Java Exception Handling

Throwable
├── Error          (OutOfMemoryError, StackOverflowError — don't catch these)
└── Exception
    ├── IOException, SQLException, ...  (checked — must handle or declare)
    └── RuntimeException
        ├── NullPointerException
        ├── ArrayIndexOutOfBoundsException
        ├── ClassCastException
        ├── ArithmeticException
        └── IllegalArgumentException

public class ExceptionDemo {
    public static void main(String[] args) {
        // Basic try-catch-finally
        try {
            int result = 10 / 0;  // ArithmeticException
            System.out.println(result);
        } catch (ArithmeticException e) {
            System.out.println("Caught: " + e.getMessage());  // / by zero
        } finally {
            System.out.println("finally always runs");
        }

        // Multi-catch (Java 7+) — handle multiple exceptions in one block
        String[] arr = {"10", "abc", null};
        for (String s : arr) {
            try {
                int n = Integer.parseInt(s);
                System.out.println("Parsed: " + n);
            } catch (NumberFormatException | NullPointerException e) {
                System.out.println("Error parsing '" + s + "': " + e.getClass().getSimpleName());
            }
        }

        // throws — declare checked exceptions
        try {
            readFile("missing.txt");
        } catch (java.io.IOException e) {
            System.out.println("File error: " + e.getMessage());
        }
    }

    // throws declares that this method may throw a checked exception
    static void readFile(String path) throws java.io.IOException {
        throw new java.io.IOException("File not found: " + path);
    }
}

import java.io.*;

// Custom checked exception
class InsufficientFundsException extends Exception {
    private double amount;

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

    public double getAmount() { return amount; }
}

public class CustomException {

    static void withdraw(double balance, double amount)
            throws InsufficientFundsException {
        if (amount > balance) {
            throw new InsufficientFundsException(amount - balance);
        }
        System.out.printf("Withdrew $%.2f. Remaining: $%.2f%n",
                          amount, balance - amount);
    }

    public static void main(String[] args) {
        // Custom exception
        try {
            withdraw(100.0, 150.0);
        } catch (InsufficientFundsException e) {
            System.out.println(e.getMessage());
            System.out.printf("You need $%.2f more.%n", e.getAmount());
        }

        // try-with-resources (Java 7+) — auto-closes resources
        // The resource must implement AutoCloseable
        try (BufferedReader br = new BufferedReader(new StringReader("line1\nline2"))) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        // br is automatically closed here — no finally needed
    }
}