笔试-2020/6/8

一、菜单树的实现

数据库表：

ID	Name	Parent_id
M1	编程语言
M2	办公工具
M1_1	前端语言	M1
M1_2	后端语言	M1
M2_1	Excel	M2
M2_2	Word	M2
M1_1_1	JS	M1_1
M1_2_1	Java	M1_2
M1_2_2	C++	M1_2

 

 

 

 

 

 

 

 

 

 

 

 

实体类：

public class Menu {
    private String id;
    private String name;
    private String parentId;
    private List<Menu> childMenus;

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

    public String getParentId() {
        return parentId;
    }

    public void setParentId(String parentId) {
        this.parentId = parentId;
    }

    public String getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public List<Menu> getChildMenus() {
        return childMenus;
    }

    public void setChildMenus(List<Menu> childMenus) {
        this.childMenus = childMenus;
    }

    @Override
    public String toString() {
        return "Menu{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                ", childMenus=" + childMenus +
                '}';
    }
}

获取整理好的菜单列表：

public static void main(String[] args) {
    List<Menu> list = new ArrayList<Menu>();
    list.add(new Menu("M1", "编程语言", null));
    list.add(new Menu("M2", "办公工具", null));
    list.add(new Menu("M1_1", "前端语言", "M1"));
    list.add(new Menu("M1_2", "后端语言", "M1"));
    list.add(new Menu("M2_1", "Excel", "M2"));
    list.add(new Menu("M2_1", "Word", "M2"));
    list.add(new Menu("M1_1_1", "JavaScript", "M1_1"));
    list.add(new Menu("M1_2_1", "Java", "M1_2"));
    list.add(new Menu("M1_2_1", "C++", "M1_2"));

    ArrayList<Menu> newMenus = new ArrayList<Menu>();

    //获取一级菜单
    for (Menu menu:list) {
        if (menu.getParentId() == null) {
            newMenus.add(menu);
            menu.setChildMenus(getChild(menu.getId(), list));
        }
    }

    System.out.println(newMenus);

}

public static List<Menu> getChild(String id, List<Menu> list) {
    List<Menu> childMenus = new ArrayList<Menu>();
    for (Menu menu:list) {
        if (StringUtils.isNotBlank(menu.getParentId())) {
            if (menu.getParentId().equals(id)) {
                childMenus.add(menu);
            }
        }
    }

    for (Menu menu:childMenus) {
        menu.setChildMenus(getChild(menu.getId(), list));
    }

    if (childMenus.size() == 0) {
        return null;
    }


    return childMenus;
}

二、SQL语句类型

DDL（数据定义语言）：create、drop、alter

DML（数据操作语言）：select、insert、update、delete

DCL（数据控制语言）：commit、rollback、revoke、grant

三、父子类加载顺序

public class Animal {

    static {
        System.out.println("父类静态代码块");
    }

    public Animal() {
        System.out.println("父类无参构造器");
    }

    public void test() {
        System.out.println("父类方法");
    }
}

class Cat extends Animal {

    static {
        System.out.println("子类静态代码块");
    }

    public Cat() {
        System.out.println("子类无参构造器");
    }

    public void test() {
        System.out.println("子类方法");
    }

    public static void main(String[] args) {
        Cat cat = new Cat();
        cat.test();
    }
}

结果如下：

父类静态代码块
子类静态代码块
父类无参构造器
子类无参构造器
子类方法

四、单例模式实现

1、饿汉式

public class MySingleton {

    private static MySingleton instance = new MySingleton();

    private MySingleton() {

    }

    public static MySingleton getInstance() {
        return instance;
    }
}

2、懒汉式

public class MySingleton {

    private static MySingleton instance;

    private MySingleton() {

    }

    public static MySingleton getInstance() {
        if (instance == null) {
            instance = new MySingleton();
        }
        return instance;
    }
}

五、GROUP BY ... HAVING使用

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value;