java.nio.charset.Charset.defaultCharset()

So please don’t use messy, old workarounds such as this:

byte [] byteArray = {'a'};
InputStream inputStream = new ByteArrayInputStream(byteArray);
InputStreamReader reader = new InputStreamReader(inputStream);
String defaultEncoding = reader.getEncoding();

Which, unfortunately, is one of the first things you stumble upon when searching for this topic.