public class User {

    private int id;

    private String name;

    public User(int id, String name) {

        this.id = id;

        this.name = name;

    }

    @Override

    public String toString() {

        return "User{" +

                "id=" + id +

                ", name='" + name + '\'' +

                '}';

    }
　　//省略get、set方法

}

import java.util.ArrayList;

import java.util.List;

public class main {

    public static void main(String[] args) {

        User user1 = new User(1, "a1");

        User user2 = new User(2, "a2");

        List list = new ArrayList();

        list.add(user1);

        list.add(user2);

        for (int i=0; i<list.size(); i++){

            System.out.println(list.get(i));

        }

        System.out.println("*******************");

        user1.setId();

        user1.setName("a11");   //此处只是修改了user1对象的属性，不需要调用ArrayList的set()方法就可以生效

        for (int i=0; i<list.size(); i++){

            System.out.println(list.get(i));

        }

        System.out.println("*******************");

        user1 = new User(111, "a111");

        list.set(0, user1);     //user1对象为新new的对象，所以此处必须调用ArrayList的set()方法才会生效

        for (int i=0; i<list.size(); i++){

            System.out.println(list.get(i));

        }

    }

}