Exceptions and try-with-resources

The concept

An exception is a failure travelling up the call stack until somebody handles it. try { … } catch (IOException e) { … } finally { … } is the shape: the risk in the try, the handling in the catch, the clean-up in finally — and throw new IllegalArgumentException("age must be positive") is how your own code reports a bad input rather than returning a wrong number.

Checked exceptions must be declared or caught, which is why file work in Java looks noisy: throws IOException on the method, or a catch around it. try-with-resources — try (BufferedReader r = Files.newBufferedReader(path)) { … } — closes the file for you even when it throws, which is the modern replacement for a finally block.

try {
    int n = Integer.parseInt("abc");
} catch (NumberFormatException e) {
    System.out.println("Not a number: " + e.getMessage());
} finally {
    // runs whether or not an exception was thrown
}

// try-with-resources closes the reader for you, even on error.
try (BufferedReader r = new BufferedReader(new FileReader("data.txt"))) {
    String line;
    while ((line = r.readLine()) != null) System.out.println(line);
} catch (IOException e) {
    System.out.println("Could not read: " + e.getMessage());
}