public String toFriendlyString() {
        return FRIENDLY_FORMAT.format(this).toString();
    }
@Test
    public void testToFriendlyString() {
        assertEquals("1.00 BTC", COIN.toFriendlyString());
        assertEquals("1.23 BTC", valueOf(1, 23).toFriendlyString());
        assertEquals("0.001 BTC", COIN.divide(1000).toFriendlyString());
        assertEquals("-1.23 BTC", valueOf(1, 23).negate().toFriendlyString());
    }