class ContentUriModel {

    @Override
    public String toString() {
        return PREFIX + server + "/" + mediaId;
    }

}

class ContentUriModelTest {

    @Test
    public void toStringTest() {

        ContentUriModel model = new ContentUriModel("ru-matrix.org", "123");
        assertEquals("mxc://ru-matrix.org/123", model.toString());
    }

}