How Do You Create a Custom Exception in Java?

1. Introduction

In Java, a custom exception is a user-defined exception created to handle specific business logic errors.

Instead of using only built-in exceptions, you can define your own exception to make your code:

• More readable
• More meaningful
• Easier to debug

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2. Why Do We Need Custom Exceptions?

Explanation

Sometimes built-in exceptions are not enough. For example:

• Invalid user age
• Insufficient balance
• Invalid input in business logic

In such cases, custom exceptions provide clear and domain-specific error messages.

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3. Steps to Create Custom Exception

Step 1: Extend Exception Class

• For checked exception → extend Exception
• For unchecked exception → extend RuntimeException

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4. Example (Checked Custom Exception)

// Step 1: Create custom exception
class InvalidAgeException extends Exception {
    public InvalidAgeException(String message) {
        super(message);
    }
}

// Step 2: Use custom exception
public class CustomExceptionExample {
    public static void main(String[] args) {
        int age = 16;

        try {
            if (age < 18) {
                throw new InvalidAgeException("Age must be 18 or above");
            }
            System.out.println("Eligible");
        } catch (InvalidAgeException e) {
            System.out.println(e.getMessage());
        }
    }
}

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5. Explanation of Code

• InvalidAgeException extends Exception → Checked exception
• Constructor passes message to parent class
• throw new InvalidAgeException(...) → explicitly throws exception
• catch block handles it

Output:

Age must be 18 or above

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6. Example (Unchecked Custom Exception)

class InvalidAmountException extends RuntimeException {
    public InvalidAmountException(String message) {
        super(message);
    }
}

public class UncheckedCustomException {
    public static void main(String[] args) {
        int amount = -100;

        if (amount < 0) {
            throw new InvalidAmountException("Amount cannot be negative");
        }

        System.out.println("Valid amount");
    }
}

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7. Explanation

• Extends RuntimeException → Unchecked exception
• No need for try-catch (optional)
• Used for programming or validation errors

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8. Key Points

• Custom exceptions improve code clarity
• Can be checked or unchecked
• Use meaningful names (e.g., InvalidAgeException)
• Always provide a clear message

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9. Best Practices

• Use checked exceptions for recoverable conditions
• Use unchecked exceptions for logic errors
• Keep exception classes simple and focused
• Add constructors for flexibility

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10. Summary

• Custom exceptions = user-defined exceptions
• Created by extending Exception or RuntimeException
• Used for handling specific application scenarios
• Improve readability and maintainability

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