java.lang.String::hashCode() should check for count == 0 to avoid repeated stores hash = 0.
Something like this:

    public int hashCode() {
        int h = hash;
-       if (h == 0) {
+      if (h == 0 && count > 0) {
            int off = offset;
            char val[] = value;
            int len = count;

            for (int i = 0; i < len; i++) {
                h = 31*h + val[off++];
            }
            hash = h;
        }
        return h;
    }