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

graph TD
  A["Throwable"] --> B["Error (serious — don't catch: OutOfMemoryError, StackOverflowError)"]
  A --> C["Exception"]
  C --> D["IOException (checked)"]
  C --> E["SQLException (checked)"]
  C --> F["FileNotFoundException (checked)"]
  C --> G["RuntimeException (unchecked)"]
  G --> H["NullPointerException"]
  G --> I["ArrayIndexOutOfBoundsException"]
  G --> J["IllegalArgumentException"]
  G --> K["ArithmeticException"]
  G --> L["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

try {
    // risky code
} catch (FileNotFoundException e) {
    System.out.println("File not found");
} catch (IOException e) {
    System.out.println("I/O error: " + e.getMessage());
} catch (Exception e) {
    System.out.println("Unexpected error: " + e.getMessage());
}

Multi-Catch (Java 7+)

try {
    // risky code
} catch (IOException | SQLException e) {
    System.out.println("Error: " + e.getMessage());
}

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Throwing Exceptions

Throw

public static int divide(int a, int b) {
    if (b == 0) {
        throw new IllegalArgumentException("Divisor cannot be zero");
    }
    return a / b;
}

Throws Declaration

For checked exceptions, declare them in the method signature:

public static String readFile(String path) throws IOException {
    return Files.readString(Path.of(path));
}

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Custom Exceptions

public class InsufficientFundsException extends Exception {
    private final double amount;

    public InsufficientFundsException(double amount) {
        super("Insufficient funds: " + amount);
        this.amount = amount;
    }

    public double getAmount() {
        return amount;
    }
}

// Usage:
if (amount > balance) {
    throw new InsufficientFundsException(amount);
}

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Try-with-Resources (Java 7+)

Resources that implement AutoCloseable are automatically closed: