What is exception handling in Java? Explain try, catch, throw, throws, and finally.

Throwable
├── Error (JVM errors — don't catch these)
│   ├── OutOfMemoryError
│   └── StackOverflowError
└── Exception
    ├── Checked (must handle — compiler enforces)
    │   ├── IOException
    │   ├── SQLException
    │   └── FileNotFoundException
    └── Unchecked (RuntimeException — optional to handle)
        ├── NullPointerException
        ├── ArrayIndexOutOfBoundsException
        ├── IllegalArgumentException
        └── NumberFormatException

public void readFile(String path) {
    FileReader reader = null;
    try {
        reader = new FileReader(path);  // might throw FileNotFoundException
        // risky code
        int data = reader.read();       // might throw IOException

    } catch (FileNotFoundException e) {
        System.out.println("File not found: " + e.getMessage());

    } catch (IOException e) {
        System.out.println("IO error: " + e.getMessage());

    } finally {
        // Always runs — good for cleanup
        try { if (reader != null) reader.close(); } catch (IOException e) {}
    }
}

try {
    String s = null;
    s.length();  // NullPointerException

} catch (NullPointerException | IllegalArgumentException e) {
    // Handle both with single catch (Java 7+)
    System.out.println("Error: " + e.getMessage());
}

public void setAge(int age) {
    if (age < 0 || age > 150) {
        throw new IllegalArgumentException("Invalid age: " + age);
    }
    this.age = age;
}

// Caller must handle it
try {
    setAge(-5);
} catch (IllegalArgumentException e) {
    System.out.println(e.getMessage()); // "Invalid age: -5"
}

// Method declares it MIGHT throw — caller must handle
public void connectDB() throws SQLException {
    Connection conn = DriverManager.getConnection(url, user, pass);
}

// Caller handles it
try {
    connectDB();
} catch (SQLException e) {
    System.out.println("DB connection failed: " + e.getMessage());
}

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

// Usage
public void login(String user, String pass) {
    if (!isValid(user, pass)) {
        throw new InvalidLoginException("Invalid credentials for: " + user);
    }
}

// Auto-closes resources — no finally needed
try (Connection conn = getConnection();
     PreparedStatement ps = conn.prepareStatement(sql)) {
    ps.executeQuery();
} catch (SQLException e) {
    e.printStackTrace();
}

@Test
public void testLogin() {
    try {
        driver.findElement(By.id("submit")).click();
        String title = driver.getTitle();
        assertEquals("Dashboard", title);

    } catch (NoSuchElementException e) {
        fail("Submit button not found: " + e.getMessage());

    } catch (TimeoutException e) {
        fail("Page took too long to load");

    } finally {
        takeScreenshot("login_test");  // always capture screenshot
    }
}