Gson does not go with instance initializers!

Comment.java

public class Comment {
    public String author;
    public String content;

    public Comment(String author, String content) {
        this.author = author;
        this.content = content;
    }
}

Post.java

import java.util.ArrayList;
import java.util.List;


public class Post {
    public String title;
    public String content;
    public User author;
    public List<Comment> comments;

    public Post() {
        this.comments = new ArrayList<Comment>();
    }

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

    public void setUser(User user) {
        this.author = user;
    }
}

ToJsonSample.java

import com.google.gson.Gson;

public class ToJsonSample {
    /*
     *  JavaオブジェクトからJSONへの変換
     */
    public static void main(String[] args) {
        User user = new User("jeff@jmail.com", "Jeff");
        
        {
            Post newPost = new Post() {
                    {
                        setUser(user);
                        this.title = null;
                        this.content = null;
                    }
                };
            Comment comment = new Comment("comment_author", "comment_comment");
            newPost.comments.add(comment);
            newPost.comments.add(comment);
            System.out.println("Gson Post" + new Gson().toJson(newPost)); // null
            System.out.println("  This Post class is : " + newPost.getClass());  // Postではなく一時クラス
            System.out.println("  Gson Post: " + new Gson().toJson(newPost, Post.class)); // なお，Typeを指定すると正しく表示できる
            // {"author":{"email":"jeff@jmail.com","fullname":"Jeff"},"comments":[{"author":"comment_author","content":"comment_comment"},{"author":"comment_author","content":"comment_comment"}]}
        }
        {
            Post newPost = new Post();
            newPost.author = mUsers.get(0);
            Comment comment = new Comment("comment_author", "comment_comment");
            newPost.comments.add(comment);
            newPost.comments.add(comment);
            System.out.println("Gson Post: " + new Gson().toJson(newPost));
            //  {"author":{"email":"jeff@jmail.com","fullname":"Jeff"},"comments":[{"author":"comment_author","content":"comment_comment"},{"author":"comment_author","content":"comment_comment"}]}
            System.out.println("  This Post class is : " + newPost.getClass()); // Post
        }
    }
}

User.java

public class User {
    public String email;
    public String fullname;

    public User(String email, String fullname) {
        this.email = email;
        this.fullname = fullname;
    }
}