Exception Handling
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⚠️ Exception Handling

Learn how to deal with runtime errors gracefully using try-catch blocks, exceptions, and the try-with-resources statement.

1. Error vs Exception

An Exception is an unwanted or unexpected event occurring during program execution that disrupts the normal flow of instructions. Java provides a robust mechanism to handle these so the program can recover or terminate gracefully.

An Error represents serious problems that a reasonable application should not try to catch (e.g., OutOfMemoryError, StackOverflowError).

2. Exception Hierarchy

            Throwable
               / \
              /   \
          Error  Exception
                    / \
                   /   \
       IOException  RuntimeException
    SQLException      / \
                     /   \
     NullPointerException ArithmeticException
  • Checked Exceptions: Exceptions checked at compile-time (e.g., IOException). You MUST handle them using try-catch or declare them using throws.
  • Unchecked Exceptions: Exceptions occurring at runtime (subclasses of RuntimeException). Not checked at compile-time (e.g., NullPointerException).

3. Try-Catch & Finally

BasicExceptionDemo.javaJava
public class BasicExceptionDemo {
    public static void main(String[] args) {
        try {
            int result = 10 / 0; // Throws ArithmeticException
            System.out.println(result);
        } catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero!");
        } finally {
            System.out.println("Finally block always executes.");
        }
    }
}

4. Custom Exceptions & Throw/Throws

Use throw to explicitly throw an exception object. Use throws in a method signature to indicate the method might throw checked exceptions.

CustomExceptionDemo.javaJava
// Custom checked exception
class InsufficientFundsException extends Exception {
    public InsufficientFundsException(String message) {
        super(message);
    }
}

public class CustomExceptionDemo {
    static void withdraw(double balance, double amount) throws InsufficientFundsException {
        if (amount > balance) {
            throw new InsufficientFundsException("Not enough money!");
        }
        System.out.println("Withdrawal successful.");
    }

    public static void main(String[] args) {
        try {
            withdraw(100, 500);
        } catch (InsufficientFundsException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

5. Try-with-Resources (Java 7)

Automatically closes resources (like files, streams, or database connections) that implement the AutoCloseable interface, removing the need for a finally block just to close them.

TryWithResourcesDemo.javaJava
import java.io.*;

public class TryWithResourcesDemo {
    public static void main(String[] args) {
        // Scanner or Streams declared here are automatically closed
        try (BufferedReader br = new BufferedReader(new FileReader("test.txt"))) {
            System.out.println(br.readLine());
        } catch (IOException e) {
            System.out.println("File error: " + e.getMessage());
        }
    }
}