Exception Handling and I/O

Errors happen — files are missing, networks time out, users provide invalid input. Java's exception handling mechanism gives you a structured way to detect, respond to, and recover from errors. This lesson also covers reading and writing files.

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The Exception Hierarchy

Throwable
├── Error             (serious — don't catch: OutOfMemoryError, StackOverflowError)
└── Exception
      ├── IOException          (checked)
      ├── SQLException         (checked)
      ├── FileNotFoundException (checked)
      └── RuntimeException     (unchecked)
            ├── NullPointerException
            ├── ArrayIndexOutOfBoundsException
            ├── IllegalArgumentException
            ├── ArithmeticException
            └── ClassCastException

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Checked vs Unchecked Exceptions

Type	Checked	Unchecked (Runtime)
Must handle?	Yes — compiler enforces try/catch or throws	No — compiler does not enforce
Caused by	External factors (I/O, network, database)	Programming errors (null access, bad cast)
Extends	Exception (but not RuntimeException)	RuntimeException
Examples	IOException, SQLException	NullPointerException, ArithmeticException

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Try / Catch / Finally

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero: " + e.getMessage());
} finally {
    System.out.println("This always executes");
}

Multiple Catch Blocks