What is the difference between checked and unchecked exceptions?

The compiler enforces checked exceptions through the throws clause. IOException, SQLException and InterruptedException are checked. NullPointerException, IllegalArgumentException and IllegalStateException are unchecked because they usually indicate a bug rather than an expected environmental failure.

public String readFirstLine(Path path) throws IOException {
    try (BufferedReader reader = Files.newBufferedReader(path)) {
        return reader.readLine();
    }
}

Design trade-offs

• Checked exceptions force callers to acknowledge a recoverable failure
• They can clutter every layer and encourage catch-and-ignore
• Unchecked exceptions keep signatures clean but rely on documentation and validation
• Spring translates many data-access failures into unchecked DataAccessException subclasses

Use checked exceptions when the caller can plausibly recover, such as retrying a transient IO failure or asking for a different file. Use unchecked exceptions for invalid arguments and broken invariants. At an application boundary, translate infrastructure exceptions into a domain-specific unchecked exception with the original cause preserved.