Question

Consider the following code:

public class ReadingTest {

    public void readAndPrint(String usingEncoding) throws Exception {
        ByteArrayInputStream bais = new ByteArrayInputStream(new byte[]{(byte) 0xC2, (byte) 0xB5}); // 'micro' sign UTF-8 representation
        InputStreamReader isr = new InputStreamReader(bais, usingEncoding);
        char[] cbuf = new char[2];
        isr.read(cbuf);
        System.out.println(cbuf[0]+" "+(int) cbuf[0]);
    }

    public static void main(String[] argv) throws Exception {
        ReadingTest w = new ReadingTest();
        w.readAndPrint("UTF-8");
        w.readAndPrint("US-ASCII");
    }
}

Observed output:

µ 181
? 65533

Why does the second call of readAndPrint() (the one using US-ASCII) succeed? I would expect it to throw an error, since the input is not a proper character in this encoding. What is the place in the Java API or JLS which mandates this behavior?

Answer 1

The default operation when finding un-decodable bytes in the input-stream is to replace them with the Unicode Character U+FFFD REPLACEMENT CHARACTER.

If you want to change that, you can pass a CharacterDecoder to the InputStreamReader which has a different CodingErrorAction configured:

CharsetDecoder decoder = Charset.forName(usingEncoding).newDecoder();
decoder.onMalformedInput(CodingErrorAction.REPORT);
InputStreamReader isr = new InputStreamReader(bais, decoder);