Exception Handling

1. Types of Exceptions

Category	Description	Examples
Checked	Must be handled with try-catch	IOException, SQLException
Unchecked	Occur at runtime, optional to handle	NullPointerException, ArithmeticException
Errors	Serious problems, usually not handled	OutOfMemoryError, StackOverflowError

2. Try-catch Block

• Syntax:

  try {
    // Code that may throw an exception
  } catch (ExceptionType name) {
    // Code to handle the exception
  }
  

• Example:

  public class TryCatchExample {
    public static void main(String[] args) {
        try {
            int result = 10 / 0; // This causes ArithmeticException
            System.out.println("Result: " + result);
        } catch (ArithmeticException e) {
            System.out.println("Error: Cannot divide by zero.");
        }
    }
  }
  

• Output:

  Error: Cannot divide by zero.
  

3. Multiple Catch Blocks

You can catch different types of exceptions separately:

try {
    int[] arr = {1, 2, 3};
    System.out.println(arr[5]); // ArrayIndexOutOfBoundsException
} catch (ArithmeticException e) {
    System.out.println("Math error.");
} catch (ArrayIndexOutOfBoundsException e) {
    System.out.println("Invalid array index.");
}

Note: Catch blocks are priorities from top to bottom

4. Finally Block

The finally block is always executed, even if an exception occurs or not. Great for cleanup code (e.g., closing files or scanners).

Scanner sc = new Scanner(System.in);
try {
    System.out.println("Enter a number: ");
    int n = sc.nextInt();
    System.out.println("Number: " + n);
} catch (Exception e) {
    System.out.println("Invalid input.");
} finally {
    sc.close(); // Always runs
    System.out.println("Scanner closed.");
}

5. Throw Keyword

Use throw to manually raise an exception.

throw new IllegalArgumentException("Negative age not allowed");

6. Throws Keyword

Used to declare that a method may throw exceptions.

public void readFile() throws IOException {
    // read file here
}

Code demo

public class ExceptionDemo {
    public static int divide(int a, int b) throws ArithmeticException {
        if (b == 0) {
            throw new ArithmeticException("Cannot divide by zero.");
        }
        return a / b;
    }

    public static void main(String[] args) {
        try {
            int result = divide(10, 0);
            System.out.println("Result: " + result);
        } catch (ArithmeticException e) {
            System.out.println("Caught an error: " + e.getMessage());
        } finally {
            System.out.println("Division attempt complete.");
        }
    }
}

Output:

Caught an error: Cannot divide by zero.
Division attempt complete.