Custom (User-Defined) Exceptions

Java provides a vast array of built-in exceptions like NullPointerException and ArithmeticException. However, sometimes these standard exceptions do not accurately describe the specific business logic errors occurring in your application.

In such cases, you can create User-Defined Exceptions (also known as Custom Exceptions).

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Why Create Custom Exceptions?

1. Business Logic Clarity: To catch and provide specific treatment to a subset of existing Java exceptions based on business rules.
2. Custom Error Codes: You can add specific attributes (like error codes or transaction IDs) to your custom exception class for better logging and monitoring.
3. Readability: InsufficientFundsException is much easier to understand in a banking application than a generic Exception or RuntimeException.

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How to Create a Custom Exception

To create a custom exception, you simply need to create a class that extends either:

• Exception: To create a Checked Exception (the compiler forces you to handle it).
• RuntimeException: To create an Unchecked Exception (handling is optional).

Step 1: Create the Exception Class

Usually, a custom exception class should provide at least a default constructor and a constructor that accepts a String message.

// Extending Exception makes this a Checked Exception
public class InvalidAgeException extends Exception {

  // Default constructor
  public InvalidAgeException() {
    super("Age is not valid");
  }

  // Parameterized constructor
  public InvalidAgeException(String message) {
    super(message);
  }
}

Step 2: Throw the Custom Exception

Use the throw keyword to throw the exception when your business rule is violated. Because it's a checked exception, you must also use throws in the method signature.

public class LabException7 {

  // Method throws the custom exception
  static void validateAge(int age) throws InvalidAgeException {
    if (age < 18) {
      throw new InvalidAgeException(
        "Not eligible to vote. Age must be 18 or older."
      );
    } else {
      System.out.println("Welcome to the voting system.");
    }
  }

  public static void main(String args[]) {
    try {
      // Calling the method requires a try-catch block
      validateAge(15);
    } catch (InvalidAgeException e) {
      System.out.println("Caught Custom Exception: " + e.getMessage());
    }
  }
}

Output:

Caught Custom Exception: Not eligible to vote. Age must be 18 or older.

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Custom Exception with Error Codes

You can enhance your custom exceptions by adding fields and methods, such as an error code.

public class PaymentFailedException extends RuntimeException {

  private int errorCode;

  public PaymentFailedException(String message, int errorCode) {
    super(message);
    this.errorCode = errorCode;
  }

  public int getErrorCode() {
    return errorCode;
  }
}

public class LabException8 {

  static void processPayment(double amount) {
    if (amount > 10000) {
      // Throwing unchecked custom exception
      throw new PaymentFailedException("Amount exceeds daily limit", 4001);
    }
    System.out.println("Payment Successful");
  }

  public static void main(String args[]) {
    try {
      processPayment(15000);
    } catch (PaymentFailedException e) {
      System.out.println("Transaction Failed!");
      System.out.println("Reason: " + e.getMessage());
      System.out.println("Error Code: " + e.getErrorCode());
    }
  }
}

[!NOTE] Since PaymentFailedException extends RuntimeException, we did not need to declare throws PaymentFailedException in the method signature.