4.1 The Throwable Class Hierarchy

← Back to Chapter 4: Exception Handling

All exceptional situations in Java inherit from a single superclass: java.lang.Throwable.

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What is Exception Handling?

An exception is an unexpected problem that happens while a program is running.

Exception handling means:

"When something goes wrong, handle the problem properly instead of allowing the application to crash unexpectedly."

Exception Hierarchy

Throwable is the parent of Error and Exception.

                       ┌─────────────────────────┐
                       │   java.lang.Throwable   │
                       └────────────┬────────────┘
                                    │
            ┌───────────────────────┴───────────────────────┐
            ▼                                               ▼
┌────────────────────────┐                     ┌────────────────────────┐
│    java.lang.Error     │                     │  java.lang.Exception   │
│ (Unrecoverable System) │                     │ (Application Recovery) │
└───────────┬────────────┘                     └───────────┬────────────┘
            │                                              │
    ┌───────┴────────┐                    ┌────────────────┴────────────────┐
    ▼                ▼                    ▼                                 ▼
OutOfMemoryError  StackOverflowError   RuntimeException                 Checked Exceptions
                                       (Unchecked)                      (Checked at compile time)
                                       • NullPointerException           • IOException
                                       • ArithmeticException            • SQLException
                                       • IllegalArgumentException       • ClassNotFoundException

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2. Error vs Exception

Dimension	java.lang.Error	java.lang.Exception
Origin	Fatal JVM or hardware-level failures	Application logic, network, or external IO failures
Recovery	Unrecoverable: Application should terminate	Recoverable: Application should catch, log, and handle
Examples	OutOfMemoryError, StackOverflowError, NoClassDefFoundError	FileNotFoundException, IllegalArgumentException
Handling	Never catch Error in production code	Handled using try-catch blocks or propagated via throws

Never Catch java.lang.Throwable Blindly

Catching Throwable will intercept fatal JVM errors like OutOfMemoryError or ThreadDeath, preventing the JVM from shutting down cleanly or triggering diagnostic dumps.