Java - Exceptions & Error Handling

Exceptions & Error Handling

Checked vs Unchecked

• Checked: must be declared or caught (e.g., IOException, SQLException).
• Unchecked: RuntimeException and subclasses (e.g., NullPointerException, IllegalArgumentException).

try/catch/finally

try {
  risky();
} catch (SpecificException e) {
  // recover or wrap with context
} catch (Exception e) {
  // last-resort handler
} finally {
  cleanup();
}

Try-with-Resources

import java.nio.file.*;
try (var reader = Files.newBufferedReader(Path.of("file.txt"))) {
  System.out.println(reader.readLine());
} catch (java.io.IOException e) {
  e.printStackTrace();
}

Custom Exceptions

class DomainException extends RuntimeException {
  DomainException(String msg) { super(msg); }
}

Best Practices

• Throw exceptions to indicate exceptional states; don’t use for control flow.
• Prefer specific exception types; add context (IDs, parameters) to messages.
• Don’t swallow exceptions; either handle or rethrow with context.
• Log once, at the boundary; avoid duplicate logs for the same failure.

Try it

1. Wrap file reading in try-with-resources; print a helpful message if the file is missing.
2. Create a custom runtime exception and throw it when an invariant is violated.