Java 程序示例：实现了一个简单的社交媒体平台：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

class User {
    private String username;
    private String password;
    private List<Post> posts;

    public User(String username, String password) {
        this.username = username;
        this.password = password;
        posts = new ArrayList<>();
    }

    public String getUsername() {
        return username;
    }

    public void addPost(Post post) {
        posts.add(post);
    }

    public List<Post> getPosts() {
        return posts;
    }
}

class Post {
    private String content;
    private String author;
    private List<String> comments;

    public Post(String content, String author) {
        this.content = content;
        this.author = author;
        comments = new ArrayList<>();
    }

    public String getContent() {
        return content;
    }

    public String getAuthor() {
        return author;
    }

    public void addComment(String comment) {
        comments.add(comment);
    }

    public List<String> getComments() {
        return comments;
    }
}

class SocialMediaPlatform {
    private Map<String, User> users;

    public SocialMediaPlatform() {
        users = new HashMap<>();
    }

    public void addUser(User user) {
        users.put(user.getUsername(), user);
    }

    public User getUser(String username) {
        return users.get(username);
    }
}

public class Main {
    public static void main(String[] args) {
        SocialMediaPlatform platform = new SocialMediaPlatform();

        User user1 = new User("alice", "password123");
        User user2 = new User("bob", "password456");

        platform.addUser(user1);
        platform.addUser(user2);

        Post post1 = new Post("Hello, world!", "alice");
        Post post2 = new Post("I love programming.", "bob");

        user1.addPost(post1);
        user2.addPost(post2);

        post1.addComment("Nice post!");
        post2.addComment("Keep up the good work!");

        System.out.println("User: " + user1.getUsername());
        System.out.println("Posts:");
        for (Post post : user1.getPosts()) {
            System.out.println("Content: " + post.getContent());
            System.out.println("Author: " + post.getAuthor());
            System.out.println("Comments:");
            for (String comment : post.getComments()) {
                System.out.println("- " + comment);
            }
            System.out.println("-----------------------");
        }
    }
}