This post demonstrate , how to write you own custom exception in java here I have made UserNotFoundException. By naming convention, this Exception throws when user not found for provided userId. Here there are three class first, UserNotFoundException.java (custom exception) , second DAO ( UserDAOImpl.java) and third class Client application which run and gives the output of program. for simlicity throws explicitly.

Write Custom Exception

Write your custome exception , e.g. UserNotFoundException which basically used to throw when user by id not found.

UserNotFoundException.java

package com.mysoftkey.exception;

/**
 * @author ranjeet.jha
 *
 */
public class UserNotFoundException extends Exception {

	/**
	 * @param throwable
	 */
	public UserNotFoundException(Throwable throwable) {
		super(throwable);
	}

	public UserNotFoundException( String msg, Throwable throwable) {
		super(msg, throwable);
	}

	public UserNotFoundException( String msg) {
		super(msg);
	}

}

DAO (Data Access Object )

From DAO (Data Access Object ) we should throw exception if user not found by userId. Here, I throws explicitly just to demonstrate how to throws UserNotFoundException , it means just to demonstrate how to throws custom exception.

UserDAOImpl.java

package com.mysoftkey.exception;

import java.util.HashMap;
import java.util.Map;

/**
 * @author ranjeet@mysoftkey.com
 *
 */
public class UserDAOImpl {

 /**
  * @param id
  * @return
  * @throws Exception
  * @throws UserNotFoundException
  */
  public Map getUserById(int id) throws Exception, UserNotFoundException {

	Map userMap = new HashMap();
	
	try {
		
		// here explicitly throwing exception.
		throw new UserNotFoundException("User not found for id: " + id);
	} catch(UserNotFoundException e) {
		throw e;
	} catch (Exception e) {
	
	}
	return userMap;
 }
}

Client Class (UserManager.)

UserManager.java

package com.mysoftkey.exception;

import java.util.Map;

/**
 * @author ranjeet
 *
 */
public class UserManager {

 /**
  * @param args
  */
 public static void main(String[] args) {
	UserDAOImpl dao = new UserDAOImpl();
	try {
		System.out.println("befor calling to pull user by id.");
		Map userMap = dao.getUserById(10);
		System.out.println(userMap.toString());
		System.out.println("After calling to pull user by id.");
	} catch (Exception e) {
		// here we get same message which I set while throwing of UserNotFoundException
		System.err.println("messg: "+ e.getMessage());
		e.printStackTrace();
	}
 } 

}

Output of Program

messg: User not found for id: 10
befor calling to pull user by id.
com.mysoftkey.exception.UserNotFoundException: User not found for id: 10
	at com.mysoftkey.exception.UserDAOImpl.getUserById(UserDAOImpl.java:28)
	at com.mysoftkey.exception.UserManager.main(UserManager.java:21)

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