Java Exception Handling - try catch throw throws finally

1. What is an Exception?

An exception is an abnormal event that disrupts the normal flow of a program at runtime. Java's exception handling mechanism lets you detect, handle, and recover from such events without crashing the program.

Java exception handling overview showing exception hierarchy, try-catch-finally, throw, throws, custom exceptions, and resource management
Java exception handling connects detection, recovery, propagation, and resource cleanup.

Java distinguishes three categories of throwable problems:

Category Superclass Checked? Examples When Occurs
Checked Exception Exception Yes — compiler enforces handling IOException, SQLException, ParseException Predictable failures: file not found, bad URL
Unchecked Exception RuntimeException No — optional to handle NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException Programming bugs / logic errors
Error Error No — not meant to be caught OutOfMemoryError, StackOverflowError Serious JVM problems — typically unrecoverable

Checked vs Unchecked: Checked exceptions must be either caught (try-catch) or declared in the method signature (throws). Unchecked exceptions (RuntimeException subclasses) have no such requirement — handling is optional.

2. Exception Hierarchy

All exceptions and errors in Java descend from java.lang.Throwable. Understanding this hierarchy helps you choose which exceptions to catch and how broadly. 

java.lang.Object
  +-- java.lang.Throwable
        +-- java.lang.Error               (Errors — don't catch these)
        |     +-- OutOfMemoryError
        |     +-- StackOverflowError
        |     +-- VirtualMachineError
        +-- java.lang.Exception           (Checked exceptions)
              +-- IOException
              |     +-- FileNotFoundException
              |     +-- EOFException
              +-- SQLException
              +-- ParseException
              +-- ClassNotFoundException
              +-- RuntimeException        (Unchecked exceptions)
                    +-- NullPointerException
                    +-- ArrayIndexOutOfBoundsException
                    +-- ClassCastException
                    +-- NumberFormatException
                    +-- IllegalArgumentException
                    +-- IllegalStateException
                    +-- ArithmeticException
                    +-- UnsupportedOperationException
Exception Type Common Cause
NullPointerException Unchecked Calling a method on a null reference
ArrayIndexOutOfBoundsException Unchecked Accessing array index outside valid range
NumberFormatException Unchecked Integer.parseInt("abc") — non-numeric string
ClassCastException Unchecked Invalid type casting: (String) new Integer(1)
ArithmeticException Unchecked Division by zero: 10 / 0
FileNotFoundException Checked Opening a file that doesn't exist
IOException Checked General I/O failure (network, disk)
SQLException Checked Database query or connection failure
StackOverflowError Error Infinite or deeply nested recursion

3. try-catch Block

The try block contains code that might throw an exception. The catch block handles it. Java checks catch blocks top to bottom — put more specific exceptions before more general ones. 

public class TryCatchDemo {

    // Basic try-catch
    public static void basicDemo() {
        try {
            int result = 10 / 0;          // throws ArithmeticException
            System.out.println(result);   // never reached
        } catch (ArithmeticException e) {
            System.out.println("Caught: " + e.getMessage());  // / by zero
        }
        System.out.println("Program continues...");
    }

    // Catching specific exceptions — order matters (specific before general)
    public static void multiCatchDemo(String input, int[] arr, int index) {
        try {
            int parsed = Integer.parseInt(input);  // may throw NumberFormatException
            int value  = arr[index];               // may throw ArrayIndexOutOfBoundsException
            int result = parsed / value;           // may throw ArithmeticException
            System.out.println("Result: " + result);
        } catch (NumberFormatException e) {
            System.out.println("Bad number format: " + e.getMessage());
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("Array index error");
        }