public String toString() {
        StringBuilder sb = new StringBuilder();
        int i = 1;
        for (Identifier identifier: mIdentifiers) {
            if (i > 1) {
                sb.append(" ");
            }
            sb.append("id");
            sb.append(i);
            sb.append(": ");
            sb.append(identifier == null ? "null" : identifier.toString());
            i++;
        }
        return sb.toString();
    }
@Test
    public void testToString() {
        Region region = new Region("myRegion", Identifier.parse("1"), Identifier.parse("2"), null);
        assertEquals("id1: 1 id2: 2 id3: null", region.toString());
    }