17.6. Exception Handling and the Exception Hierarchy in Java: Creating Custom Exceptions

17.6 Exception Handling and the Exception Hierarchy in Java

Exception handling is a fundamental aspect of programming in Java, as it allows a program to deal with abnormal situations in a controlled way. An exception is an event that occurs during the execution of a program and that interrupts the normal flow of instructions. Java provides a robust exception handling system that helps you create more reliable and maintainable programs.

Exception Hierarchy in Java

In Java, all exception classes are descendants of the Throwable class. From there, there are two main subclasses: Error and Exception. Errors are rare situations that are not expected to be caught or handled by the program, such as Java virtual machine (JVM) problems. Exceptions are conditions that an application might want to catch.

Exceptions in Java are categorized into two large groups:

• Checked Exceptions: These are exceptions that need to be handled or declared in the code. They are checked at compile time. Examples include IOException and SQLException.
• Unchecked Exceptions: These are exceptions that do not necessarily need to be handled or declared. They are checked at run time. Examples include RuntimeException, NullPointerException and ArithmeticException.

Exception handling in Java is mainly done through four keywords: try, catch, finally and throw .

Creating Custom Exceptions

Although Java provides many exception classes that cover a wide range of error situations, it is sometimes necessary to create custom exceptions. This is done by extending the Exception class or any of its subclasses.

To create a custom exception:

1. Create a new class that inherits from Exception or RuntimeException, depending on whether you want a checked or unchecked exception.
2. Add a constructor that accepts an error message as a parameter and passes that message to the base class constructor.
3. Optionally, you can add other constructors or methods as needed for your exception.

Example of a custom exception:

public class CustomException extends Exception {
    public CustomException(String message) {
        super(message);
    }
}

To throw this exception, you use the throw keyword:

public void someMethod() throws CustomException {
    if (someCondition) {
        throw new CustomException("Error description");
    }
}

To catch this exception, you can use a try-catch block:

try {
    someMethod();
} catch (CustomException e) {
    System.out.println(e.getMessage());
}

Good Practices in Exception Handling

When it comes to creating and using custom exceptions, there are several best practices to follow:

• Use custom exceptions sparingly: Before creating a new custom exception, consider whether one of Java's standard exceptions can be used.
• Provide informative error messages: Error messages should provide sufficient detail so that the problem can be identified and resolved.
• Document exceptions: If a method can throw a custom exception, it should be clearly documented so that users of the class know what to expect.
• Do not use exceptions for flow control: Exceptions should be used for error conditions and not as a form of normal program flow control.
• Avoid catching the generic Exception class: This may accidentally catch exceptions you didn't intend to handle.

In summary, exception handling and creating custom exceptions are powerful tools in Java that help you create robust and reliable programs. By following best practices and understanding the exception hierarchy, programmers can write code that effectively handles unexpected situations and improves the overall quality of the software.