What are Checked and Unchecked Exceptions in Java?

Java exceptions are categorized into Checked (compile-time) and Unchecked (runtime) exceptions.

1. Checked Exceptions

Checked at compile time. Must be handled using try-catch or declared with throws.

import java.io.*;

public class CheckedExceptionExample {
    public static void main(String[] args) {
        try {
            FileReader file = new FileReader("test.txt");
            BufferedReader br = new BufferedReader(file);
            System.out.println(br.readLine());
            br.close();
        } catch (FileNotFoundException e) {
            System.out.println("File not found: " + e.getMessage());
        } catch (IOException e) {
            System.out.println("IO Error: " + e.getMessage());
        }
    }
}

// Alternative: Using throws declaration
class CheckedWithThrows {
    public static void readFile() throws IOException {
        FileReader file = new FileReader("test.txt");
        BufferedReader br = new BufferedReader(file);
        System.out.println(br.readLine());
        br.close();
    }
}

Common Checked Exceptions

• IOException - Input/Output operations
• SQLException - Database access errors
• ClassNotFoundException - Class not found
• InterruptedException - Thread interruption

2. Unchecked Exceptions (Runtime Exceptions)

Not checked at compile time. Occur due to programming errors.

public class UncheckedExceptionExample {
    public static void main(String[] args) {
        // ArithmeticException
        try {
            int result = 10 / 0;
        } catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero");
        }
        
        // NullPointerException
        String str = null;
        try {
            System.out.println(str.length());
        } catch (NullPointerException e) {
            System.out.println("String is null");
        }
        
        // ArrayIndexOutOfBoundsException
        int[] arr = new int[3];
        try {
            arr[5] = 10;
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("Index out of bounds");
        }
        
        // NumberFormatException
        try {
            int num = Integer.parseInt("abc");
        } catch (NumberFormatException e) {
            System.out.println("Invalid number format");
        }
    }
}

Common Unchecked Exceptions

• NullPointerException - Accessing null reference
• ArithmeticException - Division by zero
• ArrayIndexOutOfBoundsException - Invalid array index
• NumberFormatException - Invalid number conversion
• IllegalArgumentException - Invalid argument passed

Custom Exception Examples

Custom Checked Exception

class InsufficientBalanceException extends Exception {
    public InsufficientBalanceException(String message) {
        super(message);
    }
}

class BankAccount {
    double balance = 1000;
    
    void withdraw(double amount) throws InsufficientBalanceException {
        if (amount > balance) {
            throw new InsufficientBalanceException("Insufficient balance. Available: " + balance);
        }
        balance -= amount;
        System.out.println("Withdrawn: " + amount + ", Remaining: " + balance);
    }
}

Custom Unchecked Exception

class InvalidAgeException extends RuntimeException {
    public InvalidAgeException(String message) {
        super(message);
    }
}

class Voter {
    void checkAge(int age) {
        if (age < 18) {
            throw new InvalidAgeException("Age " + age + " is not valid for voting");
        }
        System.out.println("Valid voter");
    }
}

Comparison Table

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