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本文链接：https://blog.youkuaiyun.com/chadcao/article/details/133219809

文章介绍了如何在Java中创建城市类和扩展其为树状结构，通过GetList方法获取城市列表，GetTree方法将列表转换为树形结构，以展示地区层级关系。

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1）创建城市类和树状结构城市类

//城市类
class City {
    private int id;
    private String name;
    private int upperid;

    public City(int id, String name, int upperid) {
        this.id = id;
        this.name = name;
        this.upperid = upperid;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getUpperId() {
        return upperid;
    }


    public void setId(int _id) {
        this.id = _id;
    }

    public void setName(String _name) {
        this.name = _name;
    }

    public void setUpperId(int _upperid) {
        this.upperid = _upperid;
    }
}
//树状结构城市类
class TreeNode extends City {
    private List<TreeNode> children = new ArrayList<TreeNode>();

    public List<TreeNode> getChildren() {
        return children;
    }

    public void setChildren(List<TreeNode> _children) {
        this.children = _children;
    }

    public TreeNode(int id, String name, int upperid) {
        super(id, name,upperid);
    }
}

2）初始化数据，创建将列表集合转化成树集合

// 获取城市列表
private List<City> GetList() {
    List<City> list = new ArrayList<>();
    list.add(new City(1, "浙江", 0));
    list.add(new City(2, "江苏", 0));
    list.add(new City(3, "福建", 0));
    list.add(new City(11, "杭州", 1));
    list.add(new City(12, "嘉兴", 1));
    list.add(new City(21, "苏州", 2));
    list.add(new City(22, "无锡", 2));
    list.add(new City(31, "福州", 3));
    list.add(new City(32, "厦门", 3));
    list.add(new City(111, "萧山", 11));
    list.add(new City(112, "临安", 11));
    list.add(new City(121, "嘉善", 12));
    list.add(new City(122, "桐乡", 12));
    list.add(new City(211, "吴江", 21));
    list.add(new City(212, "常熟", 21));
    return list;
}
// 获取城市树状结构
private List<TreeNode> GetTree(int upperId, List<City> list) {
    return list.stream()
            .filter(a -> a.getUpperId() == upperId)
            .map(child -> {
                TreeNode node = new TreeNode(
                        child.getId(),
                        child.getName(),
                        child.getUpperId()
                );
                List<TreeNode> myChildren = GetTree(child.getId(), list);
                node.setChildren(myChildren);
                return node;
            })
            .collect(Collectors.toList());
}

3）调用

List<City> list = GetList();
List<TreeNode> tree = GetTree(0, list);