迭代器及匿名内部类 跟 List何时用new何时用null

迭代器及匿名内部类 和 List何时用new何时用null
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List<Integer> pageIdList = new ArrayList<Integer>();            for (AppPage appPage : appPageResult.getAppPageList()) {                pageIdList.add(appPage.getId());            }                        List<UserPlayFlowResult> friendIdList = null;            if(!CollectionUtils.isEmpty(pageIdList)){                friendIdList = userPlayService.getFriendPlayByPageIds(userId, pageIdList, EXPIRED_DAYS);            }