Exceptions Summary

Exception Handling Basics

• Mechanism to handle runtime errors
• Uses try-catch-finally blocks
• Allows separation of error handling from main logic
• Example:

  try {
    // code that might throw exception
    readFromFile(filename);
  } catch (FileNotFoundException e) {
    // handle the exception
    System.err.println("File not found: " + e.getMessage());
  } finally {
    // cleanup code, always executed
    closeFile();
  }
  

Types of Exceptions

1. Checked Exceptions:

   • Must be declared or caught
   • Extend Exception class
   • Example: IOException

2. Unchecked Exceptions:

   • Don't need to be declared
   • Extend RuntimeException
   • Example: NullPointerException

3. Errors:

   • Serious problems
   • Not meant to be caught
   • Example: OutOfMemoryError

Throwing Exceptions

• Use throw keyword
• Can throw new exceptions
• Must declare checked exceptions
• Example:

  public void readFile(String fileName) throws IOException {
    throw new IOException("File not found");
  }
  

Creating Custom Exceptions

• Extend Exception or RuntimeException
• Add constructors and fields as needed
• Example:

  class InvalidCircleException extends IllegalArgumentException {
    public InvalidCircleException(String message) {
      super(message);
    }
  }
  

Exception Handling Best Practices

• Only catch exceptions you can handle
• Don't catch and ignore exceptions
• Clean up resources in finally block
• Use specific exception types
• Document exceptions in Javadoc
• Don't use exceptions for flow control

Common Mistakes to Avoid

• Empty catch blocks
• Catching Exception (too general)
• Not cleaning up resources
• Throwing wrong exception type
• Example of bad practice:

  try {
    // code
  } catch (Exception e) {
    // empty catch block - DON'T DO THIS
  }
  

Try-with-Resources

• Automatically closes resources
• For classes implementing AutoCloseable
• Example:

  try (FileReader reader = new FileReader(file)) {
    // use reader
  } catch (IOException e) {
    // handle exception
  }
  // reader automatically closed