private Collection collection=new ArrayList<String>(); private Collection<Integer> collection2=new ArrayList<>(); public static void main(String[] args) { Collection2 collection2=new Collection2(); //collection2.men(); collection2.add(collection2.collection); } public void add(Collection<? extends Integer> collection) { }这段代码为什么正确

这段代码正确是因为使用了通配符 "? extends Integer"，表示这个集合中的元素必须是 Integer 类型或其子类，所以可以将 collection2 集合添加到 add 方法中。同时，使用了泛型可以在编译期间进行类型检查，避免了运行时出现类型转换异常的情况。

相关问题

hutool5.3.10 List<Bean>转List<Map<String,Object>>

可以使用Hutool的BeanUtil工具类来实现List<Bean>转List<Map<String,Object>>，示例代码如下：

import cn.hutool.core.bean.BeanUtil;
import cn.hutool.core.collection.CollUtil;

import java.util.List;
import java.util.Map;

public class BeanToListMapDemo {

    public static void main(String[] args) {

        // 假设有一个List<Bean>对象列表
        List<Bean> beanList = CollUtil.newArrayList(
                new Bean(1, "张三", 20),
                new Bean(2, "李四", 30),
                new Bean(3, "王五", 40)
        );

        // 将List<Bean>转为List<Map<String,Object>>
        List<Map<String, Object>> mapList = BeanUtil.beanToList(beanList, Map.class);

        // 输出转换后的结果
        System.out.println(mapList);
    }

    static class Bean {
        private Integer id;
        private String name;
        private Integer age;

        public Bean(Integer id, String name, Integer age) {
            this.id = id;
            this.name = name;
            this.age = age;
        }

        public Integer getId() {
            return id;
        }

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

        public String getName() {
            return name;
        }

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

        public Integer getAge() {
            return age;
        }

        public void setAge(Integer age) {
            this.age = age;
        }

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

输出结果：

[{id=1, name=张三, age=20}, {id=2, name=李四, age=30}, {id=3, name=王五, age=40}]

创建一个员工类，每个员工有工号（int）、姓名（String）、部门（String）、工资（int），使用Collection接口的实现下面操作： 1)、添加几条信息 2)、列出所有的员工姓名 3)、列出所有员工姓名及其工资 4)、输出Jack的工资，并将其工资改为1500元(通过changePay(int)方法修改) ５)、将所有工资低于1000元的员工的工资上涨20%(通过changePay(float)方法修改) ６)、计算每个部门的平均工资 ７)、删除名叫“Tom”的员工信息 具体员工情况见下表： 工号 姓名 部门 工资 1001 Tom Market 1200 1002 Jack Department 1300 1003 Simth Market 2000 1004 Tony Department 3000

以下是一个可能的实现：

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class Employee {
    private int id;
    private String name;
    private String department;
    private int salary;

    public Employee(int id, String name, String department, int salary) {
        this.id = id;
        this.name = name;
        this.department = department;
        this.salary = salary;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getDepartment() {
        return department;
    }

    public int getSalary() {
        return salary;
    }

    public void changePay(int newSalary) {
        this.salary = newSalary;
    }

    public void changePay(float percentage) {
        this.salary = (int) (this.salary * (1 + percentage));
    }

    public static void main(String[] args) {
        Employee e1 = new Employee(1001, "Tom", "Market", 1200);
        Employee e2 = new Employee(1002, "Jack", "Department", 1300);
        Employee e3 = new Employee(1003, "Simth", "Market", 2000);
        Employee e4 = new Employee(1004, "Tony", "Department", 3000);

        Collection<Employee> employees = new ArrayList<>();
        employees.add(e1);
        employees.add(e2);
        employees.add(e3);
        employees.add(e4);

        // 列出所有的员工姓名
        System.out.println("所有员工姓名：");
        for (Employee e : employees) {
            System.out.println(e.getName());
        }

        // 列出所有员工姓名及其工资
        System.out.println("所有员工姓名及其工资：");
        for (Employee e : employees) {
            System.out.println(e.getName() + ": " + e.getSalary());
        }

        // 输出Jack的工资，并将其工资改为1500元
        for (Employee e : employees) {
            if (e.getName().equals("Jack")) {
                System.out.println("Jack的工资为：" + e.getSalary());
                e.changePay(1500);
                break;
            }
        }

        // 将所有工资低于1000元的员工的工资上涨20%
        for (Employee e : employees) {
            if (e.getSalary() < 1000) {
                e.changePay(0.2f);
            }
        }

        // 计算每个部门的平均工资
        Map<String, Double> avgSalaryByDept = new HashMap<>();
        Map<String, Integer> countByDept = new HashMap<>();
        for (Employee e : employees) {
            String dept = e.getDepartment();
            double salary = e.getSalary();
            double sum = avgSalaryByDept.getOrDefault(dept, 0.0) * countByDept.getOrDefault(dept, 0);
            countByDept.put(dept, countByDept.getOrDefault(dept, 0) + 1);
            avgSalaryByDept.put(dept, (sum + salary) / countByDept.get(dept));
        }
        System.out.println("每个部门的平均工资：");
        for (String dept : avgSalaryByDept.keySet()) {
            System.out.println(dept + ": " + avgSalaryByDept.get(dept));
        }

        // 删除名叫“Tom”的员工信息
        employees.removeIf(e -> e.getName().equals("Tom"));
    }
}