Custom Exception in Java

Last Updated: 06 March, 2023

An exception is an unexpected event that occurs during program execution and affects the current flow of the program execution, which can cause the program to terminate abnormally.

Java provides many built-in exception classes but also allows us to create our own exception class, which is known as a Custom Exception or User-defined Exception in Java. The Custom Exception class is basically a subclass of the built-in class Exception, and it throws that exception using the "throw" keyword.

A Java custom exception is used to customize the exception according to the needs of the user.

Syntax of Custom Exception

public class CustomExceptionClassName extends Exception { // code }

Why do we need to create custom exceptions?

Although Java's built-in exceptions cover almost all of the general types of exceptions that may occur in a Java program, there are times when we need to customise the exception according to the requirement.

There are the following reasons to use custom exceptions:

• To catch and provide specific treatment to a subset of existing Java exceptions.
• Business logic exceptions: These are the exceptions related to business logic and workflow. It is useful for the application's users or the developers to understand the exact problem.

To create a custom exception, we will create an exception class that extends java.lang.Exception class.

Consider the example given below, where we created a custom exception named InvalidRollNumberException.

Example of Custom (User defined) Exception in Java

// Custom Exception class class InvalidRollNumberException extends Exception { public InvalidRollNumberException(String msg) { super(msg); } } class Student { // Fetching student record by roll number void getStudentByRollNo(int rollNo) { try { if (rollNo > 20) { throw new InvalidRollNumberException("ERROR: Please pass valid roll number."); } else { System.out.println("Student record is available."); } } catch (InvalidRollNumberException e) { System.out.println(e.getMessage()); } } } public class CustomExceptionExample { public static void main(String args[]) { Student studentObj = new Student(); studentObj.getStudentByRollNo(35); // Passing invalid value studentObj.getStudentByRollNo(14); // Passing valid value } }

Output

ERROR: Please pass valid roll number.
Student record is available.