Lesson 01 · Exceptions & the Hierarchy

Objectives

After this lesson you will be able to:

• Place Throwable, Error, Exception, and RuntimeException in the hierarchy.
• Tell checked from unchecked exceptions and apply the handle-or-declare rule.
• Recognize common built-in exceptions and what triggers them.

The Throwable hierarchy

Only a Throwable can be thrown or caught. The tree splits into two branches:

Throwable
├── Error              (unchecked — serious JVM problems; don't catch)
└── Exception          (checked, EXCEPT...)
    └── RuntimeException  (unchecked — programming errors)

• Error (e.g. OutOfMemoryError, StackOverflowError) — abnormal conditions you normally can't recover from. Unchecked.
• RuntimeException (e.g. NullPointerException, IllegalArgumentException, ArrayIndexOutOfBoundsException) — bugs. Unchecked.
• Other Exceptions (e.g. IOException, SQLException) — recoverable conditions. Checked.

Checked vs unchecked

A checked exception must be handled or declared (the handle-or-declare rule): either catch it or list it in the method's throws clause, or the code won't compile. Unchecked exceptions (RuntimeException and Error and their subclasses) need neither.

void read() throws IOException {          // checked — must declare or catch
    Files.readString(Path.of("a.txt"));
}
int parse(String s) {                     // NumberFormatException is unchecked — no throws needed
    return Integer.parseInt(s);
}

Exam trap

NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException, ArithmeticException, NumberFormatException, and IllegalArgumentException are all unchecked (RuntimeException subclasses). IOException and SQLException are checked. Know which is which — the exam tests whether a method needs a throws clause.

Throwing and the throws clause

throw raises an exception; throws (on the signature) declares what a method may propagate.

void check(int age) {
    if (age < 0) throw new IllegalArgumentException("age < 0");   // unchecked
}

Gotcha

An overriding method may not declare new or broader checked exceptions than the method it overrides — but it may declare fewer, narrower, or any unchecked ones. This pairs with the override rules from Module 03.

Key Takeaways

• Everything thrown is a Throwable: Error and RuntimeException are unchecked; other Exceptions are checked.
• Checked exceptions follow handle-or-declare; unchecked ones don't.
• Common unchecked: NPE, ArrayIndexOutOfBounds, ClassCast, Arithmetic, NumberFormat, IllegalArgument. Common checked: IOException, SQLException.
• An override can't add broader checked exceptions, only narrower/fewer (or unchecked).

Lesson Quiz

Lesson Quiz · Exceptions & the Hierarchy0 / 5

1. Which exception is CHECKED?

   • ANullPointerException
   • BIOException
   • CArithmeticException
   • DNumberFormatException

2. Does Integer.parseInt require a throws clause on the caller?

   • AYes — NumberFormatException is checked
   • BNo — NumberFormatException is unchecked
   • COnly on Java 21
   • DOnly inside a try

3. Where does RuntimeException sit?

   • ASubclass of Error
   • BSubclass of Exception
   • CSubclass of Throwable directly
   • DTop of the hierarchy

4. An overriding method may declare...

   • AA broader checked exception than the parent
   • BA new checked exception the parent didn't
   • COnly narrower/fewer checked exceptions (or unchecked)
   • DNo exceptions at all, ever

5. Should you normally catch an Error such as StackOverflowError?

   • AYes, always
   • BNo — Errors signal unrecoverable JVM problems
   • COnly in finally
   • DOnly if checked

---

Next: try / catch / finally. Run the matching code in labs/src/main/java/com/jse21/m04_exceptions/.