protected String keyToString(byte key) {
        return Byte.toString(key);
    }
Helper method called by {@link #toString()} in order to convert a single map key into a string.
This is protected to allow subclasses to override the appearance of a given key.
jraft-rheakv/rheakv-core/src/main/java/com/alipay/sofa/jraft/rhea/util/ByteObjectHashMap.java