What are the key exception classes in Java?

In Java, the exception hierarchy starts with the class Throwable. It has two main subclasses: Error and Exception.

Error:

• Represents serious problems that usually should not be caught by the application.
• Occur due to the virtual machine (JVM) or environment issues.
• Examples: OutOfMemoryError, StackOverflowError.

Exception:

• Represents conditions that the application can and should handle.
• Divided into two types:
  • Checked Exceptions:
    • Must be caught (try-catch) or declared in the method signature (throws).
    • The compiler enforces their handling.
    • Usually represent predictable but undesirable conditions that can occur at runtime.
    • Examples: IOException, FileNotFoundException, SQLException.
  • Unchecked Exceptions:
    • Inherit from RuntimeException.
    • Do not require mandatory handling or declaration.
    • Often indicate logical errors or incorrect API usage.
    • Examples: NullPointerException, ArrayIndexOutOfBoundsException, IllegalArgumentException.

Here are the main exception classes:

• java.lang.Throwable (Base class)
  • java.lang.Error (Unrecoverable errors)
    • java.lang.OutOfMemoryError
    • java.lang.StackOverflowError
  • java.lang.Exception (Recoverable exceptions)
    • java.io.IOException (Checked)
      • java.io.FileNotFoundException (Checked)
    • java.sql.SQLException (Checked)
    • java.lang.RuntimeException (Unchecked)
      • java.lang.NullPointerException
      • java.lang.IllegalArgumentException
      • java.lang.IndexOutOfBoundsException

Comparison table:

Feature	Checked Exception	Unchecked Exception (RuntimeException)	Error
Inheritance	Exception (except RuntimeException and its subclasses)	RuntimeException	Error
Mandatory handling/declaration	Yes	No	No
Cause	Predictable failures	Programming errors, incorrect usage	Serious JVM/environment issues
Recovery	Often possible	Rare (indicates a bug)	Almost impossible

Example of exception handling:

// Example of handling checked and unchecked exceptions
public class ExceptionHandlingDemo {

    public void readFile(String fileName) {
        try {
            // Attempt to read a file - may throw FileNotFoundException (checked)
            java.io.BufferedReader reader = new java.io.BufferedReader(new java.io.FileReader(fileName));
            String line = reader.readLine();
            while (line != null) {
                System.out.println(line);
                line = reader.readLine();
            }
            reader.close(); // Also may throw IOException (checked)

            // Example of unchecked exception (may occur if fileName == null)
            int length = fileName.length(); // NullPointerException (unchecked)

        } catch (java.io.FileNotFoundException e) {
            // Handle FileNotFoundException
            System.err.println("Error: File not found - " + e.getMessage());
        } catch (java.io.IOException e) {
            // Handle general I/O errors
            System.err.println("File read error: " + e.getMessage());
        } catch (java.lang.NullPointerException e) {
            // Handle NullPointerException - optional but good for debugging
            System.err.println("Error: Null reference used - " + e.getMessage());
        }
    }

    public static void main(String[] args) {
        ExceptionHandlingDemo demo = new ExceptionHandlingDemo();
        demo.readFile("non_existent_file.txt"); // Will throw FileNotFoundException
        demo.readFile(null); // May throw NullPointerException
    }
}