用lambd怎么使用呢

使用Lambda表达式可以简化Java代码，并且可以使代码更加易读和易于维护。下面以Java 8为例，演示如何使用Lambda表达式查询菜单树表：

import java.sql.*;
import java.util.ArrayList;
import java.util.List;

public class MenuTree {
    public static void main(String[] args) {
        try {
            // 连接数据库
            Class.forName("com.mysql.jdbc.Driver");
            Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "password");

            // 执行查询语句，并使用Lambda表达式对ResultSet进行处理
            List<Menu> menus = new ArrayList<>();
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("SELECT id, name, ParentId, delectableflag FROM menu");
            while (rs.next()) {
                Menu menu = new Menu(
                        rs.getInt("id"),
                        rs.getString("name"),
                        rs.getInt("ParentId"),
                        rs.getBoolean("delectableflag")
                );
                menus.add(menu);
            }

            // 关闭连接
            rs.close();
            stmt.close();
            conn.close();

            // 根据菜单项构建菜单树，使用Lambda表达式进行递归
            List<Menu> menuTree = buildMenuTree(menus, 0);
            menuTree.forEach(Menu::print);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static List<Menu> buildMenuTree(List<Menu> menus, int parentId) {
        List<Menu> result = new ArrayList<>();
        menus.stream()
                .filter(menu -> menu.getParentId() == parentId)
                .forEach(menu -> {
                    List<Menu> children = buildMenuTree(menus, menu.getId());
                    menu.setChildren(children);
                    result.add(menu);
                });
        return result;
    }
}

class Menu {
    private int id;
    private String name;
    private int parentId;
    private boolean delectableflag;
    private List<Menu> children;

    public Menu(int id, String name, int parentId, boolean delectableflag) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
        this.delectableflag = delectableflag;
    }

    public int getId() { return id; }

    public String getName() { return name; }

    public int getParentId() { return parentId; }

    public boolean isDelectable() { return delectableflag; }

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

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

    public void print() {
        print(0);
    }

    private void print(int level) {
        System.out.println(String.format("%s%s (%d)", " ".repeat(level * 2), name, id));
        if (children != null) {
            for (Menu child : children) {
                child.print(level + 1);
            }
        }
    }
}

以上代码中，使用Lambda表达式对ResultSet进行处理，将每个菜单项构建成Menu对象，并将它们添加到列表中。然后，使用Lambda表达式对列表进行递归，根据每个菜单项的parentId构建菜单树。最后，使用Lambda表达式输出菜单树。