Java8List转Map的value冲突重复将value合并成List解决

原文链接: https://www.cnblogs.com/coding88/p/12748945.html

本来是一个菜单集合，然后按父节点分组，将子节点全部合并到一个list中

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List<SysMenu> menuList = new ArrayList<>(); Map<String, List<SysMenu>> menuMap = menutList.stream().collect(Collectors.toMap(SysMenu::getParentId, menuObj -> Lists.newArrayList(menuObj), (List<SysMenu> newValueList, List<SysMenu> oldValueList) -> {     oldValueList.addAll(newValueList);     return oldValueList; }));