Create a custom exception in Java

Following are the steps to create custom exception.

(1) create a custom exception class in Java;
(2) throw our custom Java exception;
(3) catch our custom exception; and
(4) look at the output from our custom exception when we print a stack trace.

To create a custom exception class, all you have to do is extend the Java Exception class, and create a simple constructor:

/**
 * My custom exception class.
 */
class CustomException extends Exception
{
  public CustomException(String message)
  {
    super(message);
  }
}

A method to throw our custom Java exception


here's a small example class with a method named getUser that will throw our custom exception (CustomException) if the method is given the value of zero as a parameter.

/**
 * Our test class to demonstrate our custom exception.
 */
class User
{
  public String getUser(int i) throws CustomException
  {
    if (i == 0)
    {
      // throw our custom exception
      throw new CustomException("zero ...");
    }
    else
    {
      return "user";
    }
  }
}

To test our custom Java exception. In our main method, we'll create a new instance of our User class, then call the getUser method with the value of zero, which makes that method throw our custom Java exception:

/**
 * A class to test (throw) the custom exception we've created.
 *
 */
public class CustomExceptionExample
{
  public static void main(String[] args)
  {
    // create a new User
    User user= new User();
    
    try
    {
      // intentionally throw our custom exception by
      // calling getUser with a zero
      String userStr= user.getUser(0);
    }
    catch (CustomException e)
    {
      // print the stack trace
      e.printStackTrace();
    }
  }
}