    int opt; 
    System.out.println("\n custom_exception"); 
    Scanner sc=new Scanner(System.in); 
    System.out.println("\n 1.digit_exception \n2.alphabet_exception"); 
    System.out.println("\n enter a option"); 
    try 
    { 
      opt=sc.nextInt(); 
      switch(opt) 
      { 
        case 1:  
         String str=sc.next();  
         char []ar1=str.toCharArray();   
         for(char i:ar1)  
         {  
           if(Character.isDigit(i))  
           {  
             continue;  
           }  
           else  
           {  
             throw new alphabet_exception("bad key:alphabets are not allowed");  
           }  
         }  
         break; 
        case 2:  
         String str1=sc.next();  
         char []ar2=str1.toCharArray(); 
         for(char i:ar2)  
         {  
           if(Character.isDigit(i))  
           {  
             throw new digit_exception();  
           }  
         }  
         break;  
       } //switch  
     } //try  
     catch(alphabet_exception e)  
     { 
       System.out.println(e);  
     }  
     catch(digit_exception e)  
     {   
       System.out.println(e);             
     }    
   } //void 
} //class 

D:\Java_Programs>java CustomException

 custom_exception

 1.digit_exception
2.alphabet_exception

 enter a option
1
dfdg
alphabet_exception: bad key:alphabets are not allowed

D:\Java_Programs>java CustomException

 custom_exception

 1.digit_exception
2.alphabet_exception

 enter a option
2
232
digit_exception: bad key:digits are not allowed