Behaviour of Bytestring.utf8() with bytes that aren't valid UTF-8

Is there a defined behaviour for ByteString.utf8(...) for sequences of bytes which are not valid UTF-8?

I’ve tried the following:

bytes[] invalidUtf8 = {(byte)0xFF}; // 0xFF will never appear in valid UTF-8
ByteString b = ByteString.of(invalidUtf8);
System.out.println(b.utf8()); // �

I was expecting an exception of some kind (though I could very will be incorrect with my “invalid” UTF-8). Is there a defined behaviour in this case?

[cketti]

Is it a safe assumption that if the result of ByteString.utf8() contains the unicode replacement character then the underlying bytes are not valid UTF-8?

Not really. The replacement character is a valid codepoint that can legitimately be part of a string. See for example @swankjesse’s comment above.

[swankjesse]

You can check with Okio by encoding and decoding and comparing. Valid UTF-8 will roundtrip without changes; invalid UTF-8 won’t.