throw vs throws: raising and declaring

throw raises an exception; throws declares it

throw is an executable statement that raises an exception:

public static double divide(int a, int b) {
    if (b == 0) throw new ArithmeticException("divisor is zero");
    return (double) a / b;
}

throws is a method-signature clause that declares that a method might propagate a checked exception:

public String readConfig(String path) throws IOException {
    return Files.readString(Path.of(path));  // IOException from Files is checked
}

Callers of readConfig must now either: - catch (IOException e) { ... } — handle it, or - themselves declare throws IOException — pass it up

This is the handle-or-declare rule for checked exceptions.

Wrapping a lower-level exception as the cause lets higher-level code use a domain exception while preserving the original stack trace:

public User loadUser(int id) throws ServiceException {
    try {
        return db.query(id);
    } catch (SQLException e) {
        throw new ServiceException("could not load user " + id, e);  // e is the cause
    }
}

The cause is accessible via getCause() on the thrown exception.