Don't return null in java development

Do not return null

Counterexample

##Positive example

When calling methods elsewhere, avoid unnecessary null pointers

optional null

//获取子目录列表
public List<CatalogueTreeNode> getChild(String pid) {
            if (V.isEmpty(pid)) {
            pid = BasicDic.TEMPORARY_DIRECTORY_ROOT;
        }
        CatalogueTreeNode node = treeNodeMap.get(pid);
 
        return Optional.ofNullable(node)
                .map(CatalogueTreeNode::getChild)
                .orElse(Collections.emptyList());
    }

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