assert(false) vs RuntimeException?

Question

I'm reading the source of XWalkUIClientInternal and I ran into the following code:

    switch(type) {
        case JAVASCRIPT_ALERT:
            return onJsAlert(view, url, message, result);
        case JAVASCRIPT_CONFIRM:
            return onJsConfirm(view, url, message, result);
        case JAVASCRIPT_PROMPT:
            return onJsPrompt(view, url, message, defaultValue, result);
        case JAVASCRIPT_BEFOREUNLOAD:
            // Reuse onJsConfirm to show the dialog.
            return onJsConfirm(view, url, message, result);
        default:
            break;
    }
    assert(false);
    return false;

I've never really seen this technique nor really thought about it before, but I guess this essentially means "this is unreachable code and should not happen ever", and crash the app no matter what. Although technically you can do that with a Throwable, as long as it's not caught.

So my question is, which one is better and why, assert(false) or throwing a RuntimeException, or maybe an Error?

Answer 1

The biggest difference between

assert false;

(The parenthesis are not needed, assert is not a function but a statement.) and

throw new RuntimeException();

is that the assertion can be disabled. Actually, it is disabled by default unless the JVM is started with the -ea (“enable assertions”) flag. If assertions are enabled, assert false will unconditionally throw an AssertionError which derives from Error. But since assertions can be disabled, there are two problems,

• the error might go undetected and
• control flow analysis requires a dummy return statement after the assert (which is mostly clutter).

Therefore, in the above case, I'd certainly go with an explicit (and more concise)

throw new AssertionError("invalid type " + type);

instead of an assert followed by a dummy return.

As mentioned in the comments, this is assuming that type is an internal parameter and an invalid value indicates a bug in the logic itself. If it is an input parameter, it should be validated according to the usual rules and an IllegalArgumentException be thrown if validation fails.