Built-in and custom exceptions

 

Java provides a rich Exception Handling mechanism that allows us to:

• Use built-in exceptions for common error conditions.

• Create our own custom exceptions to represent application-specific problems.

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🔹 1. Built-in Exceptions

🔸 What are they?

These are predefined exception classes in the Java standard library (in java.lang and other packages).

🔸 Types:

Category	Examples
Unchecked Exceptions (subclass of RuntimeException)	ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException
Checked Exceptions (subclass of Exception but not RuntimeException)	IOException, SQLException, ClassNotFoundException

📘 Example: Using a Built-in Exception

public class BuiltInExample {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3};
        try {
            System.out.println(numbers[5]);  // out of bounds
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("Caught: " + e);
        }
    }
}

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🔹 2. Custom Exceptions

🔸 What are they?

Custom (user-defined) exceptions are classes you create by extending the Exception class (or its subclasses) to represent specific error conditions in your application.

✅ Why use custom exceptions?

• To provide meaningful names to exceptions.

• To handle business logic errors more cleanly.

• To keep the code more readable and modular.

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🔧 How to Create a Custom Exception

Step 1: Create a class that extends Exception or RuntimeException

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

Step 2: Use throw and throws to use it

public class CustomExceptionDemo {
    public static void main(String[] args) {
        try {
            checkAge(15);
        } catch (MyCustomException e) {
            System.out.println("Custom Exception Caught: " + e.getMessage());
        }
    }
    static void checkAge(int age) throws MyCustomException {
        if (age < 18) {
            throw new MyCustomException("Age must be 18 or above.");
        }
        System.out.println("Access granted.");
    }
}

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🔁 Comparison Table

Feature	Built-in Exceptions	Custom Exceptions
Defined by	Java API (java.lang, java.io, etc.)	Programmer/User-defined
Purpose	Handle general errors like null, IO, etc.	Handle application-specific errors
Need to write new class?	❌ No	✅ Yes
Examples	NullPointerException, IOException	InsufficientFundsException, InvalidInputException
Extend from	Exception or RuntimeException	Same

🧠 Best Practice

• Use built-in exceptions for general coding errors.

• Use custom exceptions to clearly indicate and handle business rule violations or application-specific issues.