Custom exceptions in Java

In Java, we can define our own custom exceptions to handle errors that are specific to our application or domain. Custom exceptions can be created by extending the Exception or RuntimeException class.

Here are a few reasons why you might want to use custom exceptions:

  1. Provide more specific error messages: When an exception is thrown in Java, the default error message can be too general or technical for the end user to understand. By creating custom exceptions with specific error messages, you can provide more meaningful feedback to the user.
  2. Encapsulate error-handling logic: By creating custom exceptions, you can encapsulate the error-handling logic in one place and keep it separate from the rest of your code. This can make your code more modular and easier to maintain.
  3. Make code more readable: By using custom exceptions, you can make your code more readable and expressive. For example, instead of throwing a generic Exception, you can throw a custom ValidationException when input validation fails. This can make your code more self-documenting and easier for others to understand.
  4. Enable more specific error handling: By creating custom exceptions, you can enable more specific error handling. For example, you might catch a custom FileFormatException when reading data from a file, and handle it differently than a general IOException.

Here are some places where you might use custom exceptions:

  1. Input validation: When validating user input, you might create custom exceptions to handle specific validation errors. For example, you might create a InvalidEmailException for when an email address is in an invalid format.
  2. Data access: When working with databases or other data sources, you might create custom exceptions to handle specific data access errors. For example, you might create a DataAccessException for when there is an error accessing the data source.
  3. Network communication: When working with network communication, you might create custom exceptions to handle specific communication errors. For example, you might create a ConnectionException for when there is an error establishing a network connection.

Here’s a step-by-step guide on how to create and use custom exceptions in Java:

Step 1: Define the custom exception class

To define a custom exception, we can create a new class that extends Exception or RuntimeException. For example:

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

In this example, we’ve created a new class called InvalidInputException that extends Exception. The constructor takes a String argument that will be used as the exception message.

Step 2: Throw the custom exception

To throw the custom exception, we can use the throw statement in your code. For example:

public void processInput(int input) throws InvalidInputException {
    if (input < 0) {
        throw new InvalidInputException("Input must be a positive integer.");
    }

    // rest of the code
}

In this example, we’ve defined a method called processInput that takes an integer argument. If the input is less than 0, we throw an instance of InvalidInputException with the message “Input must be a positive integer”.

Step 3: Catch the custom exception

To catch the custom exception, we can use a trycatch block like you would with any other exception. For example:

try {
    processInput(-1);
} catch (InvalidInputException e) {
    System.out.println("Caught exception: " + e.getMessage());
}

In this example, we’re calling the processInput method with an input of -1, which will throw an instance of InvalidInputException. We’re using a catch block to catch the exception and print the exception message to the console.

Step 4: Handle the custom exception

We can handle the custom exception just like any other exception in Java. For example, we can display an error message to the user or log the error. Here’s an example:

public void run() {
    try {
        processInput(-1);
    } catch (InvalidInputException e) {
        System.out.println("Error: " + e.getMessage());
        // handle the error
    }

    // rest of the code
}

In this example, we’re calling the processInput method with an input of -1, which will throw an instance of InvalidInputException. We’re using a catch block to catch the exception and display an error message to the user. We can also add additional code to handle the error, such as logging the error to a file.

Also, see the example code shell-scripting-examples in our GitHub repository. See complete examples in our GitHub repositories.

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