Custom exception classes in Java allow you to create more specific exception types for handling specific error scenarios. By extending the base Exception class or one of its subclasses, you can customize the behavior and properties of your exception.

Creating a Custom Exception Class

To create a custom exception class, you need to define a new class that extends the Exception class or one of its subclasses. For example, let's say you want to create a custom exception class for handling validation errors in a user registration system:

1public class ValidationException extends Exception {
2    public ValidationException(String message) {
3        super(message);
4    }
5}

In this example, ValidationException is a custom exception class that extends the base Exception class. It provides a single constructor that accepts a String parameter for the error message.

Throwing a Custom Exception

Once you have defined your custom exception class, you can use the throw keyword to raise an instance of your exception. For example:

1public void registerUser(String username, String password) throws ValidationException {
2    if (username.isEmpty() || password.isEmpty()) {
3        throw new ValidationException("Username and password cannot be empty.");
4    }
5    // rest of the registration logic
6}

In this example, the registerUser method throws a ValidationException if the username or password is empty. This allows the calling code to handle the exception appropriately.

Handling Custom Exceptions

Custom exceptions can be handled in a try-catch block just like any other exception. For example:

1try {
2    registerUser("johndoe", "");
3} catch (ValidationException e) {
4    System.out.println(e.getMessage());
5}

In this example, the registerUser method is called with an empty password, which throws a ValidationException. The exception is caught in the catch block, and the error message is printed to the console.

Custom exception classes allow you to create more specific exception types that convey meaningful information about the error. They help improve the readability and maintainability of your code by providing a clear and consistent way to handle different types of errors.

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public class ValidationException extends Exception {

    public ValidationException(String message) {

        super(message);

    }

}

​

public void registerUser(String username, String password) throws ValidationException {

    if (username.isEmpty() || password.isEmpty()) {

        throw new ValidationException("Username and password cannot be empty.");

    }

    // rest of the registration logic

}

​

try {

    registerUser("johndoe", "");

} catch (ValidationException e) {

    System.out.println(e.getMessage());

}