Exception Hierarchy: Throwable, Exception & RuntimeException

1Concept

All exceptions inherit from `java.lang.Throwable`. `Error` represents catastrophic system issues (OutOfMemoryError, StackOverflowError) that should not be caught. `Exception` is divided into: 1. Checked Exceptions (IOException, SQLException) enforced by compiler at build time; 2. Unchecked Exceptions (subclasses of RuntimeException like NullPointerException, IllegalArgumentException) indicating programming bugs.

2Architecture Diagram

                     [ Throwable ]
                          /   \
               [ Error ]         [ Exception ]
               (Fatal JVM)       /           \
                    [ Checked Exceptions ]   [ RuntimeException ]
                    (IOException, SQL)       (NPE, Arithmetic - Unchecked)

3Code Example

public class ExceptionHierarchyDemo {
    public static void checkAccountAge(int age) {
        if (age < 18) {
            // Unchecked exception indicating caller violation
            throw new IllegalArgumentException("Account holder must be at least 18 years old.");
        }
    }

    public static void main(String[] args) {
        try {
            checkAccountAge(16);
        } catch (IllegalArgumentException e) {
            System.err.println("Caught Unchecked Exception: " + e.getMessage());
        }
    }
}

4Expected Output

Caught Unchecked Exception: Account holder must be at least 18 years old.

5Key Takeaways

• ✓Checked exceptions must be either caught (`try-catch`) or declared (`throws`).
• ✓Do NOT catch `Throwable` or `Error`; it can mask critical JVM failures.
• ✓Modern enterprise Java favours unchecked RuntimeExceptions for business logic validation.