Java — Exceptions

Basic try-catch

public class Main {
    public static void main(String[] args) {
        try {
            int result = 10 / 0;
        } catch (ArithmeticException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

Multiple catch blocks

try {
    String s = null;
    s.length();
} catch (NullPointerException e) {
    System.out.println("Null pointer");
} catch (Exception e) {
    System.out.println("General error");
} finally {
    System.out.println("Always runs");
}

Custom exceptions

class InsufficientFundsException extends Exception {
    private double amount;

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

    double getAmount() { return amount; }
}

class BankAccount {
    private double balance;

    void withdraw(double amount) throws InsufficientFundsException {
        if (amount > balance) {
            throw new InsufficientFundsException(amount - balance);
        }
        balance -= amount;
    }
}

Try-with-resources

import java.io.*;

try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    String line;
    while ((line = br.readLine()) != null) {
        System.out.println(line);
    }
} // Auto-closed

Common exceptions

Exception	When
NullPointerException	Null reference
ArrayIndexOutOfBoundsException	Invalid array index
ClassNotFoundException	Class not found
IOException	I/O error
NumberFormatException	Invalid number format
IllegalArgumentException	Invalid argument
IllegalStateException	Invalid state

Mini Practice

Write Java code that:

1. Creates a custom exception
2. Uses try-with-resources for file operations
3. Demonstrates multi-catch with |

Up Next

In the next lesson, you'll learn about Collections — Lists, Sets, and Maps.