What is an Exception in Java?

An exception is an event that disrupts the normal flow of the program during execution. It occurs when something goes wrong — for example, dividing by zero or accessing an invalid array index.

Java handles exceptions using a powerful mechanism that helps avoid program crashes and manage errors gracefully.

Exception Hierarchy

Object
  └── Throwable
       ├── Error         (serious issues, ignored usually)
       └── Exception     (handled by the program)
             ├── Checked Exception (must handle)
             └── Unchecked Exception (runtime, optional handling)

Types of Exceptions

1. Checked Exceptions

Handled at compile-time (e.g., IOException, SQLException)

2. Unchecked Exceptions

Handled at runtime (e.g., ArithmeticException, NullPointerException)

Basic Exception Handling Syntax

try {
    // Code that may throw an exception
} catch (ExceptionType name) {
    // Code to handle the exception
} finally {
    // Code that will always execute (optional)
}

Example 1: Divide by Zero (Unchecked Exception)

public class ExceptionExample {
    public static void main(String[] args) {
        try {
            int a = 10;
            int b = 0;
            int c = a / b;  // This will throw ArithmeticException
            System.out.println("Result: " + c);
        } catch (ArithmeticException e) {
            System.out.println("Error: Cannot divide by zero!");
        } finally {
            System.out.println("Finally block always runs.");
        }
    }
}

Output:

Error: Cannot divide by zero!
Finally block always runs.

Example 2: File Handling (Checked Exception)

import java.io.*;

public class FileExample {
    public static void main(String[] args) {
        try {
            FileReader file = new FileReader("data.txt");
            BufferedReader reader = new BufferedReader(file);
            System.out.println(reader.readLine());
            reader.close();
        } catch (IOException e) {
            System.out.println("File not found or error reading file: " + e.getMessage());
        }
    }
}

Note: You must handle IOException, or the compiler will throw an error.

Keywords Summary

Custom Exception Example

You can even create your own exception class:

class MyException extends Exception {
    MyException(String msg) {
        super(msg);
    }
}

public class CustomEx {
    static void validateAge(int age) throws MyException {
        if (age < 18)
            throw new MyException("You must be 18+ to register.");
        else
            System.out.println("Valid age for registration.");
    }

    public static void main(String[] args) {
        try {
            validateAge(16);
        } catch (MyException e) {
            System.out.println("Caught custom exception: " + e.getMessage());
        }
    }
}

Output:

Caught custom exception: You must be 18+ to register.